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powerpc: 64bit optimised __clear_user

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I noticed __clear_user high up in a profile of one of my RAID stress
tests. The testcase was doing a dd from /dev/zero which ends up
calling __clear_user.

__clear_user is basically a loop with a single 4 byte store which
is horribly slow. We can do much better by aligning the desination
and doing 32 bytes of 8 byte stores in a loop.

The following testcase was used to verify the patch:

http://ozlabs.org/~anton/junkcode/stress_clear_user.c

To show the improvement in performance I ran a dd from /dev/zero
to /dev/null on a POWER7 box:

Before:

# dd if=/dev/zero of=/dev/null bs=1M count=10000
10485760000 bytes (10 GB) copied, 3.72379 s, 2.8 GB/s

After:

# time dd if=/dev/zero of=/dev/null bs=1M count=10000
10485760000 bytes (10 GB) copied, 0.728318 s, 14.4 GB/s

Over 5x faster.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
This commit is contained in:
Anton Blanchard
2012-05-27 19:54:03 +00:00
committed by Benjamin Herrenschmidt
parent d136e27326
commit 17968fbbd1
3 changed files with 144 additions and 1 deletions
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2
arch/powerpc/lib/string.S
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@@ -119,6 +119,7 @@ _GLOBAL(memchr)
2: li r3,0
blr
#ifdef CONFIG_PPC32
_GLOBAL(__clear_user)
addi r6,r3,-4
li r3,0
@@ -160,3 +161,4 @@ _GLOBAL(__clear_user)
PPC_LONG 1b,91b
PPC_LONG 8b,92b
.text
#endif