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sched: Improve latencies under load by decreasing minimum scheduling granularity 

Mathieu reported bad latencies with make -j10 kind of kbuild
workloads - which is mostly caused by us scheduling with a
too coarse granularity.

Reduce the minimum granularity some more, to make sure we
can meet the latency target.

I got the following results (make -j10 kbuild load, average of 3
runs):

 vanilla:

  maximum latency: 38278.9 µs
  average latency:  7730.1 µs

 patched:

  maximum latency: 22702.1 µs
  average latency:  6684.8 µs

Mathieu also measured it:

|
| * wakeup-latency.c (SIGEV_THREAD) with make -j10
|
| - Mainline 2.6.35.2 kernel
|
| maximum latency: 45762.1 µs
| average latency: 7348.6 µs
|
| - With only Peter's smaller min_gran (shown below):
|
| maximum latency: 29100.6 µs
| average latency: 6684.1 µs
|

Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <AANLkTi=8m4g01wZPacySoF7U0PevTNVgJoZZrHiUD-pN@mail.gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
hifive-unleashed-5.1
Ingo Molnar 2010-09-12 08:14:52 +02:00
parent 49553c2ef8
commit 0bf377bbb0
1 changed files with 3 additions and 3 deletions
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6
kernel/sched_fair.c
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@ -54,13 +54,13 @@ enum sched_tunable_scaling sysctl_sched_tunable_scaling
* Minimal preemption granularity for CPU-bound tasks:
* (default: 2 msec * (1 + ilog(ncpus)), units: nanoseconds)
*/
unsigned int sysctl_sched_min_granularity = 2000000ULL;
unsigned int normalized_sysctl_sched_min_granularity = 2000000ULL;
unsigned int sysctl_sched_min_granularity = 750000ULL;
unsigned int normalized_sysctl_sched_min_granularity = 750000ULL;
/*
* is kept at sysctl_sched_latency / sysctl_sched_min_granularity
*/
static unsigned int sched_nr_latency = 3;
static unsigned int sched_nr_latency = 8;
/*
* After fork, child runs first. If set to 0 (default) then