Re: aio is unlikely

[Jeff Garzik]

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@xxxxxxxx>
> AuthorDate: Wed May 9 02:34:58 2007 -0700
> Committer:  Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxx>
>     Cc: Ingo Molnar <mingo@xxxxxxx>
>     Cc: Benjamin LaHaise <bcrl@xxxxxxxxx>
>     Cc: Zach Brown <zach.brown@xxxxxxxxxx>
>     Cc: Ulrich Drepper <drepper@xxxxxxxxxx>
>     Cc: Christoph Hellwig <hch@xxxxxx>
>     Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>     Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> ---
>  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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