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