Messages in this thread | | | Date | Wed, 09 May 2007 18:06:58 -0400 | From | Jeff Garzik <> | Subject | Re: aio is unlikely |
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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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