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sched: prefer wakers 

Prefer tasks that wake other tasks to preempt quickly. This improves
performance because more work is available sooner.

The workload that prompted this patch was a kernel build over NFS4 (for some
curious and not understood reason we had to revert commit:
18de973530 to make any progress at all)

Without this patch a make -j8 bzImage (of x86-64 defconfig) would take
3m30-ish, with this patch we're down to 2m50-ish.

psql-sysbench/mysql-sysbench show a slight improvement in peak performance as
well, tbench and vmark seemed to not care.

It is possible to improve upon the build time (to 2m20-ish) but that seriously
destroys other benchmarks (just shows that there's more room for tinkering).

Much thanks to Mike who put in a lot of effort to benchmark things and proved
a worthy opponent with a competing patch.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2009-01-14 12:39:19 +01:00 committed by Ingo Molnar
parent 831451ac4e
commit e52fb7c097
2 changed files with 55 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

59
kernel/sched_fair.c
View File

@ -1295,16 +1295,63 @@ out:
}
#endif /* CONFIG_SMP */
static unsigned long wakeup_gran(struct sched_entity *se)
/*
* Adaptive granularity
*
* se->avg_wakeup gives the average time a task runs until it does a wakeup,
* with the limit of wakeup_gran -- when it never does a wakeup.
*
* So the smaller avg_wakeup is the faster we want this task to preempt,
* but we don't want to treat the preemptee unfairly and therefore allow it
* to run for at least the amount of time we'd like to run.
*
* NOTE: we use 2*avg_wakeup to increase the probability of actually doing one
*
* NOTE: we use *nr_running to scale with load, this nicely matches the
* degrading latency on load.
*/
static unsigned long
adaptive_gran(struct sched_entity *curr, struct sched_entity *se)
{
u64 this_run = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
u64 expected_wakeup = 2*se->avg_wakeup * cfs_rq_of(se)->nr_running;
u64 gran = 0;
if (this_run < expected_wakeup)
gran = expected_wakeup - this_run;
return min_t(s64, gran, sysctl_sched_wakeup_granularity);
}
static unsigned long
wakeup_gran(struct sched_entity *curr, struct sched_entity *se)
{
unsigned long gran = sysctl_sched_wakeup_granularity;
if (cfs_rq_of(curr)->curr && sched_feat(ADAPTIVE_GRAN))
gran = adaptive_gran(curr, se);
/*
* More easily preempt - nice tasks, while not making it harder for
* + nice tasks.
* Since its curr running now, convert the gran from real-time
* to virtual-time in his units.
*/
if (!sched_feat(ASYM_GRAN) || se->load.weight > NICE_0_LOAD)
gran = calc_delta_fair(sysctl_sched_wakeup_granularity, se);
if (sched_feat(ASYM_GRAN)) {
/*
* By using 'se' instead of 'curr' we penalize light tasks, so
* they get preempted easier. That is, if 'se' < 'curr' then
* the resulting gran will be larger, therefore penalizing the
* lighter, if otoh 'se' > 'curr' then the resulting gran will
* be smaller, again penalizing the lighter task.
*
* This is especially important for buddies when the leftmost
* task is higher priority than the buddy.
*/
if (unlikely(se->load.weight != NICE_0_LOAD))
gran = calc_delta_fair(gran, se);
} else {
if (unlikely(curr->load.weight != NICE_0_LOAD))
gran = calc_delta_fair(gran, curr);
}
return gran;
}
@ -1331,7 +1378,7 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
if (vdiff <= 0)
return -1;
gran = wakeup_gran(curr);
gran = wakeup_gran(curr, se);
if (vdiff > gran)
return 1;

3
kernel/sched_features.h
View File

@ -1,5 +1,6 @@
SCHED_FEAT(NEW_FAIR_SLEEPERS, 1)
SCHED_FEAT(NORMALIZED_SLEEPER, 1)
SCHED_FEAT(NORMALIZED_SLEEPER, 0)
SCHED_FEAT(ADAPTIVE_GRAN, 1)
SCHED_FEAT(WAKEUP_PREEMPT, 1)
SCHED_FEAT(START_DEBIT, 1)
SCHED_FEAT(AFFINE_WAKEUPS, 1)