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diff --git a/Documentation/scheduler/sched-design-CFS.txt b/Documentation/scheduler/sched-design-CFS.txt new file mode 100644 index 0000000..88bcb87 --- /dev/null +++ b/Documentation/scheduler/sched-design-CFS.txt @@ -0,0 +1,186 @@ + +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 |
