Messages in this thread | | | Date | Tue, 23 Jul 2013 13:03:45 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v2] sched: Limit idle_balance() |
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On Mon, Jul 22, 2013 at 11:57:47AM -0700, Jason Low wrote: > On Mon, 2013-07-22 at 12:31 +0530, Srikar Dronamraju wrote: > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > index e8b3350..da2cb3e 100644 > > > --- a/kernel/sched/core.c > > > +++ b/kernel/sched/core.c > > > @@ -1348,6 +1348,8 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags) > > > else > > > update_avg(&rq->avg_idle, delta); > > > rq->idle_stamp = 0; > > > + > > > + rq->idle_duration = (rq->idle_duration + delta) / 2; > > > > Cant we just use avg_idle instead of introducing idle_duration? > > A potential issue I have found with avg_idle is that it may sometimes be > not quite as accurate for the purposes of this patch, because it is > always given a max value (default is 1000000 ns). For example, a CPU > could have remained idle for 1 second and avg_idle would be set to 1 > millisecond. Another question I have is whether we can update avg_idle > at all times without putting a maximum value on avg_idle, or increase > the maximum value of avg_idle by a lot.
The only user of avg_idle is idle_balance(); since you're building a new limiter we can completely scrap/rework avg_idle to do as you want it to. No point in having two of them.
Also, we now have rq->cfs.{blocked,runnable}_load_avg that might help with estimating if you're so inclined :-)
> > Should we take the consideration of whether a idle_balance was > > successful or not? > > I recently ran fserver on the 8 socket machine with HT-enabled and found > that load balance was succeeding at a higher than average rate, but idle > balance was still lowering performance of that workload by a lot. > However, it makes sense to allow idle balance to run longer/more often > when it has a higher success rate. > > > I am not sure whats a reasonable value for n can be, but may be we could > > try with n=3. > > Based on some of the data I collected, n = 10 to 20 provides much better > performance increases.
Right, so I'm still a bit puzzled by why this is so; maybe we're over-estimating the idle duration due to significant variance in the idle time?
Maybe we should try with something like the below to test this?
/* * http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance */ struct stats { long mean; long M2; unsigned int n; };
static void stats_update(struct stats *stats, long x) { long delta;
stats->n++; delta = x - stats->mean; stats->mean += delta / stats->n; stats->M2 += delta * (x - stats->mean); }
static long stats_var(struct stats *stats) { long variance;
if (!stats->n) return 0;
variance = stats->M2 / (stats->n - 1);
return int_sqrt(variance); }
static long stats_mean(struct stats *stats) { return stats->mean; }
> Yes, I have done quite a bit of testing with sched_migration_cost and > adjusting it does help performance when idle balance overhead is high. > But I have found that a higher value may decrease the performance during > situations where the cost of idle_balance is not high. Additionally, > when to modify this tunable and by how much to modify it by can > sometimes be unpredictable.
So the history if sched_migration_cost is that it used to be a per sd value; see also:
https://lkml.org/lkml/2008/9/4/215
Ingo wrote it initially for the O(1) scheduler and ripped it out when he did CFS. He now doesn't like it because it introduces boot-to-boot scheduling differences -- you never measure the exact numbers again.
That said, there is a case for restoring it since the one measure really doesn't do justice to larger systems.
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