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

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Date:	Wed, 21 Nov 2012 11:39:20 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	David Rientjes <rientjes@...gle.com>,
	Anton Vorontsov <anton.vorontsov@...aro.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 Wed, 21 Nov 2012 15:01:50 +0000
Mel Gorman <mgorman@...e.de> wrote:

> 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.

I hope Anton's writing all of this down ;)


The proposed API bugs me a bit.  It seems simplistic.  I need to have a
quality think about this.  Maybe the result of that think will be to
suggest an interface which can be extended in a back-compatible fashion
later on, if/when the simplistic nature becomes a problem.

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