From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E48F2C4321A for ; Thu, 27 Jun 2019 22:24:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C74F82063F for ; Thu, 27 Jun 2019 22:24:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726511AbfF0WYe (ORCPT ); Thu, 27 Jun 2019 18:24:34 -0400 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:38727 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726445AbfF0WYe (ORCPT ); Thu, 27 Jun 2019 18:24:34 -0400 Received: from dread.disaster.area (pa49-195-139-63.pa.nsw.optusnet.com.au [49.195.139.63]) by mail104.syd.optusnet.com.au (Postfix) with ESMTPS id 9603243C42B; Fri, 28 Jun 2019 08:24:31 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92) (envelope-from ) id 1hgcnc-0001na-29; Fri, 28 Jun 2019 08:23:24 +1000 Date: Fri, 28 Jun 2019 08:23:24 +1000 From: Dave Chinner To: Christoph Hellwig Cc: "Darrick J. Wong" , Damien Le Moal , Andreas Gruenbacher , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 07/12] xfs: don't preallocate a transaction for file size updates Message-ID: <20190627222324.GH7777@dread.disaster.area> References: <20190624055253.31183-1-hch@lst.de> <20190624055253.31183-8-hch@lst.de> <20190624161720.GQ5387@magnolia> <20190624231523.GC7777@dread.disaster.area> <20190625102507.GA1986@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190625102507.GA1986@lst.de> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.2 cv=P6RKvmIu c=1 sm=1 tr=0 cx=a_idp_d a=fNT+DnnR6FjB+3sUuX8HHA==:117 a=fNT+DnnR6FjB+3sUuX8HHA==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=kj9zAlcOel0A:10 a=dq6fvYVFJ5YA:10 a=7-415B0cAAAA:8 a=ae071pWMrU5jLcjm0hQA:9 a=wSn981L3eJwRu9Eu:21 a=hda4UQlomxu2gwLn:21 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Jun 25, 2019 at 12:25:07PM +0200, Christoph Hellwig wrote: > On Tue, Jun 25, 2019 at 09:15:23AM +1000, Dave Chinner wrote: > > > So, uh, how much of a hit do we take for having to allocate a > > > transaction for a file size extension? Particularly since we can > > > combine those things now? > > > > Unless we are out of log space, the transaction allocation and free > > should be largely uncontended and so it's just a small amount of CPU > > usage. i.e it's a slab allocation/free and then lockless space > > reservation/free. If we are out of log space, then we sleep waiting > > for space - the issue really comes down to where it is better to > > sleep in that case.... > > I see the general point, but we'll still have the same issue with > unwritten extent conversion and cow completions, and I don't remember > seeing any issue in that regard. These are realtively rare for small file workloads - I'm really talking about the effect of delalloc and how we've optimised allocation during writeback to merge small, cross-file writeback into much larger large physical IOs. Unwritten extents nor COW are used in these (common) cases, and if they are then the allocation patterns prevent the cross-file IO merging in the block layer and so we don't get the "hundred ioends for a hundred inodes from a single a physical IO completion" thundering heard problem.... > And we'd hit exactly that case > with random writes to preallocated or COW files, i.e. the typical image > file workload. I do see a noticable amount of IO completion overhead in the host when hitting unwritten extents in VM image workloads. I'll see if I can track the number of kworkers we're stalling in under some of these workloads, but I think it's still largely bound by the request queue depth of the IO stack inside the VM because there is no IO merging in these cases. Cheers, Dave. -- Dave Chinner david@fromorbit.com