The patch titled
Subject: fs: check bh blocknr earlier when searching lru
has been added to the -mm tree. Its filename is
fs-check-bh-blocknr-earlier-when-searching-lru.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/fs-check-bh-blocknr-earlier-when-searching-lru.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/fs-check-bh-blocknr-earlier-when-searching-lru.patch
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------------------------------------------------------
From: Zach Brown <zab@xxxxxxxxx>
Subject: 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@xxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/buffer.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff -puN fs/buffer.c~fs-check-bh-blocknr-earlier-when-searching-lru fs/buffer.c
--- a/fs/buffer.c~fs-check-bh-blocknr-earlier-when-searching-lru
+++ a/fs/buffer.c
@@ -1331,8 +1331,8 @@ lookup_bh_lru(struct block_device *bdev,
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],
_
Patches currently in -mm which might be from zab@xxxxxxxxx are
fs-check-bh-blocknr-earlier-when-searching-lru.patch
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