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=-8.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,URIBL_BLOCKED autolearn=ham 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 6F01FC48BE5 for ; Wed, 16 Jun 2021 00:18:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4B82F611CE for ; Wed, 16 Jun 2021 00:18:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231143AbhFPAUE (ORCPT ); Tue, 15 Jun 2021 20:20:04 -0400 Received: from bhuna.collabora.co.uk ([46.235.227.227]:40328 "EHLO bhuna.collabora.co.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229811AbhFPAUE (ORCPT ); Tue, 15 Jun 2021 20:20:04 -0400 Received: from [127.0.0.1] (localhost [127.0.0.1]) (Authenticated sender: krisman) with ESMTPSA id 497F91F43305 From: Gabriel Krisman Bertazi To: "Theodore Ts'o" Cc: kernel@collabora.com, linux-ext4@vger.kernel.org Subject: Re: Potential regression with iomap DIO for 4k writes Organization: Collabora References: <87lf7rkffv.fsf@collabora.com> Date: Tue, 15 Jun 2021 20:17:55 -0400 In-Reply-To: <87lf7rkffv.fsf@collabora.com> (Gabriel Krisman Bertazi's message of "Wed, 02 Jun 2021 19:35:48 -0400") Message-ID: <87zgvq7jd8.fsf@collabora.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org Gabriel Krisman Bertazi writes: > Hi, > > While I've been exploring the performance of different DIO > implementations, I've come across what seems a noticeable regression > (22% slowdown) in 4k writes in Ext4 when comparing the original > direct-io with the current iomap dio implementation, as existing on > linus/master. Perhaps you already know about this, but I'm having a > hard time understanding the root cause, in order to attempt to improve > the situation. Hi Ted, Sorry for the ping, but do you have any ideas of what we are seeing here? Thank you, > * Benchmark > > For starter, I'm comparing three kernels, built with same config and > compiler (gcc-8.4.0 (locally built)). My DUT is a Samsung SSD 970 EVO > Plus 250GB dedicated to this test (no concurrent IO). > > - Kernel 1: Commit immediately before iomap for ext4 is merged > ("f112a2fd1f59"). On the data below, this kernel is identified as > 5.4.0-original-dio. Available in a public branch at: > > > > - Kernel 2: tag 5.5 (first release with dio-iomap). In the data > below, identified as 5.5.0-old-iomap. For completeness, it is > available at: > > > > - Kernel 3: Kernel tag 5.13-rc3. In the data below, identified as > 5.13-rc3-iomap. For completeness, it is available at: > > > > I ran the fio job below with the combinations: BS=4k,16k and RW=read,write > > fio --ioengine libaio --size=2G --direct=1 --iodepth=64 --time_based=1 \ > --thread=1 --overwrite=1 --runtime=100 --output-format=terse > > For every kernel test, the file system was recreated, and the 2GB file > was pre-allocated. > > In an attempt to further isolate the problem, I tested both xfs and ext4 > in the same condition. > > The script I used is available at: > > > > * Results > > I obtained the following performance results, relative to the baseline > 5.4.0-original-dio. > > | IOPS | > | kernel | read-4k | read-16k | write-4k | write-16k | > |------------------------+------------------+-------------------+-------------------+-------------------+ > | 5.13.0-rc3-iomap-ext4 | 1.01192950082305 | 1.00026413252562 | 0.806377013901006 | 1.00020735846057 | > | 5.5.0-old-iomap-ext4 | 1.01154156662508 | 0.998753983520427 | 0.777051125458035 | 0.999937792461829 | > | 5.13.0-rc3-iomap-xfs | 1.00234888443008 | 1.00027645151444 | 1.00996172750095 | 1.00156349447934 | > | 5.5.0-old-iomap-xfs | 1.00010412786902 | 1.00202731110586 | 1.01502846821264 | 1.00149431330769 | > > > Total IO is the amount of data copied (relative to baseline). > > | TOTAL_IO > | kernel | read-4k | read-16k | write-4k | write-16k | > |------------------------+------------------+-------------------+-------------------+-------------------| > | 5.13.0-rc3-iomap-ext4 | 1.01193023173156 | 1.00026332569559 | 0.806377530301477 | 1.00014686835205 | > | 5.5.0-old-iomap-ext4 | 1.01154196621591 | 0.998758131673757 | 0.777050753425118 | 0.999902824986834 | > | 5.13.0-rc3-iomap-xfs | 1.00234893734134 | 1.00027535318322 | 1.00996437458991 | 1.00156305646789 | > | 5.5.0-old-iomap-xfs | 1.00010328564078 | 1.00202831801018 | 1.01503060595258 | 1.00149069402364 | > > With a visualization of the above data here: > > > > The only out of the ordinary result seems to be in write-4k for Ext4, > which suggests around 20% less IOPS (and total IO) for iomap in > comparison to the original DIO. This is not a one-off run, as it seems > to be consistently reproducible with more test runs in my environment. > The performance reduction also doesn't reproduce on XFS. > > I tried to limit the influence of other parts of the kernel that could > affect the behavior by comparing the kernel immediately before the > introduction of dio-iomap for ext4 with the first version with that > feature. By also observing that xfs doesn't change, I believe it to be > ext4 specific. > > I'm also publishing raw data and all related material to the link below, > in case anyone wants to tinker with my data: > > https://people.collabora.com/~krisman/dio/ > > Perhaps I'm missing something obvious. But I can't pinpoint a specific > problem with my analysis. Is this expected, given the way ext4 iomap > work? Do you have any idea of the root cause or how it can be improved? > > I will keep looking to this issue, but I'd like to share this partial > result, in case there is a problem with my analysis, or if you have any > suggestion. > > Thanks, -- Gabriel Krisman Bertazi