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    Subject[PATCH] fs : reorder some 'struct inode' fields to speedup i_size manipulations
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    On 32bits SMP platforms, 64bits i_size is protected by a seqcount 
    (i_size_seqcount).

    When i_size is read or written, i_size_seqcount is read/written as well, so it
    make sense to group these two fields together in the same cache line.

    Before this patch, accessing i_size needed 3 cache lines (2 for i_size, one
    for i_size_seqcount). After, only one cache line is needed/ (dirtied on a
    i_size change).

    This patch moves i_size_seqcount next to i_size, and also moves i_version to
    let offsetof(struct inode, i_size) being 0x40 instead of 0x3c (for 32bits
    platforms).

    For 64 bits platforms, i_size_seqcount doesnt exist, and the move of a 'long
    i_version' should not introduce a new hole because of padding.

    Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
    --- linux-2.6.19-rc6/include/linux/fs.h 2006-11-23 10:47:53.000000000 +0100
    +++ linux-2.6.19-rc6-ed/include/linux/fs.h 2006-11-23 11:13:36.000000000 +0100
    @@ -548,12 +548,15 @@ struct inode {
    uid_t i_uid;
    gid_t i_gid;
    dev_t i_rdev;
    + unsigned long i_version;
    loff_t i_size;
    +#ifdef __NEED_I_SIZE_ORDERED
    + seqcount_t i_size_seqcount;
    +#endif
    struct timespec i_atime;
    struct timespec i_mtime;
    struct timespec i_ctime;
    unsigned int i_blkbits;
    - unsigned long i_version;
    blkcnt_t i_blocks;
    unsigned short i_bytes;
    spinlock_t i_lock; /* i_blocks, i_bytes, maybe i_size */
    @@ -598,9 +601,6 @@ struct inode {
    void *i_security;
    #endif
    void *i_private; /* fs or device private pointer */
    -#ifdef __NEED_I_SIZE_ORDERED
    - seqcount_t i_size_seqcount;
    -#endif
    };

    /*
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