[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>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 50be290d24d2..ab04b4f9b0db 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -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 */
 };