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,UNPARSEABLE_RELAY,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 C8388C3A5A2 for ; Fri, 23 Aug 2019 13:09:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A47C021848 for ; Fri, 23 Aug 2019 13:09:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389118AbfHWNJ0 (ORCPT ); Fri, 23 Aug 2019 09:09:26 -0400 Received: from out30-45.freemail.mail.aliyun.com ([115.124.30.45]:54469 "EHLO out30-45.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727894AbfHWNJ0 (ORCPT ); Fri, 23 Aug 2019 09:09:26 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R131e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04426;MF=joseph.qi@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0TaDOlSd_1566565734; Received: from JosephdeMacBook-Pro.local(mailfrom:joseph.qi@linux.alibaba.com fp:SMTPD_---0TaDOlSd_1566565734) by smtp.aliyun-inc.com(127.0.0.1); Fri, 23 Aug 2019 21:08:54 +0800 Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO reads" To: Dave Chinner Cc: "Theodore Y. Ts'o" , Jan Kara , Joseph Qi , Andreas Dilger , Ext4 Developers List , Xiaoguang Wang , Liu Bo References: <20190728225122.GG7777@dread.disaster.area> <960bb915-20cc-26a0-7abc-bfca01aa39c0@gmail.com> <20190815151336.GO14313@quack2.suse.cz> <075fd06f-b0b4-4122-81c6-e49200d5bd17@linux.alibaba.com> <20190816145719.GA3041@quack2.suse.cz> <20190820160805.GB10232@mit.edu> <20190822054001.GT7777@dread.disaster.area> <20190823101623.GV7777@dread.disaster.area> From: Joseph Qi Message-ID: <707b1a60-00f0-847e-02f9-e63d20eab47e@linux.alibaba.com> Date: Fri, 23 Aug 2019 21:08:53 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190823101623.GV7777@dread.disaster.area> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On 19/8/23 18:16, Dave Chinner wrote: > On Fri, Aug 23, 2019 at 03:57:02PM +0800, Joseph Qi wrote: >> Hi Dave, >> >> On 19/8/22 13:40, Dave Chinner wrote: >>> On Wed, Aug 21, 2019 at 09:04:57AM +0800, Joseph Qi wrote: >>>> Hi Ted， >>>> >>>> On 19/8/21 00:08, Theodore Y. Ts'o wrote: >>>>> On Tue, Aug 20, 2019 at 11:00:39AM +0800, Joseph Qi wrote: >>>>>> >>>>>> I've tested parallel dio reads with dioread_nolock, it doesn't have >>>>>> significant performance improvement and still poor compared with reverting >>>>>> parallel dio reads. IMO, this is because with parallel dio reads, it take >>>>>> inode shared lock at the very beginning in ext4_direct_IO_read(). >>>>> >>>>> Why is that a problem? It's a shared lock, so parallel threads should >>>>> be able to issue reads without getting serialized? >>>>> >>>> The above just tells the result that even mounting with dioread_nolock, >>>> parallel dio reads still has poor performance than before (w/o parallel >>>> dio reads). >>>> >>>>> Are you using sufficiently fast storage devices that you're worried >>>>> about cache line bouncing of the shared lock? Or do you have some >>>>> other concern, such as some other thread taking an exclusive lock? >>>>> >>>> The test case is random read/write described in my first mail. And >>> >>> Regardless of dioread_nolock, ext4_direct_IO_read() is taking >>> inode_lock_shared() across the direct IO call. And writes in ext4 >>> _always_ take the inode_lock() in ext4_file_write_iter(), even >>> though it gets dropped quite early when overwrite && dioread_nolock >>> is set. But just taking the lock exclusively in write fro a short >>> while is enough to kill all shared locking concurrency... >>> >>>> from my preliminary investigation, shared lock consumes more in such >>>> scenario. >>> >>> If the write lock is also shared, then there should not be a >>> scalability issue. The shared dio locking is only half-done in ext4, >>> so perhaps comparing your workload against XFS would be an >>> informative exercise... >> >> I've done the same test workload on xfs, it behaves the same as ext4 >> after reverting parallel dio reads and mounting with dioread_lock. > > Ok, so the problem is not shared locking scalability ('cause that's > what XFS does and it scaled fine), the problem is almost certainly > that ext4 is using exclusive locking during writes... > Agree. Maybe I've misled you in my previous mails.I meant shared lock makes worse in case of mixed random read/write, since we would always take inode lock during write. And it also conflicts with dioread_nolock. It won't take any inode lock before with dioread_nolock during read, but now it always takes a shared lock. Thanks, Joseph