Commit 3f4bb1f4 authored Sep 06, 2005 by Eric Dumazet Committed by Linus Torvalds Sep 07, 2005

[PATCH] struct dentry: place d_hash close to d_parent and d_name to speedup lookups



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>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 0811bab2

include/linux/dcache.h

+2 −2

Original line number	Diff line number	Diff line
		@@ -88,8 +88,9 @@ struct dentry {
		* negative */
		/*
		* The next three fields are touched by __d_lookup. Place them here
		* so they all fit in a 16-byte range, with 16-byte alignment.
		* so they all fit in a cache line.
		*/
		struct hlist_node d_hash; /* lookup hash list */
		struct dentry d_parent; / parent directory */
		struct qstr d_name;

		@@ -103,7 +104,6 @@ struct dentry {
		void d_fsdata; / fs-specific data */
		struct rcu_head d_rcu;
		struct dcookie_struct d_cookie; / cookie, if any */
		struct hlist_node d_hash; /* lookup hash list */
		int d_mounted;
		unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
		};