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=-0.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 55036C433E0 for ; Wed, 8 Jul 2020 14:57:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 36E5620720 for ; Wed, 8 Jul 2020 14:57:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729922AbgGHO56 (ORCPT ); Wed, 8 Jul 2020 10:57:58 -0400 Received: from mail.itouring.de ([188.40.134.68]:57354 "EHLO mail.itouring.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729206AbgGHO56 (ORCPT ); Wed, 8 Jul 2020 10:57:58 -0400 Received: from tux.applied-asynchrony.com (p5ddd79e0.dip0.t-ipconnect.de [93.221.121.224]) by mail.itouring.de (Postfix) with ESMTPSA id B6ECE4160341; Wed, 8 Jul 2020 16:57:56 +0200 (CEST) Received: from [192.168.100.223] (ragnarok.applied-asynchrony.com [192.168.100.223]) by tux.applied-asynchrony.com (Postfix) with ESMTP id 63009F01600; Wed, 8 Jul 2020 16:57:56 +0200 (CEST) Subject: Re: [PATCH v2] btrfs: speedup mount time with readahead chunk tree To: dsterba@suse.cz, Robbie Ko , linux-btrfs@vger.kernel.org References: <20200707035944.15150-1-robbieko@synology.com> <20200707192511.GE16141@twin.jikos.cz> <3b3f9eb4-96ef-d039-5d86-a4c165e6d993@synology.com> <20200708140455.GA28832@twin.jikos.cz> From: =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= Organization: Applied Asynchrony, Inc. Message-ID: Date: Wed, 8 Jul 2020 16:57:56 +0200 MIME-Version: 1.0 In-Reply-To: <20200708140455.GA28832@twin.jikos.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 2020-07-08 16:04, David Sterba wrote: > On Wed, Jul 08, 2020 at 10:19:22AM +0800, Robbie Ko wrote: >> David Sterba 於 2020/7/8 上午3:25 寫道: >>> On Tue, Jul 07, 2020 at 11:59:44AM +0800, robbieko wrote: >>>> From: Robbie Ko >>>> >>>> When mounting, we always need to read the whole chunk tree, >>>> when there are too many chunk items, most of the time is >>>> spent on btrfs_read_chunk_tree, because we only read one >>>> leaf at a time. >>>> >>>> It is unreasonable to limit the readahead mechanism to a >>>> range of 64k, so we have removed that limit. >>>> >>>> In addition we added reada_maximum_size to customize the >>>> size of the pre-reader, The default is 64k to maintain the >>>> original behavior. >>>> >>>> So we fix this by used readahead mechanism, and set readahead >>>> max size to ULLONG_MAX which reads all the leaves after the >>>> key in the node when reading a level 1 node. >>> The readahead of chunk tree is a special case as we know we will need >>> the whole tree, in all other cases the search readahead needs is >>> supposed to read only one leaf. >> >> If, in most cases, readahead requires that only one leaf be read, then >> reada_ maximum_size should be nodesize instead of 64k, or use >> reada_maximum_ nr (default:1) seems better. >> >>> >>> For that reason I don't want to touch the current path readahead logic >>> at all and do the chunk tree readahead in one go instead of the >>> per-search. >> >> I don't know why we don't make the change to readahead, because the current >> readahead is limited to the logical address in 64k is very unreasonable, >> and there is a good chance that the logical address of the next leaf >> node will >> not appear in 64k, so the existing readahead is almost useless. > > I see and it seems that the assumption about layout and chances > succesfuly read blocks ahead is not valid. The logic of readahead could > be improved but that would need more performance evaluation. FWIW I gave this a try and see the following numbers, averaged over multiple mount/unmount cycles on spinning rust: without patch : ~2.7s with patch : ~4.5s ..ahem.. -h