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- =============
- CFS Scheduler
- =============
-
-
-1. OVERVIEW
-
-CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
-scheduler implemented by Ingo Molnar and merged in Linux 2.6.23. It is the
-replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
-code.
-
-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 --- i.e., actually in parallel.
-
-On real hardware, we can run only a single task at once, so we have to
-introduce the concept of "virtual runtime." The virtual runtime of a task
-specifies when its next timeslice would start execution on the ideal
-multi-tasking CPU described above. In practice, the virtual runtime of a task
-is its actual runtime normalized to the total number of running tasks.
-
-
-
-2. FEW IMPLEMENTATION DETAILS
-
-In CFS the virtual runtime is expressed and tracked via the per-task
-p->se.vruntime (nanosec-unit) value. This way, it's possible to accurately
-timestamp and measure the "expected CPU time" a task should have gotten.
-
-[ small detail: on "ideal" hardware, at any time all tasks would have the same
- p->se.vruntime value --- i.e., tasks would execute simultaneously and 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->se.vruntime value and it is thus
-very simple: it always tries to run the task with the smallest p->se.vruntime
-value (i.e., the task which executed least so far). 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.
-
-
-
-3. THE RBTREE
-
-CFS's design is quite radical: it does not use the old data structures for the
-runqueues, but 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
-previous vanilla scheduler and RSDL/SD are affected).
-
-CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
-increasing value tracking the smallest vruntime among all tasks in the
-runqueue. The total amount of work done by the system is tracked using
-min_vruntime; that value is used to place newly activated entities on the left
-side of the tree as much as possible.
-
-The total number of running tasks in the runqueue is accounted through the
-rq->cfs.load value, which is the sum of the weights of the tasks queued on the
-runqueue.
-
-CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
-p->se.vruntime key (there is a subtraction using rq->cfs.min_vruntime to
-account for possible wraparounds). CFS picks the "leftmost" task from this
-tree and sticks to it.
-As the system progresses forwards, the executed 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.
-
-Summing up, CFS works like this: it 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 added to
-p->se.vruntime. Once p->se.vruntime gets high 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.
-
-
-
-4. SOME FEATURES OF CFS
-
-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" in the
-way the previous scheduler had, and has no heuristics whatsoever. There is
-only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):
-
- /proc/sys/kernel/sched_min_granularity_ns
-
-which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
-"server" (i.e., 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
-than the previous vanilla scheduler: both types of workloads are isolated much
-more aggressively.
-
-SMP load-balancing has been reworked/sanitized: 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.
-
-
-
-5. Scheduling policies
-
-CFS implements three scheduling policies:
-
- - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
- policy that is used for regular tasks.
-
- - SCHED_BATCH: Does not preempt nearly as often as regular tasks
- would, thereby allowing tasks to run longer and make better use of
- caches but at the cost of interactivity. This is well suited for
- batch jobs.
-
- - SCHED_IDLE: This is even weaker than nice 19, but its not a true
- idle timer scheduler in order to avoid to get into priority
- inversion problems which would deadlock the machine.
-
-SCHED_FIFO/_RR are implemented in sched/rt.c and are as specified by
-POSIX.
-
-The command chrt from util-linux-ng 2.13.1.1 can set all of these except
-SCHED_IDLE.
-
-
-
-6. SCHEDULING CLASSES
-
-The new CFS scheduler has been designed in such a way to introduce "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 too much about them.
-
-sched/fair.c implements the CFS scheduler described above.
-
-sched/rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
-the previous vanilla scheduler did. It uses 100 runqueues (for all 100 RT
-priority levels, instead of 140 in the previous scheduler) and it needs no
-expired array.
-
-Scheduling classes are implemented through the sched_class structure, which
-contains hooks to functions that must be called whenever an interesting event
-occurs.
-
-This is the (partial) list of the hooks:
-
- - enqueue_task(...)
-
- Called when a task enters a runnable state.
- It puts the scheduling entity (task) into the red-black tree and
- increments the nr_running variable.
-
- - dequeue_task(...)
-
- When a task is no longer runnable, this function is called to keep the
- corresponding scheduling entity out of the red-black tree. It decrements
- the nr_running variable.
-
- - yield_task(...)
-
- This function is basically just a dequeue followed by an enqueue, unless the
- compat_yield sysctl is turned on; in that case, it places the scheduling
- entity at the right-most end of the red-black tree.
-
- - check_preempt_curr(...)
-
- This function checks if a task that entered the runnable state should
- preempt the currently running task.
-
- - pick_next_task(...)
-
- This function chooses the most appropriate task eligible to run next.
-
- - set_curr_task(...)
-
- This function is called when a task changes its scheduling class or changes
- its task group.
-
- - task_tick(...)
-
- This function is mostly called from time tick functions; it might lead to
- process switch. This drives the running preemption.
-
-
-
-
-7. GROUP SCHEDULER EXTENSIONS 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_CGROUP_SCHED strives to achieve exactly that. It lets tasks to be
-grouped and divides CPU time fairly among such groups.
-
-CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
-SCHED_RR) tasks.
-
-CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
-SCHED_BATCH) tasks.
-
- These options need CONFIG_CGROUPS to be defined, and let the administrator
- create arbitrary groups of tasks, using the "cgroup" pseudo filesystem. See
- Documentation/cgroups/cgroups.txt for more information about this filesystem.
-
-When CONFIG_FAIR_GROUP_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.
-
- # mount -t tmpfs cgroup_root /sys/fs/cgroup
- # mkdir /sys/fs/cgroup/cpu
- # mount -t cgroup -ocpu none /sys/fs/cgroup/cpu
- # cd /sys/fs/cgroup/cpu
-
- # 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