Commit 163da958 authored Feb 12, 2007 by Eric Dumazet Committed by Linus Torvalds Feb 12, 2007

[PATCH] FS: speed up rw_verify_area()



oprofile hunting showed a stall in rw_verify_area(), because of triple
indirection and potential cache misses.
(file->f_path.dentry->d_inode->i_flock)

By moving initialization of 'struct inode' pointer before the pos/count
sanity tests, we allow the compiler and processor to perform two loads by
anticipation, reducing stall, without prefetch() hints.  Even x86 arch has
enough registers to not use temporary variables and not increase text size.

I validated this patch running a bench and studied oprofile changes, and
absolute perf of the test program.

Results of my epoll_pipe_bench (source available on request) on a Pentium-M
1.6 GHz machine

Before :
# ./epoll_pipe_bench -l 30 -t 20
Avg: 436089 evts/sec read_count=8843037 write_count=8843040 21.218390 samples
per call
(best value out of 10 runs)

After :
# ./epoll_pipe_bench -l 30 -t 20
Avg: 470980 evts/sec read_count=9549871 write_count=9549894 21.216694 samples
per call
(best value out of 10 runs)

oprofile CPU_CLK_UNHALTED events gave a reduction from 5.3401 % to 2.5851 %
for the rw_verify_area() function.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 2a10387e

fs/read_write.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -197,13 +197,13 @@ int rw_verify_area(int read_write, struct file file, loff_t ppos, size_t count
		struct inode *inode;
		loff_t pos;

		inode = file->f_path.dentry->d_inode;
		if (unlikely((ssize_t) count < 0))
		goto Einval;
		pos = *ppos;
		if (unlikely((pos < 0) \|\| (loff_t) (pos + count) < 0))
		goto Einval;

		inode = file->f_path.dentry->d_inode;
		if (unlikely(inode->i_flock && MANDATORY_LOCK(inode))) {
		int retval = locks_mandatory_area(
		read_write == READ ? FLOCK_VERIFY_READ : FLOCK_VERIFY_WRITE,

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