linux-kernel - Re: [RFC, PATCH] SLAB : [NUMA] keep nodeid in struct page instead of struct slab

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Message-Id: <20070321163635.82697f08.dada1@cosmosbay.com>
Date:	Wed, 21 Mar 2007 16:36:35 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Christoph Lameter <christoph@...eter.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCH] SLAB : [NUMA] keep nodeid in struct page instead
 of struct slab

On Wed, 21 Mar 2007 07:41:46 -0700 (PDT)
Christoph Lameter <christoph@...eter.com> wrote:

> On Wed, 21 Mar 2007, Eric Dumazet wrote:
> 
> > In order to avoid a cache miss in kmem_cache_free() on NUMA and reduce hot path length, we could exploit the following common facts.
> > 
> > 1) MAX_NUMNODES <= 64
> 
> MAX_NUMNODES == 1024 on IA64 and may be on x86_64 in the future.

So what ? I am very happy for you Christoph.

Average linux machines (ie more than 99.99 % on this planet) are using MAX_NUMNODES <= 64

If you read my patch, your lovely 1024 nodes machines will run as before : No change at all for big machines.

>  
> > 2) alignment of 'struct kmem_cache *' can be >= 64
> > 
> > The following patch changes the page->lru.next to contain not only the 
> > 'struct kmem_cache *' pointer, but also the nodeid in the low order 
> > bits.
> 
> Nack.
> 
> I would suggest you audit slab and make sure that we never fall back and 
> always enter a slab on the node of page_to_nid(page). If 
> all allocations use GFP_THISNODE then you should be safe and then we can 
> remove the nodeid from the slab structure and simply use the available 
> node information in the page struct.
> 

Last time I checked 'struct page', they was no nodeid in it.

I cooked this patch because you told me it was possible to have a 'slab nodeid' different than page_to_nid(obj). Now you want me to prove the contrary. Me confused.

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