linux-kernel - Re: [PATCH 1/11] ksm: allow trees per NUMA node

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Message-ID: <alpine.LNX.2.00.1301281707430.4947@eggly.anvils>
Date:	Mon, 28 Jan 2013 17:17:24 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Petr Holasek <pholasek@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/11] ksm: allow trees per NUMA node

On Mon, 28 Jan 2013, Andrew Morton wrote:
> On Fri, 25 Jan 2013 17:54:53 -0800 (PST)
> Hugh Dickins <hughd@...gle.com> wrote:
> 
> > --- mmotm.orig/Documentation/vm/ksm.txt	2013-01-25 14:36:31.724205455 -0800
> > +++ mmotm/Documentation/vm/ksm.txt	2013-01-25 14:36:38.608205618 -0800
> > @@ -58,6 +58,13 @@ sleep_millisecs  - how many milliseconds
> >                     e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
> >                     Default: 20 (chosen for demonstration purposes)
> >  
> > +merge_across_nodes - specifies if pages from different numa nodes can be merged.
> > +                   When set to 0, ksm merges only pages which physically
> > +                   reside in the memory area of same NUMA node. It brings
> > +                   lower latency to access to shared page. Value can be
> > +                   changed only when there is no ksm shared pages in system.
> > +                   Default: 1
> > +
> 
> The explanation doesn't really tell the operator whether or not to set
> merge_across_nodes for a particular machine/workload.
> 
> I guess most people will just shrug, turn the thing on and see if it
> improved things, but that's rather random.

Right.  I don't think we can tell them which is going to be better,
but surely we could do a better job of hinting at the tradeoffs.

I think we expect large NUMA machines with lots of memory to want the
better NUMA behavior of !merge_across_nodes, but machines with more
limited memory across short-distance NUMA nodes, to prefer the greater
deduplication of merge_across nodes.

Petr, do you have a more informative text for this?

Hugh
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