From: Eric Dumazet <dada1@cosmosbay.com>
To: Pekka J Enberg <penberg@cs.helsinki.fi>
Cc: Pavel Emelianov <xemul@sw.ru>, Andrew Morton <akpm@osdl.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
devel@openvz.org, Kirill Korotaev <dev@openvz.org>,
linux-mm@kvack.org
Subject: Re: [PATCH] Show slab memory usage on OOM and SysRq-M
Date: Wed, 18 Apr 2007 14:07:52 +0200 [thread overview]
Message-ID: <20070418140752.02084f34.dada1@cosmosbay.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0704180915520.11160@sbz-30.cs.Helsinki.FI>
On Wed, 18 Apr 2007 09:17:19 +0300 (EEST)
Pekka J Enberg <penberg@cs.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
WARNING: multiple messages have this Message-ID (diff)
From: Eric Dumazet <dada1@cosmosbay.com>
To: Pekka J Enberg <penberg@cs.helsinki.fi>
Cc: Pavel Emelianov <xemul@sw.ru>, Andrew Morton <akpm@osdl.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
devel@openvz.org, Kirill Korotaev <dev@openvz.org>,
linux-mm@kvack.org
Subject: Re: [PATCH] Show slab memory usage on OOM and SysRq-M
Date: Wed, 18 Apr 2007 14:07:52 +0200 [thread overview]
Message-ID: <20070418140752.02084f34.dada1@cosmosbay.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0704180915520.11160@sbz-30.cs.Helsinki.FI>
On Wed, 18 Apr 2007 09:17:19 +0300 (EEST)
Pekka J Enberg <penberg@cs.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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next prev parent reply other threads:[~2007-04-18 12:08 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-04-17 12:55 [PATCH] Show slab memory usage on OOM and SysRq-M Pavel Emelianov
2007-04-17 12:55 ` Pavel Emelianov
2007-04-17 13:22 ` Pekka Enberg
2007-04-17 13:22 ` Pekka Enberg
2007-04-17 13:50 ` Pavel Emelianov
2007-04-17 13:50 ` Pavel Emelianov
2007-04-17 14:02 ` Pekka J Enberg
2007-04-17 14:02 ` Pekka J Enberg
2007-04-17 14:21 ` Pavel Emelianov
2007-04-17 14:21 ` Pavel Emelianov
2007-04-17 15:12 ` Eric Dumazet
2007-04-17 15:12 ` Eric Dumazet
2007-04-18 6:17 ` Pekka J Enberg
2007-04-18 6:17 ` Pekka J Enberg
2007-04-18 12:07 ` Eric Dumazet [this message]
2007-04-18 12:07 ` Eric Dumazet
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