Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759362AbXEIWHP (ORCPT ); Wed, 9 May 2007 18:07:15 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754012AbXEIWHE (ORCPT ); Wed, 9 May 2007 18:07:04 -0400 Received: from srv5.dvmed.net ([207.36.208.214]:49217 "EHLO mail.dvmed.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751648AbXEIWHC (ORCPT ); Wed, 9 May 2007 18:07:02 -0400 Message-ID: <46424602.3090404@garzik.org> Date: Wed, 09 May 2007 18:06:58 -0400 From: Jeff Garzik User-Agent: Thunderbird 1.5.0.10 (X11/20070302) MIME-Version: 1.0 To: Linux Kernel Mailing List CC: Andrew Morton , Linus Torvalds , suparna@in.ibm.com, Ingo Molnar Subject: Re: aio is unlikely References: <200705092101.l49L1CF1023363@hera.kernel.org> In-Reply-To: <200705092101.l49L1CF1023363@hera.kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Score: -4.3 (----) X-Spam-Report: SpamAssassin version 3.1.8 on srv5.dvmed.net summary: Content analysis details: (-4.3 points, 5.0 required) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2261 Lines: 57 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 > AuthorDate: Wed May 9 02:34:58 2007 -0700 > Committer: Linus Torvalds > 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 > Cc: Ingo Molnar > Cc: Benjamin LaHaise > Cc: Zach Brown > Cc: Ulrich Drepper > Cc: Christoph Hellwig > Signed-off-by: Andrew Morton > Signed-off-by: Linus Torvalds > --- > 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/