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[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>
This commit is contained in:
Eric Dumazet 2005-09-06 15:18:16 -07:00 committed by Linus Torvalds
parent 0811bab24f
commit 3f4bb1f419
1 changed files with 2 additions and 2 deletions
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4
include/linux/dcache.h
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@ -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 */
};