Re: [Jackit-devel] Re: Statistical methods for latency profiling

[Ingo Molnar <mingo@redhat.com>]

On Sun, 1 Aug 2004, Lee Revell wrote:

> So stressing the filesystem moves the center to the right a bit, from
> 6-7 to 9-10, and *drastically* lengthens the 'tail'.

basically each codepath has a typical latency distribution, and when a
workload uses multiple codepaths then the latencies get intermixed almost
linearly.

> These numbers suggest to me that a lot of the latencies from 47 usecs
> and up are caused by one code path, because they are so uniformly
> distributed over the upper part of the histogram.  The prime suspect of
> course being the ide io completions.  I tested this theory by lowering
> max_sectors_kb from 64 to 32:

> These numbers all point to the ide sg completion code as the only thing
> on the system generating latencies over ~42 usecs.

yep, that's a fair assumption. Once the IO-APIC irq-redirection problems
are solved i'll try to further thread the IDE completion IRQ to remove
that ~100 usecs latency.

	Ingo