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Fri, 17 Jan 2025 11:56:57 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id w3LPGolFimd+NwAAD6G6ig (envelope-from ); Fri, 17 Jan 2025 11:56:57 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id F24D9A08E0; Fri, 17 Jan 2025 12:56:52 +0100 (CET) Date: Fri, 17 Jan 2025 12:56:52 +0100 From: Jan Kara To: Vlastimil Babka Cc: Joanne Koong , lsf-pc@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, "Matthew Wilcox (Oracle)" Subject: Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Improving large folio writeback performance Message-ID: References: <73eb82d2-1a43-4e88-a5e3-6083a04318c1@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <73eb82d2-1a43-4e88-a5e3-6083a04318c1@suse.cz> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 278F91C000A X-Stat-Signature: t5f9cp9t4eg1rs6h51854ufkm3m3t98h X-Rspam-User: X-HE-Tag: 1737115018-80446 X-HE-Meta: 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 aOuSJcUd HJ0pQ2AFZIeiJ+M5SBpIskiaCnQF/V3HEGmaAPBkTWm6eBGr7bxIzC2FPPY/WZiPjGCDhDmPxEhBFGybnrEC93SboKTss5t3qYIfnOb2AeWP7RwBOzWFTaHDaKf1wa35OgzIgVY3amxV9/VDlBQdaTOyvnz4TX9KqnTgV241GD2TVQwVkTuOTjwbu9TtVdDHkTMU1kIz5RhVZo0lefwAi68hNNfJZpJmPIyLmNx/zAgRReiUlcBtkfsm6go+xf7OxSLmSuFoJBds5slEnv300/LSy8iG9mIF0GRCmR9HDox2snVaHlivp4cwLAx6YD1KLD8IRyLyJiJciZ0DDB7vP1ncDPw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000021, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri 17-01-25 12:40:15, Vlastimil Babka wrote: > On 1/15/25 01:50, Joanne Koong wrote: > > Hi all, > > > > I would like to propose a discussion topic about improving large folio > > writeback performance. As more filesystems adopt large folios, it > > becomes increasingly important that writeback is made to be as > > performant as possible. There are two areas I'd like to discuss: > > > > > > == Granularity of dirty pages writeback == > > Currently, the granularity of writeback is at the folio level. If one > > byte in a folio is dirty, the entire folio will be written back. This > > becomes unscalable for larger folios and significantly degrades > > performance, especially for workloads that employ random writes. > > > > One idea is to track dirty pages at a smaller granularity using a > > 64-bit bitmap stored inside the folio struct where each bit tracks a > > smaller chunk of pages (eg for 2 MB folios, each bit would track 32k > > pages), and only write back dirty chunks rather than the entire folio. > > I think this might be tricky in some cases? I.e. with 2 MB and pmd-mapped > folio, it's possible to write-protect only the whole pmd, not individual 32k > chunks in order to catch the first write to a chunk to mark it dirty. Definitely. Once you map a folio through PMD entry, you have no other option than consider whole 2MB dirty. But with PTE mappings or modifications through syscalls you can do more fine-grained dirtiness tracking and there're enough cases like that that it pays off. Honza -- Jan Kara SUSE Labs, CR