Message-ID: <431721F0.6090402@cosmosbay.com>
Date: Thu, 01 Sep 2005 17:44:48 +0200
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
Subject: [PATCH] : struct dentry : place d_hash close to d_parent and d_name
 to speedup lookups
References: <20050901035542.1c621af6.akpm@osdl.org>
In-Reply-To: <20050901035542.1c621af6.akpm@osdl.org>
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dentry cache uses sophisticated RCU technology (and prefetching if available) 
but touches 2 cache lines per dentry during hlist lookup.

This patch moves d_hash in the same cache line than d_parent and d_name fields 
so that :

1) One cache line is needed instead of two.
2) the hlist_for_each_rcu() prefetching has a chance to bring all the needed 
data in advance, not only the part that includes d_hash.next.

I also changed one old comment that was wrong for 64bits.

A further optimisation would be to separate dentry in two parts, one that is 
mostly read, and one writen (d_count/d_lock) to avoid false sharing on 
SMP/NUMA but this would need different field placement depending on 32bits or 
64bits platform.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>


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