linux-kernel - Re: [RFC] SLAB : NUMA cache_free_alien() very expensive because of virt_to

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Message-ID: <4600DB6E.40107@cosmosbay.com>
Date:	Wed, 21 Mar 2007 08:14:54 +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] SLAB : NUMA cache_free_alien() very expensive because of
 virt_to_slab(objp); nodeid = slabp->nodeid;

Christoph Lameter a écrit :
> On Wed, 21 Mar 2007, Eric Dumazet wrote:
> 
>> The fast path is to put the pointer, into the cpu array cache. This object
>> might be given back some cycles later, because of a kmem_cache_alloc() : No
>> need to access the two cache lines (struct page, struct slab)
> 
> If you do that then the slab will no longer return objects from the 
> desired nodes. The assumption is that cpu array objects are from the local 
> node.

Me confused.

How the following could be wrong ?

static inline int cache_free_alien(struct kmem_cache *cachep, void *objp)
{
int mynode = numa_node_id();
int objnode = virt_to_nid(objp); // or whatever

if (mynode == objnode)
	return 0;
...
}

If numa_node_id() is equal to the node of the page containing the first byte 
of the object, then object is on the local node. Or what ?

Thank you
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