Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262230AbVC2JWb (ORCPT ); Tue, 29 Mar 2005 04:22:31 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262256AbVC2JWa (ORCPT ); Tue, 29 Mar 2005 04:22:30 -0500 Received: from smtp200.mail.sc5.yahoo.com ([216.136.130.125]:41302 "HELO smtp200.mail.sc5.yahoo.com") by vger.kernel.org with SMTP id S262230AbVC2JVv (ORCPT ); Tue, 29 Mar 2005 04:21:51 -0500 Message-ID: <42491E2C.2070702@yahoo.com.au> Date: Tue, 29 Mar 2005 19:21:48 +1000 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.5) Gecko/20050105 Debian/1.7.5-1 X-Accept-Language: en MIME-Version: 1.0 To: Jens Axboe CC: "Chen, Kenneth W" , linux-kernel@vger.kernel.org Subject: Re: [patch] use cheaper elv_queue_empty when unplug a device References: <200503290253.j2T2rqg25691@unix-os.sc.intel.com> <20050329080646.GE16636@suse.de> <42491DBE.6020303@yahoo.com.au> In-Reply-To: <42491DBE.6020303@yahoo.com.au> Content-Type: multipart/mixed; boundary="------------080707080503080008020607" Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2133 Lines: 66 This is a multi-part message in MIME format. --------------080707080503080008020607 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Nick Piggin wrote: > I haven't used a big disk array (or tried any simulation), but I'll > attach the patch if you're looking into that area. > Oh, and this one removes a memory barrier. I think we (Jens and I) agreed this is valid. Whether or not you'll notice a difference is another story ;) --------------080707080503080008020607 Content-Type: text/plain; name="blk-no-mb.patch" Content-Transfer-Encoding: 7bit Content-Disposition: inline; filename="blk-no-mb.patch" This memory barrier is not needed because the waitqueue will only get waiters on it in the following situations: rq->count has exceeded the threshold - however all manipulations of ->count are performed under the runqueue lock, and so we will correctly pick up any waiter. Memory allocation for the request fails. In this case, there is no additional help provided by the memory barrier. We are guaranteed to eventually wake up waiters because the request allocation mempool guarantees that if the mem allocation for a request fails, there must be some requests in flight. They will wake up waiters when they are retired. --- linux-2.6-npiggin/drivers/block/ll_rw_blk.c | 1 - 1 files changed, 1 deletion(-) diff -puN drivers/block/ll_rw_blk.c~blk-no-mb drivers/block/ll_rw_blk.c --- linux-2.6/drivers/block/ll_rw_blk.c~blk-no-mb 2005-03-29 18:58:19.000000000 +1000 +++ linux-2.6-npiggin/drivers/block/ll_rw_blk.c 2005-03-29 19:20:10.000000000 +1000 @@ -1828,7 +1828,6 @@ static void __freed_request(request_queu clear_queue_congested(q, rw); if (rl->count[rw] + 1 <= q->nr_requests) { - smp_mb(); if (waitqueue_active(&rl->wait[rw])) wake_up(&rl->wait[rw]); _ --------------080707080503080008020607-- - 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/