linux-kernel - Re: Scheduler benchmarks

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Date:	Tue, 18 Sep 2007 05:45:15 -0400
From:	"Rob Hussey" <robjhussey@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, ck@....kolivas.org
Subject: Re: Scheduler benchmarks - a follow-up

On 9/18/07, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Rob Hussey <robjhussey@...il.com> wrote:
>
> > The obligatory graphs:
> > http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_lat_ctx_benchmark.png
> > http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_hackbench_benchmark.png
> > http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_pipe-test_benchmark.png
>
> btw., it's likely that if you turn off CONFIG_PREEMPT for .21 and for
> .22-ck1 they'll improve a bit too - so it's not fair to put the .23
> !PREEMPT numbers on the graph as the PREEMPT numbers of the other
> kernels. (it shows the .23 scheduler being faster than it really is)
>

The graphs are really just to show where the new numbers fit in. Plus,
I was too lazy to run all the numbers again.

> > A cursory glance suggests that performance wrt lat_ctx and hackbench
> > has increased (lower numbers), but degraded quite a lot for pipe-test.
> > The numbers for pipe-test are extremely stable though, while the
> > numbers for hackbench are more erratic (which isn't saying much since
> > the original numbers gave nearly a straight line). I'm still willing
> > to try out any more ideas.
>
> the pipe-test behavior looks like an outlier. !PREEMPT only removes code
> (which makes the code faster), so this could be a cache layout artifact.
> (or perhaps we preempt at a different point which is disadvantageous to
> caching?) Pipe-test is equivalent to "lat_ctx -s 0 2" so if there was a
> genuine slowdown it would show up in the lat_ctx graph - but the graph
> shows a speedup.
>

Interestingly, every set of lat_ctx -s 0 2 numbers I run on the
!PREEMPT kernel are on average higher than with PREEMPT (around 2.84
for !PREEMPT and 2.4 for PREEMPT). Anything higher than around 2 or 3
(such as lat_ctx -s 0 8) gives lower average numbers for !PREEMPT.

Regards,
Rob
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