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    Subject[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.

    with the patch this time :)

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

    --- linux-2.6.13/include/linux/dcache.h 2005-08-29 01:41:01.000000000 +0200
    +++ linux-2.6.13-ed/include/linux/dcache.h 2005-09-01 17:22:32.000000000 +0200
    @@ -88,8 +88,9 @@
    * 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 @@
    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 */
    };
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