Commit 60f91826 authored Oct 24, 2017 by Kemi Wang Committed by Jens Axboe Feb 02, 2018

buffer: Avoid setting buffer bits that are already set



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 <ak@linux.intel.com>
Tested-by: Kemi Wang <kemi.wang@intel.com>
Signed-off-by: Kemi Wang <kemi.wang@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent bea99a50

include/linux/buffer_head.h

+4 −1

Original line number	Diff line number	Diff line
		@@ -81,10 +81,13 @@ 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) \
		{ \
		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) \

Admin message