linux-kernel - Re: [PATCH] Show slab memory usage on OOM and SysRq-M

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Message-Id: <20070418140752.02084f34.dada1@cosmosbay.com>
Date:	Wed, 18 Apr 2007 14:07:52 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	Pavel Emelianov <xemul@...ru>, Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org, Kirill Korotaev <dev@...nvz.org>,
	linux-mm@...ck.org
Subject: Re: [PATCH] Show slab memory usage on OOM and SysRq-M

On Wed, 18 Apr 2007 09:17:19 +0300 (EEST)
Pekka J Enberg <penberg@...helsinki.fi> wrote:

> On Tue, 17 Apr 2007, Eric Dumazet wrote:
> > This nr_pages should be in struct kmem_list3, not in struct kmem_cache, 
> > or else you defeat NUMA optimizations if touching a field in kmem_cache 
> > at kmem_getpages()/kmem_freepages() time.
> 
> We already touch ->flags, ->gfpflags, and ->gfporder in kmem_getpages(). 
> Sorry for my ignorance, but how is this different?
> 

Those fields are read. Thats OK, because several CPUS might share all those without problem.

But modifying one field in kmem_cache would invalidate one cache line for all cpus that would have to reload it later.

This is what we call "false sharing" or cache line ping pongs


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