linux-kernel - Re: [Patch] Scale pidhash_shift/pidhash_size up based on num_possible

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Date:	Fri, 01 Aug 2008 11:27:20 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Robin Holt <holt@....com>
Cc:	linux-kernel@...r.kernel.org, Pavel Emelyanov <xemul@...nvz.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Patch] Scale pidhash_shift/pidhash_size up based on num_possible_cpus().

Robin Holt <holt@....com> writes:

> On Thu, Jul 31, 2008 at 03:04:56PM -0700, Eric W. Biederman wrote:
>> Robin Holt <holt@....com> writes:
>> 
>> > Like so???
>> >
>> > I have not tested this yet.
>> 
>> Looks reasonable to me.
>> 
>> In what circumstances was the lookup in the pid hash table with
>> long changes causing a performance slowdown?.  We don't perform
>> a lot of lookups.
>
> It was initially detected while profiling 'ps' on a 2048p machine that
> had 13 kernel threads per cpu.  We added a couple more device drivers
> which added additional threads.  We then started a pthread-on-process
> MPI job which had 2048 ranks each with 4 threads (test-case from
> customer job).  There were misc other processes out there which brought
> our task count up to approx 63k.  Larger page size helped the problem
> (went from 16k to 64k).

Large page size?  Do you mean larger hash size?

What were you measuring that showed improvement with the large hash size?

Eric
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