Messages in this thread |  | | | Date | Sun, 12 Sep 2010 16:34:54 -0400 | | From | Mathieu Desnoyers <> | | Subject | Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr_running |
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* Peter Zijlstra (peterz@infradead.org) wrote: > On Sat, 2010-09-11 at 13:48 -0700, Linus Torvalds wrote: > > On Sat, Sep 11, 2010 at 1:36 PM, Peter Zijlstra <peterz@infradead.org> wrote: > > > > > >From what I can make up: > > > > > > LAT=`cat /proc/sys/kernel/sched_latency_ns`; > > > echo $((LAT/8)) > /proc/sys/kernel/sched_min_granularity_ns > > > > > > will give you pretty much the same result as Mathieu's patch. > > > > Or perhaps not. The point being that Mathieu's patch seems to do this > > dynamically based on number of runnable threads per cpu. Which seems > > to be a good idea. > > > > IOW, this part: > > > > - if (delta_exec < sysctl_sched_min_granularity) > > + if (delta_exec < __sched_gran(cfs_rq->nr_running)) > > > > seems to be a rather fundamental change, and looks at least > > potentially interesting. It seems to make conceptual sense to take the > > number of running tasks into account at that point, no? > > We used to have something like that a long while back, we nixed it > because of the division and replaced it with floor(__sched_gran) (ie. > the smallest value it would ever give). > > Smaller values are better for latency, larger values are better for > throughput. So introducing __sched_gran() in order to provide larger > values doesn't make sense to me.
__sched_gran() provides large values for small nr_running, and smaller values for larger nr_running.
So for systems with few threads running, we have good throughput (and there does not seem to be much latency issues there). However for a system with a larger number of running threads, __sched_gran() dynamically reduces the granularity.
> > > And I don't like how you dismissed the measured latency improvement. > > And yes, I do think latency matters. A _lot_. > > OK, we'll make it better and sacrifice some throughput, can do, no > problem.
My approach try to get lower latencies without sacrificing throughput when there are few threads running, which IMHO is the common case where throughput really matters. A system running tons of threads should already expect some sort of throughput degradation anyway, so we might as well favor low-latency rather than throughput on those systems running lots of threads.
> > > And no, I'm not saying that Mathieu's patch is necessarily good. I > > haven't tried it myself. I don't have _that_ kind of opinion. The > > opinion I do have is that I think it's sad how you dismissed things > > out of hand - and seem to _continue_ to dismiss them without > > apparently actually having looked at the patch at all. > > Let me draw you a picture of what this patch looks like to me: > > * is slice length, + is period length > > Patch (sched_latency = 10, sched_min_gran = 10/3)
Hrm, in sched_fair.c, sysctl_sched_latency is set to 6000000ULL. So this would be a sched_latency = 6, or am I missing something ? With a sched_latency of 6, the jump you show below in your graph disappears. So what have I missed ? I agree with the shape of your graph, I'm just trying to understand why you have a sched_latency of 10.
Thanks,
Mathieu
> > > 30 | + > | > | > | + > | > | > | > | > | > | > 20 | > | > | > | > | > | > | > | > | > | > 10 | * + + + + + + + > | > | > | > | > | * > | > | * * * * * * * * > | * * > | * * > 0 +--------------------------------------------------------- > 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 > > > Normal (sched_latency = 10, sched_min_gran = 10/3) > > > 30 | + > | > | > | + > | > | > | > | + > | > | > 20 | + > | > | > | + > | > | > | > | + > | > | > 10 | * + + > | > | > | > | > | * > | > | * * * * * * * * * * * * * * > | > | > 0 +--------------------------------------------------------- > 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 > > > > Normal (sched_latency = 10, sched_min_gran = 10/8) > > 30 | > | > | > | > | > | > | > | > | > | > 20 | > | > | > | > | > | + > | + > | + > | > | + > 10 | * + + + + + + + > | > | > | > | > | * > | > | * * > | * * > | * * * * * * * * > 0 +--------------------------------------------------------- > 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 > > > > > > > > > > >
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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