linux-kernel - Re: [patch -mm] cpusets: add memory_slab

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Message-ID: <alpine.DEB.2.00.0903090206010.24605@chino.kir.corp.google.com>
Date:	Mon, 9 Mar 2009 02:12:21 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>,
	Paul Menage <menage@...gle.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch -mm] cpusets: add memory_slab_hardwall flag

On Mon, 9 Mar 2009, KOSAKI Motohiro wrote:

> Hmmm,
> this description only explay how to implement this.
> but no explain why this patch is useful.
> 
> Could you please who and why need it?
> 

The change to Documentation/cgroups/cpusets.txt should have explained it.

This is for two cases: true memory isolation (now including slab 
allocations at the object level) and NUMA optimizations.

Prior to this change, it was possible for slabs to be allocated in a 
cpuset while its objects were largely consumed by disjoint cpusets.  We 
can fix that by only allocating objects from slabs that are found on 
current->mems_allowed.  While this incurs a performance penalty, some 
users may find that true isolation outweighs the cache optimizations.

It is also helpful for long-lived objects that require NUMA affinity to a 
certain cpu or group of cpus.  That is, after all, the reasoning behind 
cpusets in the first place.  If slab objects were all allocated from a 
node with remote affinity to the cpus that will be addressing it, it 
negates a significant advantage that cpusets provides to the user.
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