FreeBSD/Linux Kernel Cross Reference
sys/cam/cam_iosched.c

     /*-
      * CAM IO Scheduler Interface
      *
      * SPDX-License-Identifier: BSD-2-Clause
      *
      * Copyright (c) 2015 Netflix, Inc.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     * $FreeBSD$
     */
    
    #include "opt_cam.h"
    #include "opt_ddb.h"
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include <sys/param.h>
    
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/bio.h>
    #include <sys/lock.h>
    #include <sys/malloc.h>
    #include <sys/mutex.h>
    #include <sys/sbuf.h>
    #include <sys/sysctl.h>
    
    #include <cam/cam.h>
    #include <cam/cam_ccb.h>
    #include <cam/cam_periph.h>
    #include <cam/cam_xpt_periph.h>
    #include <cam/cam_xpt_internal.h>
    #include <cam/cam_iosched.h>
    
    #include <ddb/ddb.h>
    
    static MALLOC_DEFINE(M_CAMSCHED, "CAM I/O Scheduler",
        "CAM I/O Scheduler buffers");
    
    static SYSCTL_NODE(_kern_cam, OID_AUTO, iosched, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
        "CAM I/O Scheduler parameters");
    
    /*
     * Default I/O scheduler for FreeBSD. This implementation is just a thin-vineer
     * over the bioq_* interface, with notions of separate calls for normal I/O and
     * for trims.
     *
     * When CAM_IOSCHED_DYNAMIC is defined, the scheduler is enhanced to dynamically
     * steer the rate of one type of traffic to help other types of traffic (eg
     * limit writes when read latency deteriorates on SSDs).
     */
    
    #ifdef CAM_IOSCHED_DYNAMIC
    
    static bool do_dynamic_iosched = true;
    SYSCTL_BOOL(_kern_cam_iosched, OID_AUTO, dynamic, CTLFLAG_RD | CTLFLAG_TUN,
        &do_dynamic_iosched, 1,
        "Enable Dynamic I/O scheduler optimizations.");
    
    /*
     * For an EMA, with an alpha of alpha, we know
     *      alpha = 2 / (N + 1)
     * or
     *      N = 1 + (2 / alpha)
     * where N is the number of samples that 86% of the current
     * EMA is derived from.
     *
     * So we invent[*] alpha_bits:
     *      alpha_bits = -log_2(alpha)
     *      alpha = 2^-alpha_bits
     * So
     *      N = 1 + 2^(alpha_bits + 1)
     *
     * The default 9 gives a 1025 lookback for 86% of the data.
     * For a brief intro: https://en.wikipedia.org/wiki/Moving_average
     *
     * [*] Steal from the load average code and many other places.
     * Note: See computation of EMA and EMVAR for acceptable ranges of alpha.
    */
   static int alpha_bits = 9;
   SYSCTL_INT(_kern_cam_iosched, OID_AUTO, alpha_bits, CTLFLAG_RW | CTLFLAG_TUN,
       &alpha_bits, 1,
       "Bits in EMA's alpha.");
   
   /*
    * Different parameters for the buckets of latency we keep track of. These are all
    * published read-only since at present they are compile time constants.
    *
    * Bucket base is the upper bounds of the first latency bucket. It's currently 20us.
    * With 20 buckets (see below), that leads to a geometric progression with a max size
    * of 5.2s which is safeily larger than 1s to help diagnose extreme outliers better.
    */
   #ifndef BUCKET_BASE
   #define BUCKET_BASE ((SBT_1S / 50000) + 1)      /* 20us */
   #endif
   static sbintime_t bucket_base = BUCKET_BASE;
   SYSCTL_SBINTIME_USEC(_kern_cam_iosched, OID_AUTO, bucket_base_us, CTLFLAG_RD,
       &bucket_base,
       "Size of the smallest latency bucket");
   
   /*
    * Bucket ratio is the geometric progression for the bucket. For a bucket b_n
    * the size of bucket b_n+1 is b_n * bucket_ratio / 100.
    */
   static int bucket_ratio = 200;  /* Rather hard coded at the moment */
   SYSCTL_INT(_kern_cam_iosched, OID_AUTO, bucket_ratio, CTLFLAG_RD,
       &bucket_ratio, 200,
       "Latency Bucket Ratio for geometric progression.");
   
   /*
    * Number of total buckets. Starting at BUCKET_BASE, each one is a power of 2.
    */
   #ifndef LAT_BUCKETS
   #define LAT_BUCKETS 20  /* < 20us < 40us ... < 2^(n-1)*20us >= 2^(n-1)*20us */
   #endif
   static int lat_buckets = LAT_BUCKETS;
   SYSCTL_INT(_kern_cam_iosched, OID_AUTO, buckets, CTLFLAG_RD,
       &lat_buckets, LAT_BUCKETS,
       "Total number of latency buckets published");
   
   /*
    * Read bias: how many reads do we favor before scheduling a write
    * when we have a choice.
    */
   static int default_read_bias = 0;
   SYSCTL_INT(_kern_cam_iosched, OID_AUTO, read_bias, CTLFLAG_RWTUN,
       &default_read_bias, 0,
       "Default read bias for new devices.");
   
   struct iop_stats;
   struct cam_iosched_softc;
   
   int iosched_debug = 0;
   
   typedef enum {
           none = 0,                               /* No limits */
           queue_depth,                    /* Limit how many ops we queue to SIM */
           iops,                           /* Limit # of IOPS to the drive */
           bandwidth,                      /* Limit bandwidth to the drive */
           limiter_max
   } io_limiter;
   
   static const char *cam_iosched_limiter_names[] =
       { "none", "queue_depth", "iops", "bandwidth" };
   
   /*
    * Called to initialize the bits of the iop_stats structure relevant to the
    * limiter. Called just after the limiter is set.
    */
   typedef int l_init_t(struct iop_stats *);
   
   /*
    * Called every tick.
    */
   typedef int l_tick_t(struct iop_stats *);
   
   /*
    * Called to see if the limiter thinks this IOP can be allowed to
    * proceed. If so, the limiter assumes that the IOP proceeded
    * and makes any accounting of it that's needed.
    */
   typedef int l_iop_t(struct iop_stats *, struct bio *);
   
   /*
    * Called when an I/O completes so the limiter can update its
    * accounting. Pending I/Os may complete in any order (even when
    * sent to the hardware at the same time), so the limiter may not
    * make any assumptions other than this I/O has completed. If it
    * returns 1, then xpt_schedule() needs to be called again.
    */
   typedef int l_iodone_t(struct iop_stats *, struct bio *);
   
   static l_iop_t cam_iosched_qd_iop;
   static l_iop_t cam_iosched_qd_caniop;
   static l_iodone_t cam_iosched_qd_iodone;
   
   static l_init_t cam_iosched_iops_init;
   static l_tick_t cam_iosched_iops_tick;
   static l_iop_t cam_iosched_iops_caniop;
   static l_iop_t cam_iosched_iops_iop;
   
   static l_init_t cam_iosched_bw_init;
   static l_tick_t cam_iosched_bw_tick;
   static l_iop_t cam_iosched_bw_caniop;
   static l_iop_t cam_iosched_bw_iop;
   
   struct limswitch {
           l_init_t        *l_init;
           l_tick_t        *l_tick;
           l_iop_t         *l_iop;
           l_iop_t         *l_caniop;
           l_iodone_t      *l_iodone;
   } limsw[] =
   {
           {       /* none */
                   .l_init = NULL,
                   .l_tick = NULL,
                   .l_iop = NULL,
                   .l_iodone= NULL,
           },
           {       /* queue_depth */
                   .l_init = NULL,
                   .l_tick = NULL,
                   .l_caniop = cam_iosched_qd_caniop,
                   .l_iop = cam_iosched_qd_iop,
                   .l_iodone= cam_iosched_qd_iodone,
           },
           {       /* iops */
                   .l_init = cam_iosched_iops_init,
                   .l_tick = cam_iosched_iops_tick,
                   .l_caniop = cam_iosched_iops_caniop,
                   .l_iop = cam_iosched_iops_iop,
                   .l_iodone= NULL,
           },
           {       /* bandwidth */
                   .l_init = cam_iosched_bw_init,
                   .l_tick = cam_iosched_bw_tick,
                   .l_caniop = cam_iosched_bw_caniop,
                   .l_iop = cam_iosched_bw_iop,
                   .l_iodone= NULL,
           },
   };
   
   struct iop_stats {
           /*
            * sysctl state for this subnode.
            */
           struct sysctl_ctx_list  sysctl_ctx;
           struct sysctl_oid       *sysctl_tree;
   
           /*
            * Information about the current rate limiters, if any
            */
           io_limiter      limiter;        /* How are I/Os being limited */
           int             min;            /* Low range of limit */
           int             max;            /* High range of limit */
           int             current;        /* Current rate limiter */
           int             l_value1;       /* per-limiter scratch value 1. */
           int             l_value2;       /* per-limiter scratch value 2. */
   
           /*
            * Debug information about counts of I/Os that have gone through the
            * scheduler.
            */
           int             pending;        /* I/Os pending in the hardware */
           int             queued;         /* number currently in the queue */
           int             total;          /* Total for all time -- wraps */
           int             in;             /* number queued all time -- wraps */
           int             out;            /* number completed all time -- wraps */
           int             errs;           /* Number of I/Os completed with error --  wraps */
   
           /*
            * Statistics on different bits of the process.
            */
                   /* Exp Moving Average, see alpha_bits for more details */
           sbintime_t      ema;
           sbintime_t      emvar;
           sbintime_t      sd;             /* Last computed sd */
   
           uint32_t        state_flags;
   #define IOP_RATE_LIMITED                1u
   
           uint64_t        latencies[LAT_BUCKETS];
   
           struct cam_iosched_softc *softc;
   };
   
   typedef enum {
           set_max = 0,                    /* current = max */
           read_latency,                   /* Steer read latency by throttling writes */
           cl_max                          /* Keep last */
   } control_type;
   
   static const char *cam_iosched_control_type_names[] =
       { "set_max", "read_latency" };
   
   struct control_loop {
           /*
            * sysctl state for this subnode.
            */
           struct sysctl_ctx_list  sysctl_ctx;
           struct sysctl_oid       *sysctl_tree;
   
           sbintime_t      next_steer;             /* Time of next steer */
           sbintime_t      steer_interval;         /* How often do we steer? */
           sbintime_t      lolat;
           sbintime_t      hilat;
           int             alpha;
           control_type    type;                   /* What type of control? */
           int             last_count;             /* Last I/O count */
   
           struct cam_iosched_softc *softc;
   };
   
   #endif
   
   struct cam_iosched_softc {
           struct bio_queue_head bio_queue;
           struct bio_queue_head trim_queue;
                                   /* scheduler flags < 16, user flags >= 16 */
           uint32_t        flags;
           int             sort_io_queue;
           int             trim_goal;              /* # of trims to queue before sending */
           int             trim_ticks;             /* Max ticks to hold trims */
           int             last_trim_tick;         /* Last 'tick' time ld a trim */
           int             queued_trims;           /* Number of trims in the queue */
   #ifdef CAM_IOSCHED_DYNAMIC
           int             read_bias;              /* Read bias setting */
           int             current_read_bias;      /* Current read bias state */
           int             total_ticks;
           int             load;                   /* EMA of 'load average' of disk / 2^16 */
   
           struct bio_queue_head write_queue;
           struct iop_stats read_stats, write_stats, trim_stats;
           struct sysctl_ctx_list  sysctl_ctx;
           struct sysctl_oid       *sysctl_tree;
   
           int             quanta;                 /* Number of quanta per second */
           struct callout  ticker;                 /* Callout for our quota system */
           struct cam_periph *periph;              /* cam periph associated with this device */
           uint32_t        this_frac;              /* Fraction of a second (1024ths) for this tick */
           sbintime_t      last_time;              /* Last time we ticked */
           struct control_loop cl;
           sbintime_t      max_lat;                /* when != 0, if iop latency > max_lat, call max_lat_fcn */
           cam_iosched_latfcn_t    latfcn;
           void            *latarg;
   #endif
   };
   
   #ifdef CAM_IOSCHED_DYNAMIC
   /*
    * helper functions to call the limsw functions.
    */
   static int
   cam_iosched_limiter_init(struct iop_stats *ios)
   {
           int lim = ios->limiter;
   
           /* maybe this should be a kassert */
           if (lim < none || lim >= limiter_max)
                   return EINVAL;
   
           if (limsw[lim].l_init)
                   return limsw[lim].l_init(ios);
   
           return 0;
   }
   
   static int
   cam_iosched_limiter_tick(struct iop_stats *ios)
   {
           int lim = ios->limiter;
   
           /* maybe this should be a kassert */
           if (lim < none || lim >= limiter_max)
                   return EINVAL;
   
           if (limsw[lim].l_tick)
                   return limsw[lim].l_tick(ios);
   
           return 0;
   }
   
   static int
   cam_iosched_limiter_iop(struct iop_stats *ios, struct bio *bp)
   {
           int lim = ios->limiter;
   
           /* maybe this should be a kassert */
           if (lim < none || lim >= limiter_max)
                   return EINVAL;
   
           if (limsw[lim].l_iop)
                   return limsw[lim].l_iop(ios, bp);
   
           return 0;
   }
   
   static int
   cam_iosched_limiter_caniop(struct iop_stats *ios, struct bio *bp)
   {
           int lim = ios->limiter;
   
           /* maybe this should be a kassert */
           if (lim < none || lim >= limiter_max)
                   return EINVAL;
   
           if (limsw[lim].l_caniop)
                   return limsw[lim].l_caniop(ios, bp);
   
           return 0;
   }
   
   static int
   cam_iosched_limiter_iodone(struct iop_stats *ios, struct bio *bp)
   {
           int lim = ios->limiter;
   
           /* maybe this should be a kassert */
           if (lim < none || lim >= limiter_max)
                   return 0;
   
           if (limsw[lim].l_iodone)
                   return limsw[lim].l_iodone(ios, bp);
   
           return 0;
   }
   
   /*
    * Functions to implement the different kinds of limiters
    */
   
   static int
   cam_iosched_qd_iop(struct iop_stats *ios, struct bio *bp)
   {
   
           if (ios->current <= 0 || ios->pending < ios->current)
                   return 0;
   
           return EAGAIN;
   }
   
   static int
   cam_iosched_qd_caniop(struct iop_stats *ios, struct bio *bp)
   {
   
           if (ios->current <= 0 || ios->pending < ios->current)
                   return 0;
   
           return EAGAIN;
   }
   
   static int
   cam_iosched_qd_iodone(struct iop_stats *ios, struct bio *bp)
   {
   
           if (ios->current <= 0 || ios->pending != ios->current)
                   return 0;
   
           return 1;
   }
   
   static int
   cam_iosched_iops_init(struct iop_stats *ios)
   {
   
           ios->l_value1 = ios->current / ios->softc->quanta;
           if (ios->l_value1 <= 0)
                   ios->l_value1 = 1;
           ios->l_value2 = 0;
   
           return 0;
   }
   
   static int
   cam_iosched_iops_tick(struct iop_stats *ios)
   {
           int new_ios;
   
           /*
            * Allow at least one IO per tick until all
            * the IOs for this interval have been spent.
            */
           new_ios = (int)((ios->current * (uint64_t)ios->softc->this_frac) >> 16);
           if (new_ios < 1 && ios->l_value2 < ios->current) {
                   new_ios = 1;
                   ios->l_value2++;
           }
   
           /*
            * If this a new accounting interval, discard any "unspent" ios
            * granted in the previous interval.  Otherwise add the new ios to
            * the previously granted ones that haven't been spent yet.
            */
           if ((ios->softc->total_ticks % ios->softc->quanta) == 0) {
                   ios->l_value1 = new_ios;
                   ios->l_value2 = 1;
           } else {
                   ios->l_value1 += new_ios;
           }
   
           return 0;
   }
   
   static int
   cam_iosched_iops_caniop(struct iop_stats *ios, struct bio *bp)
   {
   
           /*
            * So if we have any more IOPs left, allow it,
            * otherwise wait. If current iops is 0, treat that
            * as unlimited as a failsafe.
            */
           if (ios->current > 0 && ios->l_value1 <= 0)
                   return EAGAIN;
           return 0;
   }
   
   static int
   cam_iosched_iops_iop(struct iop_stats *ios, struct bio *bp)
   {
           int rv;
   
           rv = cam_iosched_limiter_caniop(ios, bp);
           if (rv == 0)
                   ios->l_value1--;
   
           return rv;
   }
   
   static int
   cam_iosched_bw_init(struct iop_stats *ios)
   {
   
           /* ios->current is in kB/s, so scale to bytes */
           ios->l_value1 = ios->current * 1000 / ios->softc->quanta;
   
           return 0;
   }
   
   static int
   cam_iosched_bw_tick(struct iop_stats *ios)
   {
           int bw;
   
           /*
            * If we're in the hole for available quota from
            * the last time, then add the quantum for this.
            * If we have any left over from last quantum,
            * then too bad, that's lost. Also, ios->current
            * is in kB/s, so scale.
            *
            * We also allow up to 4 quanta of credits to
            * accumulate to deal with burstiness. 4 is extremely
            * arbitrary.
            */
           bw = (int)((ios->current * 1000ull * (uint64_t)ios->softc->this_frac) >> 16);
           if (ios->l_value1 < bw * 4)
                   ios->l_value1 += bw;
   
           return 0;
   }
   
   static int
   cam_iosched_bw_caniop(struct iop_stats *ios, struct bio *bp)
   {
           /*
            * So if we have any more bw quota left, allow it,
            * otherwise wait. Note, we'll go negative and that's
            * OK. We'll just get a little less next quota.
            *
            * Note on going negative: that allows us to process
            * requests in order better, since we won't allow
            * shorter reads to get around the long one that we
            * don't have the quota to do just yet. It also prevents
            * starvation by being a little more permissive about
            * what we let through this quantum (to prevent the
            * starvation), at the cost of getting a little less
            * next quantum.
            *
            * Also note that if the current limit is <= 0,
            * we treat it as unlimited as a failsafe.
            */
           if (ios->current > 0 && ios->l_value1 <= 0)
                   return EAGAIN;
   
           return 0;
   }
   
   static int
   cam_iosched_bw_iop(struct iop_stats *ios, struct bio *bp)
   {
           int rv;
   
           rv = cam_iosched_limiter_caniop(ios, bp);
           if (rv == 0)
                   ios->l_value1 -= bp->bio_length;
   
           return rv;
   }
   
   static void cam_iosched_cl_maybe_steer(struct control_loop *clp);
   
   static void
   cam_iosched_ticker(void *arg)
   {
           struct cam_iosched_softc *isc = arg;
           sbintime_t now, delta;
           int pending;
   
           callout_reset(&isc->ticker, hz / isc->quanta, cam_iosched_ticker, isc);
   
           now = sbinuptime();
           delta = now - isc->last_time;
           isc->this_frac = (uint32_t)delta >> 16;         /* Note: discards seconds -- should be 0 harmless if not */
           isc->last_time = now;
   
           cam_iosched_cl_maybe_steer(&isc->cl);
   
           cam_iosched_limiter_tick(&isc->read_stats);
           cam_iosched_limiter_tick(&isc->write_stats);
           cam_iosched_limiter_tick(&isc->trim_stats);
   
           cam_iosched_schedule(isc, isc->periph);
   
           /*
            * isc->load is an EMA of the pending I/Os at each tick. The number of
            * pending I/Os is the sum of the I/Os queued to the hardware, and those
            * in the software queue that could be queued to the hardware if there
            * were slots.
            *
            * ios_stats.pending is a count of requests in the SIM right now for
            * each of these types of I/O. So the total pending count is the sum of
            * these I/Os and the sum of the queued I/Os still in the software queue
            * for those operations that aren't being rate limited at the moment.
            *
            * The reason for the rate limiting bit is because those I/Os
            * aren't part of the software queued load (since we could
            * give them to hardware, but choose not to).
            *
            * Note: due to a bug in counting pending TRIM in the device, we
            * don't include them in this count. We count each BIO_DELETE in
            * the pending count, but the periph drivers collapse them down
            * into one TRIM command. That one trim command gets the completion
            * so the counts get off.
            */
           pending = isc->read_stats.pending + isc->write_stats.pending /* + isc->trim_stats.pending */;
           pending += !!(isc->read_stats.state_flags & IOP_RATE_LIMITED) * isc->read_stats.queued +
               !!(isc->write_stats.state_flags & IOP_RATE_LIMITED) * isc->write_stats.queued /* +
               !!(isc->trim_stats.state_flags & IOP_RATE_LIMITED) * isc->trim_stats.queued */ ;
           pending <<= 16;
           pending /= isc->periph->path->device->ccbq.total_openings;
   
           isc->load = (pending + (isc->load << 13) - isc->load) >> 13; /* see above: 13 -> 16139 / 200/s = ~81s ~1 minute */
   
           isc->total_ticks++;
   }
   
   static void
   cam_iosched_cl_init(struct control_loop *clp, struct cam_iosched_softc *isc)
   {
   
           clp->next_steer = sbinuptime();
           clp->softc = isc;
           clp->steer_interval = SBT_1S * 5;       /* Let's start out steering every 5s */
           clp->lolat = 5 * SBT_1MS;
           clp->hilat = 15 * SBT_1MS;
           clp->alpha = 20;                        /* Alpha == gain. 20 = .2 */
           clp->type = set_max;
   }
   
   static void
   cam_iosched_cl_maybe_steer(struct control_loop *clp)
   {
           struct cam_iosched_softc *isc;
           sbintime_t now, lat;
           int old;
   
           isc = clp->softc;
           now = isc->last_time;
           if (now < clp->next_steer)
                   return;
   
           clp->next_steer = now + clp->steer_interval;
           switch (clp->type) {
           case set_max:
                   if (isc->write_stats.current != isc->write_stats.max)
                           printf("Steering write from %d kBps to %d kBps\n",
                               isc->write_stats.current, isc->write_stats.max);
                   isc->read_stats.current = isc->read_stats.max;
                   isc->write_stats.current = isc->write_stats.max;
                   isc->trim_stats.current = isc->trim_stats.max;
                   break;
           case read_latency:
                   old = isc->write_stats.current;
                   lat = isc->read_stats.ema;
                   /*
                    * Simple PLL-like engine. Since we're steering to a range for
                    * the SP (set point) that makes things a little more
                    * complicated. In addition, we're not directly controlling our
                    * PV (process variable), the read latency, but instead are
                    * manipulating the write bandwidth limit for our MV
                    * (manipulation variable), analysis of this code gets a bit
                    * messy. Also, the MV is a very noisy control surface for read
                    * latency since it is affected by many hidden processes inside
                    * the device which change how responsive read latency will be
                    * in reaction to changes in write bandwidth. Unlike the classic
                    * boiler control PLL. this may result in over-steering while
                    * the SSD takes its time to react to the new, lower load. This
                    * is why we use a relatively low alpha of between .1 and .25 to
                    * compensate for this effect. At .1, it takes ~22 steering
                    * intervals to back off by a factor of 10. At .2 it only takes
                    * ~10. At .25 it only takes ~8. However some preliminary data
                    * from the SSD drives suggests a reasponse time in 10's of
                    * seconds before latency drops regardless of the new write
                    * rate. Careful observation will be required to tune this
                    * effectively.
                    *
                    * Also, when there's no read traffic, we jack up the write
                    * limit too regardless of the last read latency.  10 is
                    * somewhat arbitrary.
                    */
                   if (lat < clp->lolat || isc->read_stats.total - clp->last_count < 10)
                           isc->write_stats.current = isc->write_stats.current *
                               (100 + clp->alpha) / 100;   /* Scale up */
                   else if (lat > clp->hilat)
                           isc->write_stats.current = isc->write_stats.current *
                               (100 - clp->alpha) / 100;   /* Scale down */
                   clp->last_count = isc->read_stats.total;
   
                   /*
                    * Even if we don't steer, per se, enforce the min/max limits as
                    * those may have changed.
                    */
                   if (isc->write_stats.current < isc->write_stats.min)
                           isc->write_stats.current = isc->write_stats.min;
                   if (isc->write_stats.current > isc->write_stats.max)
                           isc->write_stats.current = isc->write_stats.max;
                   if (old != isc->write_stats.current &&  iosched_debug)
                           printf("Steering write from %d kBps to %d kBps due to latency of %jdus\n",
                               old, isc->write_stats.current,
                               (uintmax_t)((uint64_t)1000000 * (uint32_t)lat) >> 32);
                   break;
           case cl_max:
                   break;
           }
   }
   #endif
   
   /*
    * Trim or similar currently pending completion. Should only be set for
    * those drivers wishing only one Trim active at a time.
    */
   #define CAM_IOSCHED_FLAG_TRIM_ACTIVE    (1ul << 0)
                           /* Callout active, and needs to be torn down */
   #define CAM_IOSCHED_FLAG_CALLOUT_ACTIVE (1ul << 1)
   
                           /* Periph drivers set these flags to indicate work */
   #define CAM_IOSCHED_FLAG_WORK_FLAGS     ((0xffffu) << 16)
   
   #ifdef CAM_IOSCHED_DYNAMIC
   static void
   cam_iosched_io_metric_update(struct cam_iosched_softc *isc,
       sbintime_t sim_latency, int cmd, size_t size);
   #endif
   
   static inline bool
   cam_iosched_has_flagged_work(struct cam_iosched_softc *isc)
   {
           return !!(isc->flags & CAM_IOSCHED_FLAG_WORK_FLAGS);
   }
   
   static inline bool
   cam_iosched_has_io(struct cam_iosched_softc *isc)
   {
   #ifdef CAM_IOSCHED_DYNAMIC
           if (do_dynamic_iosched) {
                   struct bio *rbp = bioq_first(&isc->bio_queue);
                   struct bio *wbp = bioq_first(&isc->write_queue);
                   bool can_write = wbp != NULL &&
                       cam_iosched_limiter_caniop(&isc->write_stats, wbp) == 0;
                   bool can_read = rbp != NULL &&
                       cam_iosched_limiter_caniop(&isc->read_stats, rbp) == 0;
                   if (iosched_debug > 2) {
                           printf("can write %d: pending_writes %d max_writes %d\n", can_write, isc->write_stats.pending, isc->write_stats.max);
                           printf("can read %d: read_stats.pending %d max_reads %d\n", can_read, isc->read_stats.pending, isc->read_stats.max);
                           printf("Queued reads %d writes %d\n", isc->read_stats.queued, isc->write_stats.queued);
                   }
                   return can_read || can_write;
           }
   #endif
           return bioq_first(&isc->bio_queue) != NULL;
   }
   
   static inline bool
   cam_iosched_has_more_trim(struct cam_iosched_softc *isc)
   {
           struct bio *bp;
   
           bp = bioq_first(&isc->trim_queue);
   #ifdef CAM_IOSCHED_DYNAMIC
           if (do_dynamic_iosched) {
                   /*
                    * If we're limiting trims, then defer action on trims
                    * for a bit.
                    */
                   if (bp == NULL || cam_iosched_limiter_caniop(&isc->trim_stats, bp) != 0)
                           return false;
           }
   #endif
   
           /*
            * If we've set a trim_goal, then if we exceed that allow trims
            * to be passed back to the driver. If we've also set a tick timeout
            * allow trims back to the driver. Otherwise, don't allow trims yet.
            */
           if (isc->trim_goal > 0) {
                   if (isc->queued_trims >= isc->trim_goal)
                           return true;
                   if (isc->queued_trims > 0 &&
                       isc->trim_ticks > 0 &&
                       ticks - isc->last_trim_tick > isc->trim_ticks)
                           return true;
                   return false;
           }
   
           /* NB: Should perhaps have a max trim active independent of I/O limiters */
           return !(isc->flags & CAM_IOSCHED_FLAG_TRIM_ACTIVE) && bp != NULL;
   }
   
   #define cam_iosched_sort_queue(isc)     ((isc)->sort_io_queue >= 0 ?    \
       (isc)->sort_io_queue : cam_sort_io_queues)
   
   static inline bool
   cam_iosched_has_work(struct cam_iosched_softc *isc)
   {
   #ifdef CAM_IOSCHED_DYNAMIC
           if (iosched_debug > 2)
                   printf("has work: %d %d %d\n", cam_iosched_has_io(isc),
                       cam_iosched_has_more_trim(isc),
                       cam_iosched_has_flagged_work(isc));
   #endif
   
           return cam_iosched_has_io(isc) ||
                   cam_iosched_has_more_trim(isc) ||
                   cam_iosched_has_flagged_work(isc);
   }
   
   #ifdef CAM_IOSCHED_DYNAMIC
   static void
   cam_iosched_iop_stats_init(struct cam_iosched_softc *isc, struct iop_stats *ios)
   {
   
           ios->limiter = none;
           ios->in = 0;
           ios->max = ios->current = 300000;
           ios->min = 1;
           ios->out = 0;
           ios->errs = 0;
           ios->pending = 0;
           ios->queued = 0;
           ios->total = 0;
           ios->ema = 0;
           ios->emvar = 0;
           ios->softc = isc;
           cam_iosched_limiter_init(ios);
   }
   
   static int
   cam_iosched_limiter_sysctl(SYSCTL_HANDLER_ARGS)
   {
           char buf[16];
           struct iop_stats *ios;
           struct cam_iosched_softc *isc;
           int value, i, error;
           const char *p;
   
           ios = arg1;
           isc = ios->softc;
           value = ios->limiter;
           if (value < none || value >= limiter_max)
                   p = "UNKNOWN";
           else
                   p = cam_iosched_limiter_names[value];
   
           strlcpy(buf, p, sizeof(buf));
           error = sysctl_handle_string(oidp, buf, sizeof(buf), req);
           if (error != 0 || req->newptr == NULL)
                   return error;
   
           cam_periph_lock(isc->periph);
   
           for (i = none; i < limiter_max; i++) {
                   if (strcmp(buf, cam_iosched_limiter_names[i]) != 0)
                           continue;
                   ios->limiter = i;
                   error = cam_iosched_limiter_init(ios);
                   if (error != 0) {
                           ios->limiter = value;
                           cam_periph_unlock(isc->periph);
                           return error;
                   }
                   /* Note: disk load averate requires ticker to be always running */
                   callout_reset(&isc->ticker, hz / isc->quanta, cam_iosched_ticker, isc);
                   isc->flags |= CAM_IOSCHED_FLAG_CALLOUT_ACTIVE;
   
                   cam_periph_unlock(isc->periph);
                   return 0;
           }
   
           cam_periph_unlock(isc->periph);
           return EINVAL;
   }
   
   static int
   cam_iosched_control_type_sysctl(SYSCTL_HANDLER_ARGS)
   {
           char buf[16];
           struct control_loop *clp;
           struct cam_iosched_softc *isc;
           int value, i, error;
           const char *p;
   
           clp = arg1;
           isc = clp->softc;
           value = clp->type;
           if (value < none || value >= cl_max)
                   p = "UNKNOWN";
           else
                   p = cam_iosched_control_type_names[value];
   
           strlcpy(buf, p, sizeof(buf));
           error = sysctl_handle_string(oidp, buf, sizeof(buf), req);
           if (error != 0 || req->newptr == NULL)
                   return error;
   
           for (i = set_max; i < cl_max; i++) {
                   if (strcmp(buf, cam_iosched_control_type_names[i]) != 0)
                           continue;
                   cam_periph_lock(isc->periph);
                   clp->type = i;
                   cam_periph_unlock(isc->periph);
                   return 0;
           }
   
           return EINVAL;
   }
   
   static int
   cam_iosched_sbintime_sysctl(SYSCTL_HANDLER_ARGS)
   {
           char buf[16];
           sbintime_t value;
           int error;
           uint64_t us;
   
           value = *(sbintime_t *)arg1;
           us = (uint64_t)value / SBT_1US;
           snprintf(buf, sizeof(buf), "%ju", (intmax_t)us);
           error = sysctl_handle_string(oidp, buf, sizeof(buf), req);
           if (error != 0 || req->newptr == NULL)
                   return error;
           us = strtoul(buf, NULL, 10);
           if (us == 0)
                   return EINVAL;
           *(sbintime_t *)arg1 = us * SBT_1US;
           return 0;
   }
   
   static int
   cam_iosched_sysctl_latencies(SYSCTL_HANDLER_ARGS)
   {
           int i, error;
           struct sbuf sb;
           uint64_t *latencies;
   
           latencies = arg1;
           sbuf_new_for_sysctl(&sb, NULL, LAT_BUCKETS * 16, req);
   
           for (i = 0; i < LAT_BUCKETS - 1; i++)
                   sbuf_printf(&sb, "%jd,", (intmax_t)latencies[i]);
           sbuf_printf(&sb, "%jd", (intmax_t)latencies[LAT_BUCKETS - 1]);
           error = sbuf_finish(&sb);
           sbuf_delete(&sb);
   
           return (error);
   }
   
   static int
   cam_iosched_quanta_sysctl(SYSCTL_HANDLER_ARGS)
   {
           int *quanta;
           int error, value;
   
           quanta = (unsigned *)arg1;
           value = *quanta;
   
           error = sysctl_handle_int(oidp, (int *)&value, 0, req);
          if ((error != 0) || (req->newptr == NULL))
                  return (error);
  
          if (value < 1 || value > hz)
                  return (EINVAL);
  
          *quanta = value;
  
          return (0);
  }
  
  static void
  cam_iosched_iop_stats_sysctl_init(struct cam_iosched_softc *isc, struct iop_stats *ios, char *name)
  {
          struct sysctl_oid_list *n;
          struct sysctl_ctx_list *ctx;
  
          ios->sysctl_tree = SYSCTL_ADD_NODE(&isc->sysctl_ctx,
              SYSCTL_CHILDREN(isc->sysctl_tree), OID_AUTO, name,
              CTLFLAG_RD | CTLFLAG_MPSAFE, 0, name);
          n = SYSCTL_CHILDREN(ios->sysctl_tree);
          ctx = &ios->sysctl_ctx;
  
          SYSCTL_ADD_UQUAD(ctx, n,
              OID_AUTO, "ema", CTLFLAG_RD,
              &ios->ema,
              "Fast Exponentially Weighted Moving Average");
          SYSCTL_ADD_UQUAD(ctx, n,
              OID_AUTO, "emvar", CTLFLAG_RD,
              &ios->emvar,
              "Fast Exponentially Weighted Moving Variance");
  
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "pending", CTLFLAG_RD,
              &ios->pending, 0,
              "Instantaneous # of pending transactions");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "count", CTLFLAG_RD,
              &ios->total, 0,
              "# of transactions submitted to hardware");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "queued", CTLFLAG_RD,
              &ios->queued, 0,
              "# of transactions in the queue");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "in", CTLFLAG_RD,
              &ios->in, 0,
              "# of transactions queued to driver");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "out", CTLFLAG_RD,
              &ios->out, 0,
              "# of transactions completed (including with error)");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "errs", CTLFLAG_RD,
              &ios->errs, 0,
              "# of transactions completed with an error");
  
          SYSCTL_ADD_PROC(ctx, n,
              OID_AUTO, "limiter",
              CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_MPSAFE,
              ios, 0, cam_iosched_limiter_sysctl, "A",
              "Current limiting type.");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "min", CTLFLAG_RW,
              &ios->min, 0,
              "min resource");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "max", CTLFLAG_RW,
              &ios->max, 0,
              "max resource");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "current", CTLFLAG_RW,
              &ios->current, 0,
              "current resource");
  
          SYSCTL_ADD_PROC(ctx, n,
              OID_AUTO, "latencies",
              CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_MPSAFE,
              &ios->latencies, 0,
              cam_iosched_sysctl_latencies, "A",
              "Array of power of 2 latency from 1ms to 1.024s");
  }
  
  static void
  cam_iosched_iop_stats_fini(struct iop_stats *ios)
  {
          if (ios->sysctl_tree)
                  if (sysctl_ctx_free(&ios->sysctl_ctx) != 0)
                          printf("can't remove iosched sysctl stats context\n");
  }
  
  static void
  cam_iosched_cl_sysctl_init(struct cam_iosched_softc *isc)
  {
          struct sysctl_oid_list *n;
          struct sysctl_ctx_list *ctx;
          struct control_loop *clp;
  
          clp = &isc->cl;
          clp->sysctl_tree = SYSCTL_ADD_NODE(&isc->sysctl_ctx,
              SYSCTL_CHILDREN(isc->sysctl_tree), OID_AUTO, "control",
              CTLFLAG_RD | CTLFLAG_MPSAFE, 0, "Control loop info");
          n = SYSCTL_CHILDREN(clp->sysctl_tree);
          ctx = &clp->sysctl_ctx;
  
          SYSCTL_ADD_PROC(ctx, n,
              OID_AUTO, "type",
              CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_MPSAFE,
              clp, 0, cam_iosched_control_type_sysctl, "A",
              "Control loop algorithm");
          SYSCTL_ADD_PROC(ctx, n,
              OID_AUTO, "steer_interval",
              CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_MPSAFE,
              &clp->steer_interval, 0, cam_iosched_sbintime_sysctl, "A",
              "How often to steer (in us)");
          SYSCTL_ADD_PROC(ctx, n,
              OID_AUTO, "lolat",
              CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_MPSAFE,
              &clp->lolat, 0, cam_iosched_sbintime_sysctl, "A",
              "Low water mark for Latency (in us)");
          SYSCTL_ADD_PROC(ctx, n,
              OID_AUTO, "hilat",
              CTLTYPE_STRING | CTLFLAG_RW | CTLFLAG_MPSAFE,
              &clp->hilat, 0, cam_iosched_sbintime_sysctl, "A",
              "Hi water mark for Latency (in us)");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "alpha", CTLFLAG_RW,
              &clp->alpha, 0,
              "Alpha for PLL (x100) aka gain");
  }
  
  static void
  cam_iosched_cl_sysctl_fini(struct control_loop *clp)
  {
          if (clp->sysctl_tree)
                  if (sysctl_ctx_free(&clp->sysctl_ctx) != 0)
                          printf("can't remove iosched sysctl control loop context\n");
  }
  #endif
  
  /*
   * Allocate the iosched structure. This also insulates callers from knowing
   * sizeof struct cam_iosched_softc.
   */
  int
  cam_iosched_init(struct cam_iosched_softc **iscp, struct cam_periph *periph)
  {
  
          *iscp = malloc(sizeof(**iscp), M_CAMSCHED, M_NOWAIT | M_ZERO);
          if (*iscp == NULL)
                  return ENOMEM;
  #ifdef CAM_IOSCHED_DYNAMIC
          if (iosched_debug)
                  printf("CAM IOSCHEDULER Allocating entry at %p\n", *iscp);
  #endif
          (*iscp)->sort_io_queue = -1;
          bioq_init(&(*iscp)->bio_queue);
          bioq_init(&(*iscp)->trim_queue);
  #ifdef CAM_IOSCHED_DYNAMIC
          if (do_dynamic_iosched) {
                  bioq_init(&(*iscp)->write_queue);
                  (*iscp)->read_bias = default_read_bias;
                  (*iscp)->current_read_bias = 0;
                  (*iscp)->quanta = min(hz, 200);
                  cam_iosched_iop_stats_init(*iscp, &(*iscp)->read_stats);
                  cam_iosched_iop_stats_init(*iscp, &(*iscp)->write_stats);
                  cam_iosched_iop_stats_init(*iscp, &(*iscp)->trim_stats);
                  (*iscp)->trim_stats.max = 1;    /* Trims are special: one at a time for now */
                  (*iscp)->last_time = sbinuptime();
                  callout_init_mtx(&(*iscp)->ticker, cam_periph_mtx(periph), 0);
                  (*iscp)->periph = periph;
                  cam_iosched_cl_init(&(*iscp)->cl, *iscp);
                  callout_reset(&(*iscp)->ticker, hz / (*iscp)->quanta, cam_iosched_ticker, *iscp);
                  (*iscp)->flags |= CAM_IOSCHED_FLAG_CALLOUT_ACTIVE;
          }
  #endif
  
          return 0;
  }
  
  /*
   * Reclaim all used resources. This assumes that other folks have
   * drained the requests in the hardware. Maybe an unwise assumption.
   */
  void
  cam_iosched_fini(struct cam_iosched_softc *isc)
  {
          if (isc) {
                  cam_iosched_flush(isc, NULL, ENXIO);
  #ifdef CAM_IOSCHED_DYNAMIC
                  cam_iosched_iop_stats_fini(&isc->read_stats);
                  cam_iosched_iop_stats_fini(&isc->write_stats);
                  cam_iosched_iop_stats_fini(&isc->trim_stats);
                  cam_iosched_cl_sysctl_fini(&isc->cl);
                  if (isc->sysctl_tree)
                          if (sysctl_ctx_free(&isc->sysctl_ctx) != 0)
                                  printf("can't remove iosched sysctl stats context\n");
                  if (isc->flags & CAM_IOSCHED_FLAG_CALLOUT_ACTIVE) {
                          callout_drain(&isc->ticker);
                          isc->flags &= ~ CAM_IOSCHED_FLAG_CALLOUT_ACTIVE;
                  }
  #endif
                  free(isc, M_CAMSCHED);
          }
  }
  
  /*
   * After we're sure we're attaching a device, go ahead and add
   * hooks for any sysctl we may wish to honor.
   */
  void cam_iosched_sysctl_init(struct cam_iosched_softc *isc,
      struct sysctl_ctx_list *ctx, struct sysctl_oid *node)
  {
          struct sysctl_oid_list *n;
  
          n = SYSCTL_CHILDREN(node);
          SYSCTL_ADD_INT(ctx, n,
                  OID_AUTO, "sort_io_queue", CTLFLAG_RW | CTLFLAG_MPSAFE,
                  &isc->sort_io_queue, 0,
                  "Sort IO queue to try and optimise disk access patterns");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "trim_goal", CTLFLAG_RW,
              &isc->trim_goal, 0,
              "Number of trims to try to accumulate before sending to hardware");
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "trim_ticks", CTLFLAG_RW,
              &isc->trim_goal, 0,
              "IO Schedul qaunta to hold back trims for when accumulating");
  
  #ifdef CAM_IOSCHED_DYNAMIC
          if (!do_dynamic_iosched)
                  return;
  
          isc->sysctl_tree = SYSCTL_ADD_NODE(&isc->sysctl_ctx,
              SYSCTL_CHILDREN(node), OID_AUTO, "iosched",
              CTLFLAG_RD | CTLFLAG_MPSAFE, 0, "I/O scheduler statistics");
          n = SYSCTL_CHILDREN(isc->sysctl_tree);
          ctx = &isc->sysctl_ctx;
  
          cam_iosched_iop_stats_sysctl_init(isc, &isc->read_stats, "read");
          cam_iosched_iop_stats_sysctl_init(isc, &isc->write_stats, "write");
          cam_iosched_iop_stats_sysctl_init(isc, &isc->trim_stats, "trim");
          cam_iosched_cl_sysctl_init(isc);
  
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "read_bias", CTLFLAG_RW,
              &isc->read_bias, default_read_bias,
              "How biased towards read should we be independent of limits");
  
          SYSCTL_ADD_PROC(ctx, n,
              OID_AUTO, "quanta", CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_MPSAFE,
              &isc->quanta, 0, cam_iosched_quanta_sysctl, "I",
              "How many quanta per second do we slice the I/O up into");
  
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "total_ticks", CTLFLAG_RD,
              &isc->total_ticks, 0,
              "Total number of ticks we've done");
  
          SYSCTL_ADD_INT(ctx, n,
              OID_AUTO, "load", CTLFLAG_RD,
              &isc->load, 0,
              "scaled load average / 100");
  
          SYSCTL_ADD_U64(ctx, n,
              OID_AUTO, "latency_trigger", CTLFLAG_RW,
              &isc->max_lat, 0,
              "Latency treshold to trigger callbacks");
  #endif
  }
  
  void
  cam_iosched_set_latfcn(struct cam_iosched_softc *isc,
      cam_iosched_latfcn_t fnp, void *argp)
  {
  #ifdef CAM_IOSCHED_DYNAMIC
          isc->latfcn = fnp;
          isc->latarg = argp;
  #endif
  }
  
  /*
   * Client drivers can set two parameters. "goal" is the number of BIO_DELETEs
   * that will be queued up before iosched will "release" the trims to the client
   * driver to wo with what they will (usually combine as many as possible). If we
   * don't get this many, after trim_ticks we'll submit the I/O anyway with
   * whatever we have.  We do need an I/O of some kind of to clock the deferred
   * trims out to disk. Since we will eventually get a write for the super block
   * or something before we shutdown, the trims will complete. To be safe, when a
   * BIO_FLUSH is presented to the iosched work queue, we set the ticks time far
   * enough in the past so we'll present the BIO_DELETEs to the client driver.
   * There might be a race if no BIO_DELETESs were queued, a BIO_FLUSH comes in
   * and then a BIO_DELETE is sent down. No know client does this, and there's
   * already a race between an ordered BIO_FLUSH and any BIO_DELETEs in flight,
   * but no client depends on the ordering being honored.
   *
   * XXX I'm not sure what the interaction between UFS direct BIOs and the BUF
   * flushing on shutdown. I think there's bufs that would be dependent on the BIO
   * finishing to write out at least metadata, so we'll be fine. To be safe, keep
   * the number of ticks low (less than maybe 10s) to avoid shutdown races.
   */
  
  void
  cam_iosched_set_trim_goal(struct cam_iosched_softc *isc, int goal)
  {
  
          isc->trim_goal = goal;
  }
  
  void
  cam_iosched_set_trim_ticks(struct cam_iosched_softc *isc, int trim_ticks)
  {
  
          isc->trim_ticks = trim_ticks;
  }
  
  /*
   * Flush outstanding I/O. Consumers of this library don't know all the
   * queues we may keep, so this allows all I/O to be flushed in one
   * convenient call.
   */
  void
  cam_iosched_flush(struct cam_iosched_softc *isc, struct devstat *stp, int err)
  {
          bioq_flush(&isc->bio_queue, stp, err);
          bioq_flush(&isc->trim_queue, stp, err);
  #ifdef CAM_IOSCHED_DYNAMIC
          if (do_dynamic_iosched)
                  bioq_flush(&isc->write_queue, stp, err);
  #endif
  }
  
  #ifdef CAM_IOSCHED_DYNAMIC
  static struct bio *
  cam_iosched_get_write(struct cam_iosched_softc *isc)
  {
          struct bio *bp;
  
          /*
           * We control the write rate by controlling how many requests we send
           * down to the drive at any one time. Fewer requests limits the
           * effects of both starvation when the requests take a while and write
           * amplification when each request is causing more than one write to
           * the NAND media. Limiting the queue depth like this will also limit
           * the write throughput and give and reads that want to compete to
           * compete unfairly.
           */
          bp = bioq_first(&isc->write_queue);
          if (bp == NULL) {
                  if (iosched_debug > 3)
                          printf("No writes present in write_queue\n");
                  return NULL;
          }
  
          /*
           * If pending read, prefer that based on current read bias
           * setting.
           */
          if (bioq_first(&isc->bio_queue) && isc->current_read_bias) {
                  if (iosched_debug)
                          printf(
                              "Reads present and current_read_bias is %d queued "
                              "writes %d queued reads %d\n",
                              isc->current_read_bias, isc->write_stats.queued,
                              isc->read_stats.queued);
                  isc->current_read_bias--;
                  /* We're not limiting writes, per se, just doing reads first */
                  return NULL;
          }
  
          /*
           * See if our current limiter allows this I/O.
           */
          if (cam_iosched_limiter_iop(&isc->write_stats, bp) != 0) {
                  if (iosched_debug)
                          printf("Can't write because limiter says no.\n");
                  isc->write_stats.state_flags |= IOP_RATE_LIMITED;
                  return NULL;
          }
  
          /*
           * Let's do this: We've passed all the gates and we're a go
           * to schedule the I/O in the SIM.
           */
          isc->current_read_bias = isc->read_bias;
          bioq_remove(&isc->write_queue, bp);
          if (bp->bio_cmd == BIO_WRITE) {
                  isc->write_stats.queued--;
                  isc->write_stats.total++;
                  isc->write_stats.pending++;
          }
          if (iosched_debug > 9)
                  printf("HWQ : %p %#x\n", bp, bp->bio_cmd);
          isc->write_stats.state_flags &= ~IOP_RATE_LIMITED;
          return bp;
  }
  #endif
  
  /*
   * Put back a trim that you weren't able to actually schedule this time.
   */
  void
  cam_iosched_put_back_trim(struct cam_iosched_softc *isc, struct bio *bp)
  {
          bioq_insert_head(&isc->trim_queue, bp);
          if (isc->queued_trims == 0)
                  isc->last_trim_tick = ticks;
          isc->queued_trims++;
  #ifdef CAM_IOSCHED_DYNAMIC
          isc->trim_stats.queued++;
          isc->trim_stats.total--;                /* since we put it back, don't double count */
          isc->trim_stats.pending--;
  #endif
  }
  
  /*
   * gets the next trim from the trim queue.
   *
   * Assumes we're called with the periph lock held.  It removes this
   * trim from the queue and the device must explicitly reinsert it
   * should the need arise.
   */
  struct bio *
  cam_iosched_next_trim(struct cam_iosched_softc *isc)
  {
          struct bio *bp;
  
          bp  = bioq_first(&isc->trim_queue);
          if (bp == NULL)
                  return NULL;
          bioq_remove(&isc->trim_queue, bp);
          isc->queued_trims--;
          isc->last_trim_tick = ticks;    /* Reset the tick timer when we take trims */
  #ifdef CAM_IOSCHED_DYNAMIC
          isc->trim_stats.queued--;
          isc->trim_stats.total++;
          isc->trim_stats.pending++;
  #endif
          return bp;
  }
  
  /*
   * gets an available trim from the trim queue, if there's no trim
   * already pending. It removes this trim from the queue and the device
   * must explicitly reinsert it should the need arise.
   *
   * Assumes we're called with the periph lock held.
   */
  struct bio *
  cam_iosched_get_trim(struct cam_iosched_softc *isc)
  {
  #ifdef CAM_IOSCHED_DYNAMIC
          struct bio *bp;
  #endif
  
          if (!cam_iosched_has_more_trim(isc))
                  return NULL;
  #ifdef CAM_IOSCHED_DYNAMIC
          bp  = bioq_first(&isc->trim_queue);
          if (bp == NULL)
                  return NULL;
  
          /*
           * If pending read, prefer that based on current read bias setting. The
           * read bias is shared for both writes and TRIMs, but on TRIMs the bias
           * is for a combined TRIM not a single TRIM request that's come in.
           */
          if (do_dynamic_iosched) {
                  if (bioq_first(&isc->bio_queue) && isc->current_read_bias) {
                          if (iosched_debug)
                                  printf("Reads present and current_read_bias is %d"
                                      " queued trims %d queued reads %d\n",
                                      isc->current_read_bias, isc->trim_stats.queued,
                                      isc->read_stats.queued);
                          isc->current_read_bias--;
                          /* We're not limiting TRIMS, per se, just doing reads first */
                          return NULL;
                  }
                  /*
                   * We're going to do a trim, so reset the bias.
                   */
                  isc->current_read_bias = isc->read_bias;
          }
  
          /*
           * See if our current limiter allows this I/O. Because we only call this
           * here, and not in next_trim, the 'bandwidth' limits for trims won't
           * work, while the iops or max queued limits will work. It's tricky
           * because we want the limits to be from the perspective of the
           * "commands sent to the device." To make iops work, we need to check
           * only here (since we want all the ops we combine to count as one). To
           * make bw limits work, we'd need to check in next_trim, but that would
           * have the effect of limiting the iops as seen from the upper layers.
           */
          if (cam_iosched_limiter_iop(&isc->trim_stats, bp) != 0) {
                  if (iosched_debug)
                          printf("Can't trim because limiter says no.\n");
                  isc->trim_stats.state_flags |= IOP_RATE_LIMITED;
                  return NULL;
          }
          isc->current_read_bias = isc->read_bias;
          isc->trim_stats.state_flags &= ~IOP_RATE_LIMITED;
          /* cam_iosched_next_trim below keeps proper book */
  #endif
          return cam_iosched_next_trim(isc);
  }
  
  
  #ifdef CAM_IOSCHED_DYNAMIC
  static struct bio *
  bio_next(struct bio *bp)
  {
          bp = TAILQ_NEXT(bp, bio_queue);
          /*
           * After the first commands, the ordered bit terminates
           * our search because BIO_ORDERED acts like a barrier.
           */
          if (bp == NULL || bp->bio_flags & BIO_ORDERED)
                  return NULL;
          return bp;
  }
  
  static bool
  cam_iosched_rate_limited(struct iop_stats *ios)
  {
          return ios->state_flags & IOP_RATE_LIMITED;
  }
  #endif
  
  /*
   * Determine what the next bit of work to do is for the periph. The
   * default implementation looks to see if we have trims to do, but no
   * trims outstanding. If so, we do that. Otherwise we see if we have
   * other work. If we do, then we do that. Otherwise why were we called?
   */
  struct bio *
  cam_iosched_next_bio(struct cam_iosched_softc *isc)
  {
          struct bio *bp;
  
          /*
           * See if we have a trim that can be scheduled. We can only send one
           * at a time down, so this takes that into account.
           *
           * XXX newer TRIM commands are queueable. Revisit this when we
           * implement them.
           */
          if ((bp = cam_iosched_get_trim(isc)) != NULL)
                  return bp;
  
  #ifdef CAM_IOSCHED_DYNAMIC
          /*
           * See if we have any pending writes, room in the queue for them,
           * and no pending reads (unless we've scheduled too many).
           * if so, those are next.
           */
          if (do_dynamic_iosched) {
                  if ((bp = cam_iosched_get_write(isc)) != NULL)
                          return bp;
          }
  #endif
          /*
           * next, see if there's other, normal I/O waiting. If so return that.
           */
  #ifdef CAM_IOSCHED_DYNAMIC
          if (do_dynamic_iosched) {
                  for (bp = bioq_first(&isc->bio_queue); bp != NULL;
                       bp = bio_next(bp)) {
                          /*
                           * For the dynamic scheduler with a read bias, bio_queue
                           * is only for reads. However, without one, all
                           * operations are queued. Enforce limits here for any
                           * operation we find here.
                           */
                          if (bp->bio_cmd == BIO_READ) {
                                  if (cam_iosched_rate_limited(&isc->read_stats) ||
                                      cam_iosched_limiter_iop(&isc->read_stats, bp) != 0) {
                                          isc->read_stats.state_flags |= IOP_RATE_LIMITED;
                                          continue;
                                  }
                                  isc->read_stats.state_flags &= ~IOP_RATE_LIMITED;
                          }
                          /*
                           * There can only be write requests on the queue when
                           * the read bias is 0, but we need to process them
                           * here. We do not assert for read bias == 0, however,
                           * since it is dynamic and we can have WRITE operations
                           * in the queue after we transition from 0 to non-zero.
                           */
                          if (bp->bio_cmd == BIO_WRITE) {
                                  if (cam_iosched_rate_limited(&isc->write_stats) ||
                                      cam_iosched_limiter_iop(&isc->write_stats, bp) != 0) {
                                          isc->write_stats.state_flags |= IOP_RATE_LIMITED;
                                          continue;
                                  }
                                  isc->write_stats.state_flags &= ~IOP_RATE_LIMITED;
                          }
                          /*
                           * here we know we have a bp that's != NULL, that's not rate limited
                           * and can be the next I/O.
                           */
                          break;
                  }
          } else
  #endif
                  bp = bioq_first(&isc->bio_queue);
  
          if (bp == NULL)
                  return (NULL);
          bioq_remove(&isc->bio_queue, bp);
  #ifdef CAM_IOSCHED_DYNAMIC
          if (do_dynamic_iosched) {
                  if (bp->bio_cmd == BIO_READ) {
                          isc->read_stats.queued--;
                          isc->read_stats.total++;
                          isc->read_stats.pending++;
                  } else if (bp->bio_cmd == BIO_WRITE) {
                          isc->write_stats.queued--;
                          isc->write_stats.total++;
                          isc->write_stats.pending++;
                  }
          }
          if (iosched_debug > 9)
                  printf("HWQ : %p %#x\n", bp, bp->bio_cmd);
  #endif
          return bp;
  }
  
  /*
   * Driver has been given some work to do by the block layer. Tell the
   * scheduler about it and have it queue the work up. The scheduler module
   * will then return the currently most useful bit of work later, possibly
   * deferring work for various reasons.
   */
  void
  cam_iosched_queue_work(struct cam_iosched_softc *isc, struct bio *bp)
  {
  
          /*
           * A BIO_SPEEDUP from the uppper layers means that they have a block
           * shortage. At the present, this is only sent when we're trying to
           * allocate blocks, but have a shortage before giving up. bio_length is
           * the size of their shortage. We will complete just enough BIO_DELETEs
           * in the queue to satisfy the need. If bio_length is 0, we'll complete
           * them all. This allows the scheduler to delay BIO_DELETEs to improve
           * read/write performance without worrying about the upper layers. When
           * it's possibly a problem, we respond by pretending the BIO_DELETEs
           * just worked. We can't do anything about the BIO_DELETEs in the
           * hardware, though. We have to wait for them to complete.
           */
          if (bp->bio_cmd == BIO_SPEEDUP) {
                  off_t len;
                  struct bio *nbp;
  
                  len = 0;
                  while (bioq_first(&isc->trim_queue) &&
                      (bp->bio_length == 0 || len < bp->bio_length)) {
                          nbp = bioq_takefirst(&isc->trim_queue);
                          len += nbp->bio_length;
                          nbp->bio_error = 0;
                          biodone(nbp);
                  }
                  if (bp->bio_length > 0) {
                          if (bp->bio_length > len)
                                  bp->bio_resid = bp->bio_length - len;
                          else
                                  bp->bio_resid = 0;
                  }
                  bp->bio_error = 0;
                  biodone(bp);
                  return;
          }
  
          /*
           * If we get a BIO_FLUSH, and we're doing delayed BIO_DELETEs then we
           * set the last tick time to one less than the current ticks minus the
           * delay to force the BIO_DELETEs to be presented to the client driver.
           */
          if (bp->bio_cmd == BIO_FLUSH && isc->trim_ticks > 0)
                  isc->last_trim_tick = ticks - isc->trim_ticks - 1;
  
          /*
           * Put all trims on the trim queue. Otherwise put the work on the bio
           * queue.
           */
          if (bp->bio_cmd == BIO_DELETE) {
                  bioq_insert_tail(&isc->trim_queue, bp);
                  if (isc->queued_trims == 0)
                          isc->last_trim_tick = ticks;
                  isc->queued_trims++;
  #ifdef CAM_IOSCHED_DYNAMIC
                  isc->trim_stats.in++;
                  isc->trim_stats.queued++;
  #endif
          }
  #ifdef CAM_IOSCHED_DYNAMIC
          else if (do_dynamic_iosched && isc->read_bias != 0 &&
              (bp->bio_cmd != BIO_READ)) {
                  if (cam_iosched_sort_queue(isc))
                          bioq_disksort(&isc->write_queue, bp);
                  else
                          bioq_insert_tail(&isc->write_queue, bp);
                  if (iosched_debug > 9)
                          printf("Qw  : %p %#x\n", bp, bp->bio_cmd);
                  if (bp->bio_cmd == BIO_WRITE) {
                          isc->write_stats.in++;
                          isc->write_stats.queued++;
                  }
          }
  #endif
          else {
                  if (cam_iosched_sort_queue(isc))
                          bioq_disksort(&isc->bio_queue, bp);
                  else
                          bioq_insert_tail(&isc->bio_queue, bp);
  #ifdef CAM_IOSCHED_DYNAMIC
                  if (iosched_debug > 9)
                          printf("Qr  : %p %#x\n", bp, bp->bio_cmd);
                  if (bp->bio_cmd == BIO_READ) {
                          isc->read_stats.in++;
                          isc->read_stats.queued++;
                  } else if (bp->bio_cmd == BIO_WRITE) {
                          isc->write_stats.in++;
                          isc->write_stats.queued++;
                  }
  #endif
          }
  }
  
  /*
   * If we have work, get it scheduled. Called with the periph lock held.
   */
  void
  cam_iosched_schedule(struct cam_iosched_softc *isc, struct cam_periph *periph)
  {
  
          if (cam_iosched_has_work(isc))
                  xpt_schedule(periph, CAM_PRIORITY_NORMAL);
  }
  
  /*
   * Complete a trim request. Mark that we no longer have one in flight.
   */
  void
  cam_iosched_trim_done(struct cam_iosched_softc *isc)
  {
  
          isc->flags &= ~CAM_IOSCHED_FLAG_TRIM_ACTIVE;
  }
  
  /*
   * Complete a bio. Called before we release the ccb with xpt_release_ccb so we
   * might use notes in the ccb for statistics.
   */
  int
  cam_iosched_bio_complete(struct cam_iosched_softc *isc, struct bio *bp,
      union ccb *done_ccb)
  {
          int retval = 0;
  #ifdef CAM_IOSCHED_DYNAMIC
          if (!do_dynamic_iosched)
                  return retval;
  
          if (iosched_debug > 10)
                  printf("done: %p %#x\n", bp, bp->bio_cmd);
          if (bp->bio_cmd == BIO_WRITE) {
                  retval = cam_iosched_limiter_iodone(&isc->write_stats, bp);
                  if ((bp->bio_flags & BIO_ERROR) != 0)
                          isc->write_stats.errs++;
                  isc->write_stats.out++;
                  isc->write_stats.pending--;
          } else if (bp->bio_cmd == BIO_READ) {
                  retval = cam_iosched_limiter_iodone(&isc->read_stats, bp);
                  if ((bp->bio_flags & BIO_ERROR) != 0)
                          isc->read_stats.errs++;
                  isc->read_stats.out++;
                  isc->read_stats.pending--;
          } else if (bp->bio_cmd == BIO_DELETE) {
                  if ((bp->bio_flags & BIO_ERROR) != 0)
                          isc->trim_stats.errs++;
                  isc->trim_stats.out++;
                  isc->trim_stats.pending--;
          } else if (bp->bio_cmd != BIO_FLUSH) {
                  if (iosched_debug)
                          printf("Completing command with bio_cmd == %#x\n", bp->bio_cmd);
          }
  
          if ((bp->bio_flags & BIO_ERROR) == 0 && done_ccb != NULL &&
              (done_ccb->ccb_h.status & CAM_QOS_VALID) != 0) {
                  sbintime_t sim_latency;
                  
                  sim_latency = cam_iosched_sbintime_t(done_ccb->ccb_h.qos.periph_data);
                  
                  cam_iosched_io_metric_update(isc, sim_latency,
                      bp->bio_cmd, bp->bio_bcount);
                  /*
                   * Debugging code: allow callbacks to the periph driver when latency max
                   * is exceeded. This can be useful for triggering external debugging actions.
                   */
                  if (isc->latfcn && isc->max_lat != 0 && sim_latency > isc->max_lat)
                          isc->latfcn(isc->latarg, sim_latency, bp);
          }
                  
  #endif
          return retval;
  }
  
  /*
   * Tell the io scheduler that you've pushed a trim down into the sim.
   * This also tells the I/O scheduler not to push any more trims down, so
   * some periphs do not call it if they can cope with multiple trims in flight.
   */
  void
  cam_iosched_submit_trim(struct cam_iosched_softc *isc)
  {
  
          isc->flags |= CAM_IOSCHED_FLAG_TRIM_ACTIVE;
  }
  
  /*
   * Change the sorting policy hint for I/O transactions for this device.
   */
  void
  cam_iosched_set_sort_queue(struct cam_iosched_softc *isc, int val)
  {
  
          isc->sort_io_queue = val;
  }
  
  int
  cam_iosched_has_work_flags(struct cam_iosched_softc *isc, uint32_t flags)
  {
          return isc->flags & flags;
  }
  
  void
  cam_iosched_set_work_flags(struct cam_iosched_softc *isc, uint32_t flags)
  {
          isc->flags |= flags;
  }
  
  void
  cam_iosched_clr_work_flags(struct cam_iosched_softc *isc, uint32_t flags)
  {
          isc->flags &= ~flags;
  }
  
  #ifdef CAM_IOSCHED_DYNAMIC
  /*
   * After the method presented in Jack Crenshaw's 1998 article "Integer
   * Square Roots," reprinted at
   * http://www.embedded.com/electronics-blogs/programmer-s-toolbox/4219659/Integer-Square-Roots
   * and well worth the read. Briefly, we find the power of 4 that's the
   * largest smaller than val. We then check each smaller power of 4 to
   * see if val is still bigger. The right shifts at each step divide
   * the result by 2 which after successive application winds up
   * accumulating the right answer. It could also have been accumulated
   * using a separate root counter, but this code is smaller and faster
   * than that method. This method is also integer size invariant.
   * It returns floor(sqrt((float)val)), or the largest integer less than
   * or equal to the square root.
   */
  static uint64_t
  isqrt64(uint64_t val)
  {
          uint64_t res = 0;
          uint64_t bit = 1ULL << (sizeof(uint64_t) * NBBY - 2);
  
          /*
           * Find the largest power of 4 smaller than val.
           */
          while (bit > val)
                  bit >>= 2;
  
          /*
           * Accumulate the answer, one bit at a time (we keep moving
           * them over since 2 is the square root of 4 and we test
           * powers of 4). We accumulate where we find the bit, but
           * the successive shifts land the bit in the right place
           * by the end.
           */
          while (bit != 0) {
                  if (val >= res + bit) {
                          val -= res + bit;
                          res = (res >> 1) + bit;
                  } else
                          res >>= 1;
                  bit >>= 2;
          }
  
          return res;
  }
  
  static sbintime_t latencies[LAT_BUCKETS - 1] = {
          BUCKET_BASE <<  0,      /* 20us */
          BUCKET_BASE <<  1,
          BUCKET_BASE <<  2,
          BUCKET_BASE <<  3,
          BUCKET_BASE <<  4,
          BUCKET_BASE <<  5,
          BUCKET_BASE <<  6,
          BUCKET_BASE <<  7,
          BUCKET_BASE <<  8,
          BUCKET_BASE <<  9,
          BUCKET_BASE << 10,
          BUCKET_BASE << 11,
          BUCKET_BASE << 12,
          BUCKET_BASE << 13,
          BUCKET_BASE << 14,
          BUCKET_BASE << 15,
          BUCKET_BASE << 16,
          BUCKET_BASE << 17,
          BUCKET_BASE << 18       /* 5,242,880us */
  };
  
  static void
  cam_iosched_update(struct iop_stats *iop, sbintime_t sim_latency)
  {
          sbintime_t y, deltasq, delta;
          int i;
  
          /*
           * Keep counts for latency. We do it by power of two buckets.
           * This helps us spot outlier behavior obscured by averages.
           */
          for (i = 0; i < LAT_BUCKETS - 1; i++) {
                  if (sim_latency < latencies[i]) {
                          iop->latencies[i]++;
                          break;
                  }
          }
          if (i == LAT_BUCKETS - 1)
                  iop->latencies[i]++;     /* Put all > 8192ms values into the last bucket. */
  
          /*
           * Classic exponentially decaying average with a tiny alpha
           * (2 ^ -alpha_bits). For more info see the NIST statistical
           * handbook.
           *
           * ema_t = y_t * alpha + ema_t-1 * (1 - alpha)          [nist]
           * ema_t = y_t * alpha + ema_t-1 - alpha * ema_t-1
           * ema_t = alpha * y_t - alpha * ema_t-1 + ema_t-1
           * alpha = 1 / (1 << alpha_bits)
           * sub e == ema_t-1, b == 1/alpha (== 1 << alpha_bits), d == y_t - ema_t-1
           *      = y_t/b - e/b + be/b
           *      = (y_t - e + be) / b
           *      = (e + d) / b
           *
           * Since alpha is a power of two, we can compute this w/o any mult or
           * division.
           *
           * Variance can also be computed. Usually, it would be expressed as follows:
           *      diff_t = y_t - ema_t-1
           *      emvar_t = (1 - alpha) * (emavar_t-1 + diff_t^2 * alpha)
           *        = emavar_t-1 - alpha * emavar_t-1 + delta_t^2 * alpha - (delta_t * alpha)^2
           * sub b == 1/alpha (== 1 << alpha_bits), e == emavar_t-1, d = delta_t^2
           *        = e - e/b + dd/b + dd/bb
           *        = (bbe - be + bdd + dd) / bb
           *        = (bbe + b(dd-e) + dd) / bb (which is expanded below bb = 1<<(2*alpha_bits))
           */
          /*
           * XXX possible numeric issues
           *      o We assume right shifted integers do the right thing, since that's
           *        implementation defined. You can change the right shifts to / (1LL << alpha).
           *      o alpha_bits = 9 gives ema ceiling of 23 bits of seconds for ema and 14 bits
           *        for emvar. This puts a ceiling of 13 bits on alpha since we need a
           *        few tens of seconds of representation.
           *      o We mitigate alpha issues by never setting it too high.
           */
          y = sim_latency;
          delta = (y - iop->ema);                                 /* d */
          iop->ema = ((iop->ema << alpha_bits) + delta) >> alpha_bits;
  
          /*
           * Were we to naively plow ahead at this point, we wind up with many numerical
           * issues making any SD > ~3ms unreliable. So, we shift right by 12. This leaves
           * us with microsecond level precision in the input, so the same in the
           * output. It means we can't overflow deltasq unless delta > 4k seconds. It
           * also means that emvar can be up 46 bits 40 of which are fraction, which
           * gives us a way to measure up to ~8s in the SD before the computation goes
           * unstable. Even the worst hard disk rarely has > 1s service time in the
           * drive. It does mean we have to shift left 12 bits after taking the
           * square root to compute the actual standard deviation estimate. This loss of
           * precision is preferable to needing int128 types to work. The above numbers
           * assume alpha=9. 10 or 11 are ok, but we start to run into issues at 12,
           * so 12 or 13 is OK for EMA, EMVAR and SD will be wrong in those cases.
           */
          delta >>= 12;
          deltasq = delta * delta;                                /* dd */
          iop->emvar = ((iop->emvar << (2 * alpha_bits)) +        /* bbe */
              ((deltasq - iop->emvar) << alpha_bits) +            /* b(dd-e) */
              deltasq)                                            /* dd */
              >> (2 * alpha_bits);                                /* div bb */
          iop->sd = (sbintime_t)isqrt64((uint64_t)iop->emvar) << 12;
  }
  
  static void
  cam_iosched_io_metric_update(struct cam_iosched_softc *isc,
      sbintime_t sim_latency, int cmd, size_t size)
  {
          /* xxx Do we need to scale based on the size of the I/O ? */
          switch (cmd) {
          case BIO_READ:
                  cam_iosched_update(&isc->read_stats, sim_latency);
                  break;
          case BIO_WRITE:
                  cam_iosched_update(&isc->write_stats, sim_latency);
                  break;
          case BIO_DELETE:
                  cam_iosched_update(&isc->trim_stats, sim_latency);
                  break;
          default:
                  break;
          }
  }
  
  #ifdef DDB
  static int biolen(struct bio_queue_head *bq)
  {
          int i = 0;
          struct bio *bp;
  
          TAILQ_FOREACH(bp, &bq->queue, bio_queue) {
                  i++;
          }
          return i;
  }
  
  /*
   * Show the internal state of the I/O scheduler.
   */
  DB_SHOW_COMMAND(iosched, cam_iosched_db_show)
  {
          struct cam_iosched_softc *isc;
  
          if (!have_addr) {
                  db_printf("Need addr\n");
                  return;
          }
          isc = (struct cam_iosched_softc *)addr;
          db_printf("pending_reads:     %d\n", isc->read_stats.pending);
          db_printf("min_reads:         %d\n", isc->read_stats.min);
          db_printf("max_reads:         %d\n", isc->read_stats.max);
          db_printf("reads:             %d\n", isc->read_stats.total);
          db_printf("in_reads:          %d\n", isc->read_stats.in);
          db_printf("out_reads:         %d\n", isc->read_stats.out);
          db_printf("queued_reads:      %d\n", isc->read_stats.queued);
          db_printf("Read Q len         %d\n", biolen(&isc->bio_queue));
          db_printf("pending_writes:    %d\n", isc->write_stats.pending);
          db_printf("min_writes:        %d\n", isc->write_stats.min);
          db_printf("max_writes:        %d\n", isc->write_stats.max);
          db_printf("writes:            %d\n", isc->write_stats.total);
          db_printf("in_writes:         %d\n", isc->write_stats.in);
          db_printf("out_writes:        %d\n", isc->write_stats.out);
          db_printf("queued_writes:     %d\n", isc->write_stats.queued);
          db_printf("Write Q len        %d\n", biolen(&isc->write_queue));
          db_printf("pending_trims:     %d\n", isc->trim_stats.pending);
          db_printf("min_trims:         %d\n", isc->trim_stats.min);
          db_printf("max_trims:         %d\n", isc->trim_stats.max);
          db_printf("trims:             %d\n", isc->trim_stats.total);
          db_printf("in_trims:          %d\n", isc->trim_stats.in);
          db_printf("out_trims:         %d\n", isc->trim_stats.out);
          db_printf("queued_trims:      %d\n", isc->trim_stats.queued);
          db_printf("Trim Q len         %d\n", biolen(&isc->trim_queue));
          db_printf("read_bias:         %d\n", isc->read_bias);
          db_printf("current_read_bias: %d\n", isc->current_read_bias);
          db_printf("Trim active?       %s\n",
              (isc->flags & CAM_IOSCHED_FLAG_TRIM_ACTIVE) ? "yes" : "no");
  }
  #endif
  #endif

Cache object: d1ede4455331547a8542508211e35f74