FreeBSD/Linux Kernel Cross Reference
sys/Documentation/sched-design-CFS.txt

     
     This is the CFS scheduler.
     
     80% of CFS's design can be summed up in a single sentence: CFS basically
     models an "ideal, precise multi-tasking CPU" on real hardware.
     
     "Ideal multi-tasking CPU" is a (non-existent  :-))  CPU that has 100%
     physical power and which can run each task at precise equal speed, in
     parallel, each at 1/nr_running speed. For example: if there are 2 tasks
    running then it runs each at 50% physical power - totally in parallel.
    
    On real hardware, we can run only a single task at once, so while that
    one task runs, the other tasks that are waiting for the CPU are at a
    disadvantage - the current task gets an unfair amount of CPU time. In
    CFS this fairness imbalance is expressed and tracked via the per-task
    p->wait_runtime (nanosec-unit) value. "wait_runtime" is the amount of
    time the task should now run on the CPU for it to become completely fair
    and balanced.
    
    ( small detail: on 'ideal' hardware, the p->wait_runtime value would
      always be zero - no task would ever get 'out of balance' from the
      'ideal' share of CPU time. )
    
    CFS's task picking logic is based on this p->wait_runtime value and it
    is thus very simple: it always tries to run the task with the largest
    p->wait_runtime value. In other words, CFS tries to run the task with
    the 'gravest need' for more CPU time. So CFS always tries to split up
    CPU time between runnable tasks as close to 'ideal multitasking
    hardware' as possible.
    
    Most of the rest of CFS's design just falls out of this really simple
    concept, with a few add-on embellishments like nice levels,
    multiprocessing and various algorithm variants to recognize sleepers.
    
    In practice it works like this: the system runs a task a bit, and when
    the task schedules (or a scheduler tick happens) the task's CPU usage is
    'accounted for': the (small) time it just spent using the physical CPU
    is deducted from p->wait_runtime. [minus the 'fair share' it would have
    gotten anyway]. Once p->wait_runtime gets low enough so that another
    task becomes the 'leftmost task' of the time-ordered rbtree it maintains
    (plus a small amount of 'granularity' distance relative to the leftmost
    task so that we do not over-schedule tasks and trash the cache) then the
    new leftmost task is picked and the current task is preempted.
    
    The rq->fair_clock value tracks the 'CPU time a runnable task would have
    fairly gotten, had it been runnable during that time'. So by using
    rq->fair_clock values we can accurately timestamp and measure the
    'expected CPU time' a task should have gotten. All runnable tasks are
    sorted in the rbtree by the "rq->fair_clock - p->wait_runtime" key, and
    CFS picks the 'leftmost' task and sticks to it. As the system progresses
    forwards, newly woken tasks are put into the tree more and more to the
    right - slowly but surely giving a chance for every task to become the
    'leftmost task' and thus get on the CPU within a deterministic amount of
    time.
    
    Some implementation details:
    
     - the introduction of Scheduling Classes: an extensible hierarchy of
       scheduler modules. These modules encapsulate scheduling policy
       details and are handled by the scheduler core without the core
       code assuming about them too much.
    
     - sched_fair.c implements the 'CFS desktop scheduler': it is a
       replacement for the vanilla scheduler's SCHED_OTHER interactivity
       code.
    
       I'd like to give credit to Con Kolivas for the general approach here:
       he has proven via RSDL/SD that 'fair scheduling' is possible and that
       it results in better desktop scheduling. Kudos Con!
    
       The CFS patch uses a completely different approach and implementation
       from RSDL/SD. My goal was to make CFS's interactivity quality exceed
       that of RSDL/SD, which is a high standard to meet :-) Testing
       feedback is welcome to decide this one way or another. [ and, in any
       case, all of SD's logic could be added via a kernel/sched_sd.c module
       as well, if Con is interested in such an approach. ]
    
       CFS's design is quite radical: it does not use runqueues, it uses a
       time-ordered rbtree to build a 'timeline' of future task execution,
       and thus has no 'array switch' artifacts (by which both the vanilla
       scheduler and RSDL/SD are affected).
    
       CFS uses nanosecond granularity accounting and does not rely on any
       jiffies or other HZ detail. Thus the CFS scheduler has no notion of
       'timeslices' and has no heuristics whatsoever. There is only one
       central tunable (you have to switch on CONFIG_SCHED_DEBUG):
    
             /proc/sys/kernel/sched_granularity_ns
    
       which can be used to tune the scheduler from 'desktop' (low
       latencies) to 'server' (good batching) workloads. It defaults to a
       setting suitable for desktop workloads. SCHED_BATCH is handled by the
       CFS scheduler module too.
    
       Due to its design, the CFS scheduler is not prone to any of the
       'attacks' that exist today against the heuristics of the stock
       scheduler: fiftyp.c, thud.c, chew.c, ring-test.c, massive_intr.c all
       work fine and do not impact interactivity and produce the expected
       behavior.
   
      the CFS scheduler has a much stronger handling of nice levels and
      SCHED_BATCH: both types of workloads should be isolated much more
      agressively than under the vanilla scheduler.
   
      ( another detail: due to nanosec accounting and timeline sorting,
        sched_yield() support is very simple under CFS, and in fact under
        CFS sched_yield() behaves much better than under any other
        scheduler i have tested so far. )
   
    - sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler
      way than the vanilla scheduler does. It uses 100 runqueues (for all
      100 RT priority levels, instead of 140 in the vanilla scheduler)
      and it needs no expired array.
   
    - reworked/sanitized SMP load-balancing: the runqueue-walking
      assumptions are gone from the load-balancing code now, and
      iterators of the scheduling modules are used. The balancing code got
      quite a bit simpler as a result.
   
   
   Group scheduler extension to CFS
   ================================
   
   Normally the scheduler operates on individual tasks and strives to provide
   fair CPU time to each task. Sometimes, it may be desirable to group tasks
   and provide fair CPU time to each such task group. For example, it may
   be desirable to first provide fair CPU time to each user on the system
   and then to each task belonging to a user.
   
   CONFIG_FAIR_GROUP_SCHED strives to achieve exactly that. It lets
   SCHED_NORMAL/BATCH tasks be be grouped and divides CPU time fairly among such
   groups. At present, there are two (mutually exclusive) mechanisms to group
   tasks for CPU bandwidth control purpose:
   
           - Based on user id (CONFIG_FAIR_USER_SCHED)
                   In this option, tasks are grouped according to their user id.
           - Based on "cgroup" pseudo filesystem (CONFIG_FAIR_CGROUP_SCHED)
                   This options lets the administrator create arbitrary groups
                   of tasks, using the "cgroup" pseudo filesystem. See
                   Documentation/cgroups.txt for more information about this
                   filesystem.
   
   Only one of these options to group tasks can be chosen and not both.
   
   Group scheduler tunables:
   
   When CONFIG_FAIR_USER_SCHED is defined, a directory is created in sysfs for
   each new user and a "cpu_share" file is added in that directory.
   
           # cd /sys/kernel/uids
           # cat 512/cpu_share             # Display user 512's CPU share
           1024
           # echo 2048 > 512/cpu_share     # Modify user 512's CPU share
           # cat 512/cpu_share             # Display user 512's CPU share
           2048
           #
   
   CPU bandwidth between two users are divided in the ratio of their CPU shares.
   For ex: if you would like user "root" to get twice the bandwidth of user
   "guest", then set the cpu_share for both the users such that "root"'s
   cpu_share is twice "guest"'s cpu_share
   
   
   When CONFIG_FAIR_CGROUP_SCHED is defined, a "cpu.shares" file is created
   for each group created using the pseudo filesystem. See example steps
   below to create task groups and modify their CPU share using the "cgroups"
   pseudo filesystem
   
           # mkdir /dev/cpuctl
           # mount -t cgroup -ocpu none /dev/cpuctl
           # cd /dev/cpuctl
   
           # mkdir multimedia      # create "multimedia" group of tasks
           # mkdir browser         # create "browser" group of tasks
   
           # #Configure the multimedia group to receive twice the CPU bandwidth
           # #that of browser group
   
           # echo 2048 > multimedia/cpu.shares
           # echo 1024 > browser/cpu.shares
   
           # firefox &     # Launch firefox and move it to "browser" group
           # echo <firefox_pid> > browser/tasks
   
           # #Launch gmplayer (or your favourite movie player)
           # echo <movie_player_pid> > multimedia/tasks

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