linux-kernel - Re: [RFC 3/3] man-pages: Add man page for vmpressure

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Date:	Wed, 21 Nov 2012 15:01:50 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Leonid Moiseichuk <leonid.moiseichuk@...ia.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	John Stultz <john.stultz@...aro.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	patches@...aro.org, kernel-team@...roid.com,
	linux-man@...r.kernel.org
Subject: Re: [RFC 3/3] man-pages: Add man page for vmpressure_fd(2)

On Tue, Nov 20, 2012 at 10:12:28AM -0800, David Rientjes wrote:
> On Mon, 19 Nov 2012, Anton Vorontsov wrote:
> 
> > We try to make userland freeing resources when the system becomes low on
> > memory. Once we're short on memory, sometimes it's better to discard
> > (free) data, rather than let the kernel to drain file caches or even start
> > swapping.
> > 
> 
> To add another usecase: its possible to modify our version of malloc (or 
> any malloc) so that memory that is free()'d can be released back to the 
> kernel only when necessary, i.e. when keeping the extra memory around 
> starts to have a detremental effect on the system, memcg, or cpuset.  When 
> there is an abundance of memory available such that allocations need not 
> defragment or reclaim memory to be allocated, it can improve performance 
> to keep a memory arena from which to allocate from immediately without 
> calling the kernel.
> 

A potential third use case is a variation of the first for batch systems. If
it's running low priority tasks and a high priority task starts that
results in memory pressure then the job scheduler may decide to move the
low priority jobs elsewhere (or cancel them entirely).

A similar use case is monitoring systems running high priority workloads
that should never swap. It can be easily detected if the system starts
swapping but a pressure notification might act as an early warning system
that something is happening on the system that might cause the primary
workload to start swapping.

-- 
Mel Gorman
SUSE Labs
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