linux-kernel - Re: [PATCH] Reduce vm_stat cacheline contention in __vm_enough

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Message-ID: <alpine.DEB.2.00.1110191029570.24001@router.home>
Date:	Wed, 19 Oct 2011 10:31:54 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Dimitri Sivanich <sivanich@....com>
cc:	David Rientjes <rientjes@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH] Reduce vm_stat cacheline contention in
 __vm_enough_memory

On Wed, 19 Oct 2011, Dimitri Sivanich wrote:

> For 120 threads writing in parallel (each to it's own mountpoint), the
> threshold needs to be on the order of 1000.  At a threshold of 750, I
> start to see a slowdown of 50-60 MB/sec.
>
> For 400 threads writing in parallel, the threshold needs to be on the order
> of 2000 (although we're off by about 40 MB/sec at that point).
>
> The necessary deltas in these cases are quite a bit higher than the current
> 125 maximum (see calculate*threshold in mm/vmstat.c).
>
> I like the idea of having certain areas triggering vm_stat sync, as long
> as we know what those key areas are and how often they might be called.

You could potentially reduce the maximum necessary by applying my earlier
patch (but please reduce the counters touched to the current cacheline).
That should reduce the number of updates in the global cacheline and allow
you to reduce the very high deltas that you have to deal with now.

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