Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752316Ab2KXVKg (ORCPT ); Sat, 24 Nov 2012 16:10:36 -0500 Received: from mx1.redhat.com ([209.132.183.28]:36747 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752249Ab2KXVKe (ORCPT ); Sat, 24 Nov 2012 16:10:34 -0500 Date: Sat, 24 Nov 2012 16:09:37 -0500 (EST) From: Mikulas Patocka X-X-Sender: mpatocka@file.rdu.redhat.com To: Jeff Chua cc: Jens Axboe , Lai Jiangshan , Linus Torvalds , Jan Kara , lkml , linux-fsdevel Subject: Re: Recent kernel "mount" slow In-Reply-To: Message-ID: References: <20121120180949.GG1408@quack.suse.cz> <50AF7901.20401@kernel.dk> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2268 Lines: 60 On Sat, 24 Nov 2012, Jeff Chua wrote: > On Fri, Nov 23, 2012 at 9:24 PM, Jens Axboe wrote: > > On 2012-11-22 20:21, Linus Torvalds wrote: > >> Doesn't sound like a fsdevel issue since it seems to be independent of > >> filesystems. More like some generic block layer thing. Adding Jens > >> (and quoting the whole thing) > >> > >> Jens, any ideas? Most of your stuff came in after -rc2, which would > >> fit with the fact that most of the slowdown seems to be after -rc2 > >> according to Jeff. > > > > No ideas. Looking at what went in from my side, only the rq plug sorting > > is a core change, and that should not cause any change in behaviour for > > a single device. That's commit 975927b9. > > > >> Jeff, more bisecting would be good, though. > > > > Probably required, yes... > > > This one slows mount from 0.012s to 0.168s. > > commit 62ac665ff9fc07497ca524bd20d6a96893d11071 > Author: Mikulas Patocka > Date: Wed Sep 26 07:46:43 2012 +0200 > > blockdev: turn a rw semaphore into a percpu rw semaphore > > > There were couple of more changes to percpu-rw-semaphores after > 3.7.0-rc2 and those slows mount further from 0.168s to 0.500s. I don't > really know, but I'm suspecting these. Still bisecting. The problem there is that you either use normal semaphores and slow down I/O or you use percpu-semaphores, you don't slow down I/O, but you slow down mount. So it's better to slow down mount. (if you don't use any semaphore at all, as it was in 3.6 kernel and before, there is a race condition that can crash the kernel if someone does mount and direct I/O read on the same device at the same time) You can improve mount time if you change all occurences of synchronize_sched() in include/linux/percpu-rwsem.h to synchronize_sched_expedited(). But some people say that synchronize_sched_expedited() is bad for real time latency. (can there be something like: if (realtime) synchronize_sched(); else synchronize_sched_expedited(); ?) Mikulas -- 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/