It's expensive to set buffer flags that are already set, because that
causes a costly cache line transition.
A common case is setting the "verified" flag during ext4 writes.
This patch checks for the flag being set first.
With the AIM7/creat-clo benchmark testing on a 48G ramdisk based-on ext4
file system, we see 3.3%(15431->15936) improvement of aim7.jobs-per-min on
a 2-sockets broadwell platform.
What the benchmark does is: it forks 3000 processes, and each process do
the following:
a) open a new file
b) close the file
c) delete the file
until loop=100*1000 times.
The original patch is contributed by Andi Kleen.
Signed-off-by: Andi Kleen <[email protected]>
Signed-off-by: Kemi Wang <[email protected]>
Tested-by: Kemi Wang <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
---
include/linux/buffer_head.h | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index c8dae55..211d8f5 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -80,11 +80,14 @@ struct buffer_head {
/*
* macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
* and buffer_foo() functions.
+ * To avoid reset buffer flags that are already set, because that causes
+ * a costly cache line transition, check the flag first.
*/
#define BUFFER_FNS(bit, name) \
static __always_inline void set_buffer_##name(struct buffer_head *bh) \
{ \
- set_bit(BH_##bit, &(bh)->b_state); \
+ if (!test_bit(BH_##bit, &(bh)->b_state)) \
+ set_bit(BH_##bit, &(bh)->b_state); \
} \
static __always_inline void clear_buffer_##name(struct buffer_head *bh) \
{ \
--
2.7.4
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