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SubjectRe: aio is unlikely
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 <akpm@osdl.org>
> AuthorDate: Wed May 9 02:34:58 2007 -0700
> Committer: Linus Torvalds <torvalds@woody.linux-foundation.org>
> 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 <suparna@in.ibm.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Benjamin LaHaise <bcrl@kvack.org>
> Cc: Zach Brown <zach.brown@oracle.com>
> Cc: Ulrich Drepper <drepper@redhat.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> ---
> 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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