linux-kernel - Re: [PATCH 0/10] Reduce system disruption due to kswapd V2

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Date:	Wed, 10 Apr 2013 15:28:32 -0700 (PDT)
From:	dormando <dormando@...ia.net>
To:	Mel Gorman <mgorman@...e.de>
cc:	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Rik van Riel <riel@...hat.com>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Johannes Weiner <hannes@...xchg.org>,
	Satoru Moriya <satoru.moriya@....com>,
	Michal Hocko <mhocko@...e.cz>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/10] Reduce system disruption due to kswapd V2

> On Tue, Apr 09, 2013 at 05:27:18PM +0000, Christoph Lameter wrote:
> > One additional measure that may be useful is to make kswapd prefer one
> > specific processor on a socket. Two benefits arise from that:
> >
> > 1. Better use of cpu caches and therefore higher speed, less
> > serialization.
> >
>
> Considering the volume of pages that kswapd can scan when it's active
> I would expect that it trashes its cache anyway. The L1 cache would be
> flushed after scanning struct pages for just a few MB of memory.
>
> > 2. Reduction of the disturbances to one processor.
> >
>
> I've never checked it but I would have expected kswapd to stay on the
> same processor for significant periods of time. Have you experienced
> problems where kswapd bounces around on CPUs within a node causing
> workload disruption?

When kswapd shares the same CPU as our main process it causes a measurable
drop in response time (graphs show tiny spikes at the same time memory is
freed). Would be nice to be able to ensure it runs on a different core
than our latency sensitive processes at least. We can pin processes to
subsets of cores but I don't think there's a way to keep kswapd from
waking up on any of them?
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