From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764902AbYD2TOw (ORCPT ); Tue, 29 Apr 2008 15:14:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753810AbYD2TOk (ORCPT ); Tue, 29 Apr 2008 15:14:40 -0400 Received: from mx1.redhat.com ([66.187.233.31]:57670 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752608AbYD2TOj (ORCPT ); Tue, 29 Apr 2008 15:14:39 -0400 Date: Tue, 29 Apr 2008 15:12:21 -0400 (EDT) From: Mikulas Patocka To: Jens Axboe cc: linux-kernel@vger.kernel.org, Mike Anderson , Alasdair Graeme Kergon Subject: [PATCH] Optimize lock in queue unplugging Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Mike Anderson was doing an OLTP benchmark on a computer with 48 physical disks mapped to one logical device via device mapper. He found that there was a slowdown on request_queue->lock in function generic_unplug_device. The slowdown is caused by the fact that when some code calls unplug on the device mapper, device mapper calls unplug on all physical disks. These unplug calls take the lock, find that the queue is already unplugged, release the lock and exit. With the below patch, performance of the benchmark was increased by 18% (the whole OLTP application, not just block layer microbenchmarks). So I'm submitting this patch for upstream. I think the patch is correct, because when more threads call simultaneously plug and unplug, it is unspecified, if the queue is or isn't plugged (so the patch can't make this worse). And the caller that plugged the queue should unplug it anyway. (if it doesn't, there's 3ms timeout). Mikulas Signed-off-by: Mikulas Patocka --- block/blk-core.c | 3 +++ 1 file changed, 3 insertions(+) Index: linux-2.6.25/block/blk-core.c =================================================================== --- linux-2.6.25.orig/block/blk-core.c 2008-04-17 04:49:44.000000000 +0200 +++ linux-2.6.25/block/blk-core.c 2008-04-29 18:50:37.000000000 +0200 @@ -271,6 +271,9 @@ EXPORT_SYMBOL(__generic_unplug_device); **/ void generic_unplug_device(struct request_queue *q) { + if (likely(!blk_queue_plugged(q))) + return; + spin_lock_irq(q->queue_lock); __generic_unplug_device(q); spin_unlock_irq(q->queue_lock);