linux-kernel - Re: [PATCH v2] KSM: numa awareness sysfs knob

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Date:	Sat, 30 Jun 2012 12:43:24 +0300
From:	Izik Eidus <izik.eidus@...ellosystems.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Holasek <pholasek@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Anton Arapov <anton@...hat.com>
Subject: Re: [PATCH v2] KSM: numa awareness sysfs knob

On 06/30/2012 01:50 AM, David Rientjes wrote:
> On Fri, 29 Jun 2012, Andrew Morton wrote:
>
>>> I've tested this patch on numa machines with 2, 4 and 8 nodes and
>>> measured speed of memory access inside of KVM guests with memory pinned
>>> to one of nodes with this benchmark:
>>>
>>> http://pholasek.fedorapeople.org/alloc_pg.c
>>>
>>> Population standard deviations of access times in percentage of average
>>> were following:
>>>
>>> merge_nodes=1
>>> 2 nodes 1.4%
>>> 4 nodes 1.6%
>>> 8 nodes	1.7%
>>>
>>> merge_nodes=0
>>> 2 nodes	1%
>>> 4 nodes	0.32%
>>> 8 nodes	0.018%
>> ooh, numbers!  Thanks.
>>
> Ok, the standard deviation increases when merging pages from nodes with
> remote distance, that makes sense.  But if that's true, then you would
> restrict either the entire application to local memory with mempolicies or
> cpusets, or you would use mbind() to restrict this memory to that set of
> nodes already so that accesses, even with ksm merging, would have
> affinity.

While you are right for case you write your own custom application,
but I think the KVM guest case is little bit more problomatic in case 
the guest memory must be splitted across serval nodes.

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