linux/Documentation/scheduler/sched-design-CFS.txt
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   1                      =============
   2                      CFS Scheduler
   3                      =============
   4
   5
   61.  OVERVIEW
   7
   8CFS stands for "Completely Fair Scheduler," and is the new "desktop" process
   9scheduler implemented by Ingo Molnar and merged in Linux 2.6.23.  It is the
  10replacement for the previous vanilla scheduler's SCHED_OTHER interactivity
  11code.
  12
  1380% of CFS's design can be summed up in a single sentence: CFS basically models
  14an "ideal, precise multi-tasking CPU" on real hardware.
  15
  16"Ideal multi-tasking CPU" is a (non-existent  :-)) CPU that has 100% physical
  17power and which can run each task at precise equal speed, in parallel, each at
  181/nr_running speed.  For example: if there are 2 tasks running, then it runs
  19each at 50% physical power --- i.e., actually in parallel.
  20
  21On real hardware, we can run only a single task at once, so we have to
  22introduce the concept of "virtual runtime."  The virtual runtime of a task
  23specifies when its next timeslice would start execution on the ideal
  24multi-tasking CPU described above.  In practice, the virtual runtime of a task
  25is its actual runtime normalized to the total number of running tasks.
  26
  27
  28
  292.  FEW IMPLEMENTATION DETAILS
  30
  31In CFS the virtual runtime is expressed and tracked via the per-task
  32p->se.vruntime (nanosec-unit) value.  This way, it's possible to accurately
  33timestamp and measure the "expected CPU time" a task should have gotten.
  34
  35[ small detail: on "ideal" hardware, at any time all tasks would have the same
  36  p->se.vruntime value --- i.e., tasks would execute simultaneously and no task
  37  would ever get "out of balance" from the "ideal" share of CPU time.  ]
  38
  39CFS's task picking logic is based on this p->se.vruntime value and it is thus
  40very simple: it always tries to run the task with the smallest p->se.vruntime
  41value (i.e., the task which executed least so far).  CFS always tries to split
  42up CPU time between runnable tasks as close to "ideal multitasking hardware" as
  43possible.
  44
  45Most of the rest of CFS's design just falls out of this really simple concept,
  46with a few add-on embellishments like nice levels, multiprocessing and various
  47algorithm variants to recognize sleepers.
  48
  49
  50
  513.  THE RBTREE
  52
  53CFS's design is quite radical: it does not use the old data structures for the
  54runqueues, but it uses a time-ordered rbtree to build a "timeline" of future
  55task execution, and thus has no "array switch" artifacts (by which both the
  56previous vanilla scheduler and RSDL/SD are affected).
  57
  58CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic
  59increasing value tracking the smallest vruntime among all tasks in the
  60runqueue.  The total amount of work done by the system is tracked using
  61min_vruntime; that value is used to place newly activated entities on the left
  62side of the tree as much as possible.
  63
  64The total number of running tasks in the runqueue is accounted through the
  65rq->cfs.load value, which is the sum of the weights of the tasks queued on the
  66runqueue.
  67
  68CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the
  69p->se.vruntime key (there is a subtraction using rq->cfs.min_vruntime to
  70account for possible wraparounds).  CFS picks the "leftmost" task from this
  71tree and sticks to it.
  72As the system progresses forwards, the executed tasks are put into the tree
  73more and more to the right --- slowly but surely giving a chance for every task
  74to become the "leftmost task" and thus get on the CPU within a deterministic
  75amount of time.
  76
  77Summing up, CFS works like this: it runs a task a bit, and when the task
  78schedules (or a scheduler tick happens) the task's CPU usage is "accounted
  79for": the (small) time it just spent using the physical CPU is added to
  80p->se.vruntime.  Once p->se.vruntime gets high enough so that another task
  81becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a
  82small amount of "granularity" distance relative to the leftmost task so that we
  83do not over-schedule tasks and trash the cache), then the new leftmost task is
  84picked and the current task is preempted.
  85
  86
  87
  884.  SOME FEATURES OF CFS
  89
  90CFS uses nanosecond granularity accounting and does not rely on any jiffies or
  91other HZ detail.  Thus the CFS scheduler has no notion of "timeslices" in the
  92way the previous scheduler had, and has no heuristics whatsoever.  There is
  93only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):
  94
  95   /proc/sys/kernel/sched_min_granularity_ns
  96
  97which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
  98"server" (i.e., good batching) workloads.  It defaults to a setting suitable
  99for desktop workloads.  SCHED_BATCH is handled by the CFS scheduler module too.
 100
 101Due to its design, the CFS scheduler is not prone to any of the "attacks" that
 102exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
 103chew.c, ring-test.c, massive_intr.c all work fine and do not impact
 104interactivity and produce the expected behavior.
 105
 106The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH
 107than the previous vanilla scheduler: both types of workloads are isolated much
 108more aggressively.
 109
 110SMP load-balancing has been reworked/sanitized: the runqueue-walking
 111assumptions are gone from the load-balancing code now, and iterators of the
 112scheduling modules are used.  The balancing code got quite a bit simpler as a
 113result.
 114
 115
 116
 1175. Scheduling policies
 118
 119CFS implements three scheduling policies:
 120
 121  - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling
 122    policy that is used for regular tasks.
 123
 124  - SCHED_BATCH: Does not preempt nearly as often as regular tasks
 125    would, thereby allowing tasks to run longer and make better use of
 126    caches but at the cost of interactivity. This is well suited for
 127    batch jobs.
 128
 129  - SCHED_IDLE: This is even weaker than nice 19, but its not a true
 130    idle timer scheduler in order to avoid to get into priority
 131    inversion problems which would deadlock the machine.
 132
 133SCHED_FIFO/_RR are implemented in sched/rt.c and are as specified by
 134POSIX.
 135
 136The command chrt from util-linux-ng 2.13.1.1 can set all of these except
 137SCHED_IDLE.
 138
 139
 140
 1416.  SCHEDULING CLASSES
 142
 143The new CFS scheduler has been designed in such a way to introduce "Scheduling
 144Classes," an extensible hierarchy of scheduler modules.  These modules
 145encapsulate scheduling policy details and are handled by the scheduler core
 146without the core code assuming too much about them.
 147
 148sched/fair.c implements the CFS scheduler described above.
 149
 150sched/rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than
 151the previous vanilla scheduler did.  It uses 100 runqueues (for all 100 RT
 152priority levels, instead of 140 in the previous scheduler) and it needs no
 153expired array.
 154
 155Scheduling classes are implemented through the sched_class structure, which
 156contains hooks to functions that must be called whenever an interesting event
 157occurs.
 158
 159This is the (partial) list of the hooks:
 160
 161 - enqueue_task(...)
 162
 163   Called when a task enters a runnable state.
 164   It puts the scheduling entity (task) into the red-black tree and
 165   increments the nr_running variable.
 166
 167 - dequeue_task(...)
 168
 169   When a task is no longer runnable, this function is called to keep the
 170   corresponding scheduling entity out of the red-black tree.  It decrements
 171   the nr_running variable.
 172
 173 - yield_task(...)
 174
 175   This function is basically just a dequeue followed by an enqueue, unless the
 176   compat_yield sysctl is turned on; in that case, it places the scheduling
 177   entity at the right-most end of the red-black tree.
 178
 179 - check_preempt_curr(...)
 180
 181   This function checks if a task that entered the runnable state should
 182   preempt the currently running task.
 183
 184 - pick_next_task(...)
 185
 186   This function chooses the most appropriate task eligible to run next.
 187
 188 - set_curr_task(...)
 189
 190   This function is called when a task changes its scheduling class or changes
 191   its task group.
 192
 193 - task_tick(...)
 194
 195   This function is mostly called from time tick functions; it might lead to
 196   process switch.  This drives the running preemption.
 197
 198
 199
 200
 2017.  GROUP SCHEDULER EXTENSIONS TO CFS
 202
 203Normally, the scheduler operates on individual tasks and strives to provide
 204fair CPU time to each task.  Sometimes, it may be desirable to group tasks and
 205provide fair CPU time to each such task group.  For example, it may be
 206desirable to first provide fair CPU time to each user on the system and then to
 207each task belonging to a user.
 208
 209CONFIG_CGROUP_SCHED strives to achieve exactly that.  It lets tasks to be
 210grouped and divides CPU time fairly among such groups.
 211
 212CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and
 213SCHED_RR) tasks.
 214
 215CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and
 216SCHED_BATCH) tasks.
 217
 218   These options need CONFIG_CGROUPS to be defined, and let the administrator
 219   create arbitrary groups of tasks, using the "cgroup" pseudo filesystem.  See
 220   Documentation/cgroups/cgroups.txt for more information about this filesystem.
 221
 222When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each
 223group created using the pseudo filesystem.  See example steps below to create
 224task groups and modify their CPU share using the "cgroups" pseudo filesystem.
 225
 226        # mount -t tmpfs cgroup_root /sys/fs/cgroup
 227        # mkdir /sys/fs/cgroup/cpu
 228        # mount -t cgroup -ocpu none /sys/fs/cgroup/cpu
 229        # cd /sys/fs/cgroup/cpu
 230
 231        # mkdir multimedia      # create "multimedia" group of tasks
 232        # mkdir browser         # create "browser" group of tasks
 233
 234        # #Configure the multimedia group to receive twice the CPU bandwidth
 235        # #that of browser group
 236
 237        # echo 2048 > multimedia/cpu.shares
 238        # echo 1024 > browser/cpu.shares
 239
 240        # firefox &     # Launch firefox and move it to "browser" group
 241        # echo <firefox_pid> > browser/tasks
 242
 243        # #Launch gmplayer (or your favourite movie player)
 244        # echo <movie_player_pid> > multimedia/tasks
 245
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