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=-5.3 required=3.0 tests=BAYES_00, 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 61889C2D0A3 for ; Mon, 9 Nov 2020 17:46:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0E6B920665 for ; Mon, 9 Nov 2020 17:46:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732352AbgKIRqf (ORCPT ); Mon, 9 Nov 2020 12:46:35 -0500 Received: from mx2.suse.de ([195.135.220.15]:38598 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730092AbgKIRqb (ORCPT ); Mon, 9 Nov 2020 12:46:31 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 8279EAFD5; Mon, 9 Nov 2020 17:46:30 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id 9F692DA7D7; Mon, 9 Nov 2020 18:44:49 +0100 (CET) Date: Mon, 9 Nov 2020 18:44:49 +0100 From: David Sterba To: Qu Wenruo Cc: linux-btrfs@vger.kernel.org Subject: Re: [PATCH 27/32] btrfs: scrub: use flexible array for scrub_page::csums Message-ID: <20201109174449.GZ6756@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Qu Wenruo , linux-btrfs@vger.kernel.org References: <20201103133108.148112-1-wqu@suse.com> <20201103133108.148112-28-wqu@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201103133108.148112-28-wqu@suse.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Tue, Nov 03, 2020 at 09:31:03PM +0800, Qu Wenruo wrote: > There are several factors affecting how many checksum bytes are needed > for one scrub_page: > > - Sector size and page size > For subpage case, one page can contain several sectors, thus the csum > size will differ. > > - Checksum size > Since btrfs supports different csum size now, which can vary from 4 > bytes for CRC32 to 32 bytes for SHA256. > > So instead of using fixed BTRFS_CSUM_SIZE, now use flexible array for > scrub_page::csums, and determine the size at scrub_page allocation time. > > This does not only provide the basis for later subpage scrub support, I'd like to know more how this would help for the subpage support. > but also reduce the memory usage for default btrfs on x86_64. > As the default CRC32 only uses 4 bytes, thus we can save 28 bytes for > each scrub page. Because even with the flexible array, the allocation is from the generic slabs and scrub_page is now 128 bytes, so saving 28 bytes won't make any difference.