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=-5.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,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 E4994C433E0 for ; Thu, 9 Jul 2020 01:46:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9B04C206F6 for ; Thu, 9 Jul 2020 01:46:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=synology.com header.i=@synology.com header.b="FDdQThSH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726118AbgGIBqm (ORCPT ); Wed, 8 Jul 2020 21:46:42 -0400 Received: from mail.synology.com ([211.23.38.101]:32986 "EHLO synology.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726107AbgGIBqm (ORCPT ); Wed, 8 Jul 2020 21:46:42 -0400 Subject: Re: [PATCH v2] btrfs: speedup mount time with readahead chunk tree DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=synology.com; s=123; t=1594259200; bh=7ZwIZmK1gHLTOR9xWZC00X7aT+8VS9mveyjksyXjpB4=; h=Subject:To:References:From:Date:In-Reply-To; b=FDdQThSH6QDmK3hhfnzghyyII9kKDRCxLFvDDCvPM6HHD3fY0Ssc/h+9MU3koIMYB +v1XgSFePy4Rv2XeKNm5VNPeYoreVUczZEOkpixJDn0ZZV0cemD+Ohvti3QpL4QlOu 48wEdsdhPzA+lBtOPcNmUpzQl7/CCxzq+S5xJ3dY= To: =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= , dsterba@suse.cz, 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: Robbie Ko Message-ID: Date: Thu, 9 Jul 2020 09:46:40 +0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Synology-MCP-Status: no X-Synology-Spam-Flag: no X-Synology-Spam-Status: score=0, required 6, WHITELIST_FROM_ADDRESS 0 X-Synology-Virus-Status: no Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org Holger Hoffstätte 於 2020/7/8 下午10:57 寫道: > 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.. > I have the following two questions for you. 1. What is the version you are using? 2. Can you please measure the time of btrfs_read_chunk_tree alone? I think the problem you are having is that btrfs_read_block_groups is slowing down because it is using the wrong READA flag, which is causing a lot of useless IO's when reading the block group. This can be fixed with the following commit. btrfs: block-group: don't set the wrong READA flag for btrfs_read_block_groups() https://git.kernel.org/pub/scm/linux/kernel /git/torvalds/linux.git/commit/?h=v5.8-rc4& id=83fe9e12b0558eae519351cff00da1e06bc054d2 > -h