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

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Date:	Thu, 11 Apr 2013 22:13:16 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
Cc:	dormando <dormando@...ia.net>, 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>,
	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 Thu 11-04-13 10:10:44, Mel Gorman wrote:
> On Wed, Apr 10, 2013 at 03:28:32PM -0700, dormando wrote:
> > > 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?
> 
> I've never tried it myself but does the following work?
> 
> taskset -p MASK `pidof kswapd`

I would use pgrep rather than pidof which seem to need the whole process
name but yes this should work as kswapdN is not PF_THREAD_BOUND kernel
thread.

-- 
Michal Hocko
SUSE Labs
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