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 96AB5C3A59D for ; Thu, 22 Aug 2019 05:41:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 713B7233A0 for ; Thu, 22 Aug 2019 05:41:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728049AbfHVFl2 (ORCPT ); Thu, 22 Aug 2019 01:41:28 -0400 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:43873 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726857AbfHVFl1 (ORCPT ); Thu, 22 Aug 2019 01:41:27 -0400 Received: from dread.disaster.area (pa49-195-190-67.pa.nsw.optusnet.com.au [49.195.190.67]) by mail104.syd.optusnet.com.au (Postfix) with ESMTPS id 876F143C237; Thu, 22 Aug 2019 15:41:24 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92) (envelope-from ) id 1i0fpJ-0006Ll-VU; Thu, 22 Aug 2019 15:40:01 +1000 Date: Thu, 22 Aug 2019 15:40:01 +1000 From: Dave Chinner To: Joseph Qi Cc: "Theodore Y. Ts'o" , Jan Kara , Joseph Qi , Andreas Dilger , Ext4 Developers List , Xiaoguang Wang , Liu Bo Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO reads" Message-ID: <20190822054001.GT7777@dread.disaster.area> References: <6DADA28C-542F-45F6-ADB0-870A81ABED23@dilger.ca> <15112e38-94fe-39d6-a8e2-064ff47187d5@linux.alibaba.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 a=TR82T6zjGmBjdfWdGgpkDw==:117 a=TR82T6zjGmBjdfWdGgpkDw==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=IkcTkHD0fZMA:10 a=FmdZ9Uzk2mMA:10 a=7-415B0cAAAA:8 a=rqTOBM8hfhvl6ENb3C8A:9 a=QEXdDO2ut3YA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org 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... Cheers, Dave. -- Dave Chinner david@fromorbit.com