Documentation / scheduler / sched-design-CFS.txt
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Based on kernel version 3.9. Page generated on 2013-05-02 23:13 EST.
1 ============= 2 CFS Scheduler 3 ============= 4 5 6 1. OVERVIEW 7 8 CFS stands for "Completely Fair Scheduler," and is the new "desktop" process 9 scheduler implemented by Ingo Molnar and merged in Linux 2.6.23. It is the 10 replacement for the previous vanilla scheduler's SCHED_OTHER interactivity 11 code. 12 13 80% of CFS's design can be summed up in a single sentence: CFS basically models 14 an "ideal, precise multi-tasking CPU" on real hardware. 15 16 "Ideal multi-tasking CPU" is a (non-existent :-)) CPU that has 100% physical 17 power and which can run each task at precise equal speed, in parallel, each at 18 1/nr_running speed. For example: if there are 2 tasks running, then it runs 19 each at 50% physical power --- i.e., actually in parallel. 20 21 On real hardware, we can run only a single task at once, so we have to 22 introduce the concept of "virtual runtime." The virtual runtime of a task 23 specifies when its next timeslice would start execution on the ideal 24 multi-tasking CPU described above. In practice, the virtual runtime of a task 25 is its actual runtime normalized to the total number of running tasks. 26 27 28 29 2. FEW IMPLEMENTATION DETAILS 30 31 In CFS the virtual runtime is expressed and tracked via the per-task 32 p->se.vruntime (nanosec-unit) value. This way, it's possible to accurately 33 timestamp 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 39 CFS's task picking logic is based on this p->se.vruntime value and it is thus 40 very simple: it always tries to run the task with the smallest p->se.vruntime 41 value (i.e., the task which executed least so far). CFS always tries to split 42 up CPU time between runnable tasks as close to "ideal multitasking hardware" as 43 possible. 44 45 Most of the rest of CFS's design just falls out of this really simple concept, 46 with a few add-on embellishments like nice levels, multiprocessing and various 47 algorithm variants to recognize sleepers. 48 49 50 51 3. THE RBTREE 52 53 CFS's design is quite radical: it does not use the old data structures for the 54 runqueues, but it uses a time-ordered rbtree to build a "timeline" of future 55 task execution, and thus has no "array switch" artifacts (by which both the 56 previous vanilla scheduler and RSDL/SD are affected). 57 58 CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic 59 increasing value tracking the smallest vruntime among all tasks in the 60 runqueue. The total amount of work done by the system is tracked using 61 min_vruntime; that value is used to place newly activated entities on the left 62 side of the tree as much as possible. 63 64 The total number of running tasks in the runqueue is accounted through the 65 rq->cfs.load value, which is the sum of the weights of the tasks queued on the 66 runqueue. 67 68 CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the 69 p->se.vruntime key (there is a subtraction using rq->cfs.min_vruntime to 70 account for possible wraparounds). CFS picks the "leftmost" task from this 71 tree and sticks to it. 72 As the system progresses forwards, the executed tasks are put into the tree 73 more and more to the right --- slowly but surely giving a chance for every task 74 to become the "leftmost task" and thus get on the CPU within a deterministic 75 amount of time. 76 77 Summing up, CFS works like this: it runs a task a bit, and when the task 78 schedules (or a scheduler tick happens) the task's CPU usage is "accounted 79 for": the (small) time it just spent using the physical CPU is added to 80 p->se.vruntime. Once p->se.vruntime gets high enough so that another task 81 becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a 82 small amount of "granularity" distance relative to the leftmost task so that we 83 do not over-schedule tasks and trash the cache), then the new leftmost task is 84 picked and the current task is preempted. 85 86 87 88 4. SOME FEATURES OF CFS 89 90 CFS uses nanosecond granularity accounting and does not rely on any jiffies or 91 other HZ detail. Thus the CFS scheduler has no notion of "timeslices" in the 92 way the previous scheduler had, and has no heuristics whatsoever. There is 93 only one central tunable (you have to switch on CONFIG_SCHED_DEBUG): 94 95 /proc/sys/kernel/sched_min_granularity_ns 96 97 which 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 99 for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too. 100 101 Due to its design, the CFS scheduler is not prone to any of the "attacks" that 102 exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c, 103 chew.c, ring-test.c, massive_intr.c all work fine and do not impact 104 interactivity and produce the expected behavior. 105 106 The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH 107 than the previous vanilla scheduler: both types of workloads are isolated much 108 more aggressively. 109 110 SMP load-balancing has been reworked/sanitized: the runqueue-walking 111 assumptions are gone from the load-balancing code now, and iterators of the 112 scheduling modules are used. The balancing code got quite a bit simpler as a 113 result. 114 115 116 117 5. Scheduling policies 118 119 CFS 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. 132 133 SCHED_FIFO/_RR are implemented in sched/rt.c and are as specified by 134 POSIX. 135 136 The command chrt from util-linux-ng 2.13.1.1 can set all of these except 137 SCHED_IDLE. 138 139 140 141 6. SCHEDULING CLASSES 142 143 The new CFS scheduler has been designed in such a way to introduce "Scheduling 144 Classes," an extensible hierarchy of scheduler modules. These modules 145 encapsulate scheduling policy details and are handled by the scheduler core 146 without the core code assuming too much about them. 147 148 sched/fair.c implements the CFS scheduler described above. 149 150 sched/rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than 151 the previous vanilla scheduler did. It uses 100 runqueues (for all 100 RT 152 priority levels, instead of 140 in the previous scheduler) and it needs no 153 expired array. 154 155 Scheduling classes are implemented through the sched_class structure, which 156 contains hooks to functions that must be called whenever an interesting event 157 occurs. 158 159 This 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. 197 198 199 200 201 7. GROUP SCHEDULER EXTENSIONS TO CFS 202 203 Normally, the scheduler operates on individual tasks and strives to provide 204 fair CPU time to each task. Sometimes, it may be desirable to group tasks and 205 provide fair CPU time to each such task group. For example, it may be 206 desirable to first provide fair CPU time to each user on the system and then to 207 each task belonging to a user. 208 209 CONFIG_CGROUP_SCHED strives to achieve exactly that. It lets tasks to be 210 grouped and divides CPU time fairly among such groups. 211 212 CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and 213 SCHED_RR) tasks. 214 215 CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and 216 SCHED_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. 221 222 When CONFIG_FAIR_GROUP_SCHED is defined, a "cpu.shares" file is created for each 223 group created using the pseudo filesystem. See example steps below to create 224 task 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
- [ scheduler ]
- 00-INDEX
- sched-arch.txt
- sched-bwc.txt
- sched-design-CFS.txt
- sched-domains.txt
- sched-nice-design.txt
- sched-rt-group.txt
- sched-stats.txt
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