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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9161AC00528 for ; Tue, 1 Aug 2023 23:18:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229499AbjHAXSB (ORCPT ); Tue, 1 Aug 2023 19:18:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55350 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230261AbjHAXSA (ORCPT ); Tue, 1 Aug 2023 19:18:00 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 26BAB26AA for ; Tue, 1 Aug 2023 16:17:52 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 919746175E for ; Tue, 1 Aug 2023 23:17:51 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id ED2BFC433C8; Tue, 1 Aug 2023 23:17:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1690931871; bh=65Tp/9tJaUD3Fib6QC9NRUutewiMxa5DvWC56qPg+80=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=njms4SmUEH7HCKq24F+ry/NYlKM1jhAaE/AcCRywJYKcC7bcxz3rMBHlQj8Bg6oZS 5Bu17jLigCOoGN0Yfx951mTve+ozk5cdlcmyErMHP2UjqcoEVOxDGGh1xvABxA3qQm OrxPTDJ8I9Cihp7uPT4ECD9u7oISF37FARyKr8kd2/JeVNHk/aR+zILV2NJ2wxkeii Fm0+3BBn2XQgXNfEL2JGR4wFgsFL4aQEfH6hWqXSGE2rcQ78ZWjzAvqpQrg4D/syQU kiw77FQapqXmWtHgWowP5+qizYznXJr2HsarqH7lnHqFW1JzzA+cMHGcPOmRXl2Ftb Ge0C1XQLn/gyA== Date: Tue, 1 Aug 2023 16:17:50 -0700 From: "Darrick J. Wong" To: Omar Sandoval Cc: linux-xfs@vger.kernel.org, kernel-team@fb.com, Prashant Nema Subject: Re: [PATCH 2/6] xfs: invert the realtime summary cache Message-ID: <20230801231750.GO11352@frogsfrogsfrogs> References: <20230712224001.GV108251@frogsfrogsfrogs> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Mon, Jul 17, 2023 at 12:54:24PM -0700, Omar Sandoval wrote: > On Wed, Jul 12, 2023 at 03:40:01PM -0700, Darrick J. Wong wrote: > > On Tue, Jun 20, 2023 at 02:32:12PM -0700, Omar Sandoval wrote: > > > From: Omar Sandoval > > > > > > In commit 355e3532132b ("xfs: cache minimum realtime summary level"), I > > > added a cache of the minimum level of the realtime summary that has any > > > free extents. However, it turns out that the _maximum_ level is more > > > useful for upcoming optimizations, and basically equivalent for the > > > existing usage. So, let's change the meaning of the cache to be the > > > maximum level + 1, or 0 if there are no free extents. > > > > Hmm. If I'm reading xfs_rtmodify_summary_int right, m_rsum_cache[b] now > > tells us the maximum log2(length) of the free extents starting in > > rtbitmap block b? > > > > IOWs, let's say the cache contents are: > > > > {2, 3, 2, 15, 8} > > > > Someone asks for a 400rtx (realtime extent) allocation, so we want to > > find a free space of at least magnitude floor(log2(400)) == 8. > > > > The cache tells us that there aren't any free extents longer than 2^1 > > blocks in rtbitmap blocks 0 and 2; longer than 2^2 blocks in rtbmp block > > 1; longer than 2^7 blocks in rtbmp block 4; nor longer than 2^14 blocks > > in rtbmp block 3? > > There's a potential for an off-by-one bug here, so just to make sure > we're saying the same thing: the realtime summary for level n contains > the number of free extents starting in a bitmap block such that > floor(log2(size_in_realtime_extents)) == n. The maximum size of a free > extent in level n is therefore 2^(n + 1) - 1 realtime extents. > > So in your example, the cache is telling us that realtime bitmap blocks > 0 and 2 don't have anything free in levels 2 or above, and therefore > don't have any free extents longer than _or equal to_ 2^2. D'oh. I forgot that subtlety that the maximum size of a free extent in level n is therefore 2^(n + 1) - 1 realtime extents. > I'll try to reword the commit message and comments to make this > unambiguous. Ok, thanks. A couple of quick examples (feel free to use mine) would be helpful for descrambling my brain. :) --D