• Linus Torvalds's avatar
    vfs: fix bad hashing of dentries · d4c96061
    Linus Torvalds authored
    commit 99d263d4c5b2f541dfacb5391e22e8c91ea982a6 upstream.
    
    Josef Bacik found a performance regression between 3.2 and 3.10 and
    narrowed it down to commit bfcfaa77 ("vfs: use 'unsigned long'
    accesses for dcache name comparison and hashing"). He reports:
    
     "The test case is essentially
    
          for (i = 0; i < 1000000; i++)
                  mkdir("a$i");
    
      On xfs on a fio card this goes at about 20k dir/sec with 3.2, and 12k
      dir/sec with 3.10.  This is because we spend waaaaay more time in
      __d_lookup on 3.10 than in 3.2.
    
      The new hashing function for strings is suboptimal for <
      sizeof(unsigned long) string names (and hell even > sizeof(unsigned
      long) string names that I've tested).  I broke out the old hashing
      function and the new one into a userspace helper to get real numbers
      and this is what I'm getting:
    
          Old hash table had 1000000 entries, 0 dupes, 0 max dupes
          New hash table had 12628 entries, 987372 dupes, 900 max dupes
          We had 11400 buckets with a p50 of 30 dupes, p90 of 240 dupes, p99 of 567 dupes for the new hash
    
      My test does the hash, and then does the d_hash into a integer pointer
      array the same size as the dentry hash table on my system, and then
      just increments the value at the address we got to see how many
      entries we overlap with.
    
      As you can see the old hash function ended up with all 1 million
      entries in their own bucket, whereas the new one they are only
      distributed among ~12.5k buckets, which is why we're using so much
      more CPU in __d_lookup".
    
    The reason for this hash regression is two-fold:
    
     - On 64-bit architectures the down-mixing of the original 64-bit
       word-at-a-time hash into the final 32-bit hash value is very
       simplistic and suboptimal, and just adds the two 32-bit parts
       together.
    
       In particular, because there is no bit shuffling and the mixing
       boundary is also a byte boundary, similar character patterns in the
       low and high word easily end up just canceling each other out.
    
     - the old byte-at-a-time hash mixed each byte into the final hash as it
       hashed the path component name, resulting in the low bits of the hash
       generally being a good source of hash data.  That is not true for the
       word-at-a-time case, and the hash data is distributed among all the
       bits.
    
    The fix is the same in both cases: do a better job of mixing the bits up
    and using as much of the hash data as possible.  We already have the
    "hash_32|64()" functions to do that.
    Reported-by: default avatarJosef Bacik <jbacik@fb.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Chris Mason <clm@fb.com>
    Cc: linux-fsdevel@vger.kernel.org
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    d4c96061
dcache.c 78.8 KB