FreeBSD/Linux Kernel Cross Reference
sys/vm/vm_zeroidle.c

     /*-
      * Copyright (c) 1994 John Dyson
      * Copyright (c) 2001 Matt Dillon
      *
      * All Rights Reserved.
      * 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.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 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.
     *
     *      from: @(#)vm_machdep.c  7.3 (Berkeley) 5/13/91
     *      Utah $Hdr: vm_machdep.c 1.16.1.1 89/06/23$
     * from: FreeBSD: .../i386/vm_machdep.c,v 1.165 2001/07/04 23:27:04 dillon
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include <opt_sched.h>
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/proc.h>
    #include <sys/vmmeter.h>
    #include <sys/lock.h>
    #include <sys/mutex.h>
    #include <sys/sched.h>
    #include <sys/sysctl.h>
    #include <sys/kthread.h>
    #include <sys/unistd.h>
    
    #include <vm/vm.h>
    #include <vm/vm_param.h>
    #include <vm/vm_page.h>
    #include <vm/vm_phys.h>
    
    static int idlezero_enable_default = 0;
    /* Defer setting the enable flag until the kthread is running. */
    static int idlezero_enable = 0;
    SYSCTL_INT(_vm, OID_AUTO, idlezero_enable, CTLFLAG_RWTUN, &idlezero_enable, 0,
        "Allow the kernel to use idle cpu cycles to zero-out pages");
    /*
     * Implement the pre-zeroed page mechanism.
     */
    
    #define ZIDLE_LO(v)     ((v) * 2 / 3)
    #define ZIDLE_HI(v)     ((v) * 4 / 5)
    
    static boolean_t wakeup_needed = FALSE;
    static int zero_state;
    
    static int
    vm_page_zero_check(void)
    {
    
            if (!idlezero_enable)
                    return (0);
            /*
             * Attempt to maintain approximately 1/2 of our free pages in a
             * PG_ZERO'd state.   Add some hysteresis to (attempt to) avoid
             * generally zeroing a page when the system is near steady-state.
             * Otherwise we might get 'flutter' during disk I/O / IPC or 
             * fast sleeps.  We also do not want to be continuously zeroing
             * pages because doing so may flush our L1 and L2 caches too much.
             */
            if (zero_state && vm_page_zero_count >= ZIDLE_LO(vm_cnt.v_free_count))
                    return (0);
            if (vm_page_zero_count >= ZIDLE_HI(vm_cnt.v_free_count))
                    return (0);
            return (1);
    }
    
    static void
    vm_page_zero_idle(void)
    {
    
            mtx_assert(&vm_page_queue_free_mtx, MA_OWNED);
            zero_state = 0;
            if (vm_phys_zero_pages_idle()) {
                   if (vm_page_zero_count >= ZIDLE_HI(vm_cnt.v_free_count))
                           zero_state = 1;
           }
   }
   
   /* Called by vm_page_free to hint that a new page is available. */
   void
   vm_page_zero_idle_wakeup(void)
   {
   
           mtx_assert(&vm_page_queue_free_mtx, MA_OWNED);
           if (wakeup_needed && vm_page_zero_check()) {
                   wakeup_needed = FALSE;
                   wakeup(&zero_state);
           }
   }
   
   static void
   vm_pagezero(void __unused *arg)
   {
   
           idlezero_enable = idlezero_enable_default;
   
           mtx_lock(&vm_page_queue_free_mtx);
           for (;;) {
                   if (vm_page_zero_check()) {
                           vm_page_zero_idle();
   #ifndef PREEMPTION
                           if (sched_runnable()) {
                                   thread_lock(curthread);
                                   mi_switch(SW_VOL | SWT_IDLE, NULL);
                                   thread_unlock(curthread);
                           }
   #endif
                   } else {
                           wakeup_needed = TRUE;
                           msleep(&zero_state, &vm_page_queue_free_mtx, 0,
                               "pgzero", hz * 300);
                   }
           }
   }
   
   static void
   pagezero_start(void __unused *arg)
   {
           int error;
           struct proc *p;
           struct thread *td;
   
           error = kproc_create(vm_pagezero, NULL, &p, RFSTOPPED, 0, "pagezero");
           if (error)
                   panic("pagezero_start: error %d\n", error);
           td = FIRST_THREAD_IN_PROC(p);
           thread_lock(td);
   
           /* We're an idle task, don't count us in the load. */
           td->td_flags |= TDF_NOLOAD;
           sched_class(td, PRI_IDLE);
           sched_prio(td, PRI_MAX_IDLE);
           sched_add(td, SRQ_BORING);
           thread_unlock(td);
   }
   SYSINIT(pagezero, SI_SUB_KTHREAD_VM, SI_ORDER_ANY, pagezero_start, NULL);

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