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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 2B8CEC5DF60 for ; Thu, 7 Nov 2019 15:38:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 066BB2178F for ; Thu, 7 Nov 2019 15:38:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388065AbfKGPiZ (ORCPT ); Thu, 7 Nov 2019 10:38:25 -0500 Received: from outgoing-auth-1.mit.edu ([18.9.28.11]:52145 "EHLO outgoing.mit.edu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726231AbfKGPiZ (ORCPT ); Thu, 7 Nov 2019 10:38:25 -0500 Received: from callcc.thunk.org (ip-12-2-52-196.nyc.us.northamericancoax.com [196.52.2.12]) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id xA7FcLB9003883 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 7 Nov 2019 10:38:22 -0500 Received: by callcc.thunk.org (Postfix, from userid 15806) id 3C15F420311; Thu, 7 Nov 2019 10:38:19 -0500 (EST) Date: Thu, 7 Nov 2019 10:38:19 -0500 From: "Theodore Y. Ts'o" To: Dmitry Monakhov Cc: linux-ext4@vger.kernel.org Subject: Re: [PATCH] ext4: fix extent_status fragmentation for plain files Message-ID: <20191107153819.GI26959@mit.edu> References: <20191106122502.19986-1-dmonakhov@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191106122502.19986-1-dmonakhov@gmail.com> User-Agent: Mutt/1.12.2 (2019-09-21) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Wed, Nov 06, 2019 at 12:25:02PM +0000, Dmitry Monakhov wrote: > It is appeared that extent are not cached for inodes with depth == 0 > which result in suboptimal extent status populating inside ext4_map_blocks() > by map's result where size requested is usually smaller than extent size so > cache becomes fragmented > > # Example: I have plain file: > File size of /mnt/test is 33554432 (8192 blocks of 4096 bytes) > ext: logical_offset: physical_offset: length: expected: flags: > 0: 0.. 8191: 40960.. 49151: 8192: last,eof > > $ perf record -e 'ext4:ext4_es_*' /root/bin/fio --name=t --direct=0 --rw=randread --bs=4k --filesize=32M --size=32M --filename=/mnt/test > $ perf script | grep ext4_es_insert_extent | head -n 10 > fio 131 [000] 13.975421: ext4:ext4_es_insert_extent: dev 253,0 ino 12 es [494/1) mapped 41454 status W > fio 131 [000] 13.976467: ext4:ext4_es_insert_extent: dev 253,0 ino 12 es [6907/1) mapped 47867 status W So this is certainly bad behavior, but the original intent was to not cached extents that were in the inode's i_blocks[] array because the information was already in the inode cache, and so we could save memory but just pulling the information out of the i_blocks away and there was no need to cache the extent in the es cache. There are cases where we do need to track the extent in the es cache --- for example, if we are writing the file and we need to track its delayed allocation status. So I wonder if we might be better off defining a new flag EXT4_MAP_INROOT, which gets set by ext4_ext_map_blocks() and ext4_ind_map_blocks() if the mapping is exclusively found in the i_blocks array, and if EXT4_MAP_INROOT is set, and we don't need to set EXTENT_STATUS_DELAYED, we skip the call to ext4_es_insert_extent(). What do you think? This should significantly reduce the memory utilization of the es_cache, which would be good for low-memory workloads, and those where there are a large number of inodes that fit in the es_cache, which is probably true for most desktops, especially those belonging kernel developers. :-) - Ted