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authorLinus Torvalds <torvalds@linux-foundation.org>2014-09-13 11:30:10 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-09-13 11:30:10 -0700
commit99d263d4c5b2f541dfacb5391e22e8c91ea982a6 (patch)
parent23d0db76ffa13ffb95229946e4648568c3c29db5 (diff)
vfs: fix bad hashing of dentries
Josef Bacik found a performance regression between 3.2 and 3.10 and narrowed it down to commit bfcfaa77bdf0 ("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: Josef 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: Linus Torvalds <torvalds@linux-foundation.org>
2 files changed, 3 insertions, 4 deletions
diff --git a/fs/dcache.c b/fs/dcache.c
index d30ce699ae4b..4023e77b800e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -106,8 +106,7 @@ static inline struct hlist_bl_head *d_hash(const struct dentry *parent,
unsigned int hash)
hash += (unsigned long) parent / L1_CACHE_BYTES;
- hash = hash + (hash >> d_hash_shift);
- return dentry_hashtable + (hash & d_hash_mask);
+ return dentry_hashtable + hash_32(hash, d_hash_shift);
/* Statistics gathering. */
diff --git a/fs/namei.c b/fs/namei.c
index a996bb48dfab..229235862e50 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -34,6 +34,7 @@
#include <linux/device_cgroup.h>
#include <linux/fs_struct.h>
#include <linux/posix_acl.h>
+#include <linux/hash.h>
#include <asm/uaccess.h>
#include "internal.h"
@@ -1634,8 +1635,7 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
static inline unsigned int fold_hash(unsigned long hash)
- hash += hash >> (8*sizeof(int));
- return hash;
+ return hash_64(hash, 32);
#else /* 32-bit case */