diff options
| author | Zach Brown <zab@zabbo.net> | 2014-10-03 09:31:23 +1000 |
|---|---|---|
| committer | Stephen Rothwell <sfr@canb.auug.org.au> | 2014-10-13 14:58:14 +1100 |
| commit | 7477565454a95894489e30237445a4bd050eb74a (patch) | |
| tree | f2931b0cf206a0b2b233e309c3282762ee9fa62a /fs | |
| parent | eb8d417a4d31489ec93be40f305a89fc8cf001c6 (diff) | |
fs: check bh blocknr earlier when searching lru
It's very common for the buffer heads in the lru to have different block
numbers. By comparing the blocknr before the bdev and size we can reduce
the cost of searching in the very common case where all the entries have
the same bdev and size.
In quick hot cache cycle counting tests on a single fs workstation this
cut the cost of a miss by about 20%.
A diff of the disassembly shows the reordering of the bdev and blocknr
comparisons. This is in such a tiny loop that skipping one comparison is
a meaningful portion of the total work being done:
1628: 83 c1 01 add $0x1,%ecx
162b: 83 f9 08 cmp $0x8,%ecx
162e: 74 60 je 1690 <__find_get_block+0xa0>
1630: 89 c8 mov %ecx,%eax
1632: 65 4c 8b 04 c5 00 00 mov %gs:0x0(,%rax,8),%r8
1639: 00 00
163b: 4d 85 c0 test %r8,%r8
163e: 4c 89 c3 mov %r8,%rbx
1641: 74 e5 je 1628 <__find_get_block+0x38>
- 1643: 4d 3b 68 30 cmp 0x30(%r8),%r13
+ 1643: 4d 3b 68 18 cmp 0x18(%r8),%r13
1647: 75 df jne 1628 <__find_get_block+0x38>
- 1649: 4d 3b 60 18 cmp 0x18(%r8),%r12
+ 1649: 4d 3b 60 30 cmp 0x30(%r8),%r12
164d: 75 d9 jne 1628 <__find_get_block+0x38>
164f: 49 39 50 20 cmp %rdx,0x20(%r8)
1653: 75 d3 jne 1628 <__find_get_block+0x38>
Signed-off-by: Zach Brown <zab@zabbo.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/buffer.c | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/fs/buffer.c b/fs/buffer.c index b95b978b11ea..6c48f20eddd4 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -1333,8 +1333,8 @@ lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size) for (i = 0; i < BH_LRU_SIZE; i++) { struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]); - if (bh && bh->b_bdev == bdev && - bh->b_blocknr == block && bh->b_size == size) { + if (bh && bh->b_blocknr == block && bh->b_bdev == bdev && + bh->b_size == size) { if (i) { while (i) { __this_cpu_write(bh_lrus.bhs[i], |