Linux Kernel Mailing List wrote:
Gitweb: http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b8522ead3534c6cd06752b47a3bc380956191a2a
Commit: b8522ead3534c6cd06752b47a3bc380956191a2a
Parent: b41eeef14d7c73af6d16c7d02b7a939082a137ff
Author: Andrew Morton <[email protected]>
AuthorDate: Wed May 9 02:34:58 2007 -0700
Committer: Linus Torvalds <[email protected]>
CommitDate: Wed May 9 12:30:54 2007 -0700
aio is unlikely
Stick an unlikely() around is_aio(): I assert that most IO is synchronous.
Cc: Suparna Bhattacharya <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Cc: Zach Brown <[email protected]>
Cc: Ulrich Drepper <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
---
include/linux/aio.h | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/include/linux/aio.h b/include/linux/aio.h
index a30ef13..43dc2eb 100644
--- a/include/linux/aio.h
+++ b/include/linux/aio.h
@@ -226,7 +226,8 @@ int FASTCALL(io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
__put_ioctx(kioctx); \
} while (0)
-#define in_aio() !is_sync_wait(current->io_wait)
+#define in_aio() (unlikely(!is_sync_wait(current->io_wait)))
Please revert. Workload-dependent "likelihood" should not cause
programmers to add such markers.
This is a common misunderstanding about unlikely() and likely(). The
branch prediction used for each assumes 99% unlikely or 99% likely,
which is not true at all for workload-dependent code.
Even if only 1% of Linux users use AIO, for that 1%, the 'unlikely'
marker causes repeated branch mispredictions.
likely() and unlikely() should be used for cases where code is
likely/unlikely for EVERYBODY.
Jeff
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