• Dave Chinner's avatar
    xfs: abstract the differences in dir2/dir3 via an ops vector · 32c5483a
    Dave Chinner authored
    Lots of the dir code now goes through switches to determine what is
    the correct on-disk format to parse. It generally involves a
    "xfs_sbversion_hasfoo" check, deferencing the superblock version and
    feature fields and hence touching several cache lines per operation
    in the process. Some operations do multiple checks because they nest
    conditional operations and they don't pass the information in a
    direct fashion between each other.
    
    Hence, add an ops vector to the xfs_inode structure that is
    configured when the inode is initialised to point to all the correct
    decode and encoding operations.  This will significantly reduce the
    branchiness and cacheline footprint of the directory object decoding
    and encoding.
    
    This is the first patch in a series of conversion patches. It will
    introduce the ops structure, the setup of it and add the first
    operation to the vector. Subsequent patches will convert directory
    ops one at a time to keep the changes simple and obvious.
    
    Just this patch shows the benefit of such an approach on code size.
    Just converting the two shortform dir operations as this patch does
    decreases the built binary size by ~1500 bytes:
    
    $ size fs/xfs/xfs.o.orig fs/xfs/xfs.o.p1
       text    data     bss     dec     hex filename
     794490   96802    1096  892388   d9de4 fs/xfs/xfs.o.orig
     792986   96802    1096  890884   d9804 fs/xfs/xfs.o.p1
    $
    
    That's a significant decrease in the instruction cache footprint of
    the directory code for such a simple change, and indicates that this
    approach is definitely worth pursuing further.
    Signed-off-by: default avatarDave Chinner <dchinner@redhat.com>
    Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarBen Myers <bpm@sgi.com>
    32c5483a
xfs_da_btree.h 7.49 KB