--- /dev/null
+ =============
+ 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. 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
Code Review / kvmfornfv.git / blobdiff