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sched: new documentation about CFS

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Rewrite of the CFS documentation - because the old one was sorely
out-dated.

Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Claudio Scordino 2008-08-20 15:18:45 +02:00 committed by Ingo Molnar
parent 3fb669dd6e
commit f58e2c33ff
1 changed files with 214 additions and 149 deletions
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319
Documentation/scheduler/sched-design-CFS.txt
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@ -1,151 +1,218 @@
=============
CFS Scheduler
=============
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.
1. OVERVIEW
"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.
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.
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.
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.
( 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. )
"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.
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.
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.
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.
2. FEW IMPLEMENTATION DETAILS
Some 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.
- 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.
[ 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. ]
- sched_fair.c implements the 'CFS desktop scheduler': it is a
replacement for the vanilla scheduler's SCHED_OTHER interactivity
code.
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.
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!
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.
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):
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_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.
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.
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.
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.
( 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.
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.
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.
5. SCHEDULING CLASSES
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:
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.
- 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.
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_tree(...)
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.
- task_new(...)
The core scheduler gives the scheduling module an opportunity to manage new
task startup. The CFS scheduling module uses it for group scheduling, while
the scheduling module for a real-time task does not use it.
6. 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_GROUP_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.
At present, there are two (mutually exclusive) mechanisms to group tasks for
CPU bandwidth control purposes:
- Based on user id (CONFIG_USER_SCHED)
With this option, tasks are grouped according to their user id.
- Based on "cgroup" pseudo filesystem (CONFIG_CGROUP_SCHED)
This options needs CONFIG_CGROUPS to be defined, and 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.
When CONFIG_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
@ -155,16 +222,14 @@ each new user and a "cpu_share" file is added in that directory.
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
CPU bandwidth between two users is divided in the ratio of their CPU shares.
For example: 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
When CONFIG_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