Re: BFS vs. mainline scheduler benchmarks and measurements

[Jens Axboe]

On Wed, Sep 09 2009, Jens Axboe wrote:
> On Wed, Sep 09 2009, Mike Galbraith wrote:
> > On Wed, 2009-09-09 at 08:13 +0200, Ingo Molnar wrote:
> > > * Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> > > 
> > > > On Tue, Sep 08 2009, Peter Zijlstra wrote:
> > > > > On Tue, 2009-09-08 at 11:13 +0200, Jens Axboe wrote:
> > > > > > And here's a newer version.
> > > > > 
> > > > > I tinkered a bit with your proglet and finally found the 
> > > > > problem.
> > > > > 
> > > > > You used a single pipe per child, this means the loop in 
> > > > > run_child() would consume what it just wrote out until it got 
> > > > > force preempted by the parent which would also get woken.
> > > > > 
> > > > > This results in the child spinning a while (its full quota) and 
> > > > > only reporting the last timestamp to the parent.
> > > > 
> > > > Oh doh, that's not well thought out. Well it was a quick hack :-) 
> > > > Thanks for the fixup, now it's at least usable to some degree.
> > > 
> > > What kind of latencies does it report on your box?
> > > 
> > > Our vanilla scheduler default latency targets are:
> > > 
> > >   single-core: 20 msecs
> > >     dual-core: 40 msecs
> > >     quad-core: 60 msecs
> > >     opto-core: 80 msecs
> > > 
> > > You can enable CONFIG_SCHED_DEBUG=y and set it directly as well via 
> > > /proc/sys/kernel/sched_latency_ns:
> > > 
> > >    echo 10000000 > /proc/sys/kernel/sched_latency_ns
> > 
> > He would also need to lower min_granularity, otherwise, it'd be larger
> > than the whole latency target.
> > 
> > I'm testing right now, and one thing that is definitely a problem is the
> > amount of sleeper fairness we're giving.  A full latency is just too
> > much short term fairness in my testing.  While sleepers are catching up,
> > hogs languish.  That's the biggest issue going on.
> > 
> > I've also been doing some timings of make -j4 (looking at idle time),
> > and find that child_runs_first is mildly detrimental to fork/exec load,
> > as are buddies.
> > 
> > I'm running with the below at the moment.  (the kthread/workqueue thing
> > is just because I don't see any reason for it to exist, so consider it
> > to be a waste of perfectly good math;)
> 
> Using latt, it seems better than -rc9. The below are entries logged
> while running make -j128 on a 64 thread box. I did two runs on each, and
> latt is using 8 clients.
> 
> -rc9
>         Max                23772 usec
>         Avg                 1129 usec
>         Stdev               4328 usec
>         Stdev mean           117 usec
> 
>         Max                32709 usec
>         Avg                 1467 usec
>         Stdev               5095 usec
>         Stdev mean           136 usec
> 
> -rc9 + patch
> 
>         Max                11561 usec
>         Avg                 1532 usec
>         Stdev               1994 usec
>         Stdev mean            48 usec
> 
>         Max                 9590 usec
>         Avg                 1550 usec
>         Stdev               2051 usec
>         Stdev mean            50 usec
> 
> max latency is way down, and much smaller variation as well.

Things are much better with this patch on the notebook! I cannot compare
with BFS as that still doesn't run anywhere I want it to run, but it's
way better than -rc9-git stock. latt numbers on the notebook have 1/3
the max latency, average is lower, and stddev is much smaller too.

-- 
Jens Axboe

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