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

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Message-ID: <20080801191336.GK10501@sgi.com>
Date:	Fri, 1 Aug 2008 14:13:36 -0500
From:	Robin Holt <holt@....com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Robin Holt <holt@....com>, 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().

On Fri, Aug 01, 2008 at 11:27:20AM -0700, Eric W. Biederman wrote:
> 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?

Oops, confusing details.  That was a different problem we had been
tracking.

Sorry for the confusion,
Robin
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