Re: [patch] voluntary-preempt-2.6.8-rc4-O7

Ingo Molnar
2004-08-14T12:40:57Z

* Lee Revell <rlrevell@joe-job.com> wrote:

> > http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.8-rc4-O7
> >
>
> Some more interesting results. I am not seeing much of a difference in performance, but
> the shape of the distribution changes quite a bit:
>
> http://krustophenia.net/testresults.php?dataset=2.6.8-rc3-O5
> http://krustophenia.net/testresults.php?dataset=2.6.8-rc4-bk3-O7

nice graphs - i suspect the shape difference is due to the tracing overhead: it adds visible
(and measurable) overhead to all kernel execution paths, but since the overwhelming majority
of paths have very low latencies it doesnt degrade the max_latency results visibly. This is
good empirical data - it means what we are after a few offenders who are more than a
magnitude slower than the common paths. It also shows that most of the extra overhead is
rather in isolated functions (e.g. the memcpy's), not in some big latency path involving
many small functions.

i also suspect that if you magnified the 'sharp' nontracing graphs near zero they'd show a
similar shape to the tracing graphs.

Ingo