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

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Date:	Tue, 20 Mar 2007 18:12:35 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <christoph@...eter.com>,
	Andi Kleen <andi@...stfloor.org>
Cc:	linux kernel <linux-kernel@...r.kernel.org>
Subject: [RFC] SLAB : NUMA cache_free_alien() very expensive because of
 virt_to_slab(objp); nodeid = slabp->nodeid;

Hi

I noticed on a small x86_64 NUMA setup (2 nodes) that cache_free_alien() is very expensive.
This is because of a cache miss on struct slab.
At the time an object is freed (call to kmem_cache_free() for example), the underlying 'struct slab' is not anymore cache-hot.

struct slab *slabp = virt_to_slab(objp);
nodeid = slabp->nodeid; // cache miss

So we currently need slab only to lookup nodeid, to be able to use the cachep cpu cache, or not.

Couldn't we use something less expensive, like pfn_to_nid() ?
On x86_64 pfn_to_nid usually shares one cache line for all objects (struct memnode)

Is it possible virt_to_slab(objp)->nodeid being different from pfn_to_nid(objp) ?

Eric
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