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h=Date:From:Subject:Message-ID:MIME-Version:Content-Type; b=DEZWVjL0xyfCBeyRW70yV9z+bT8dLBgE3Cr2B2R1FFbqVgTk3+cS79TVtarhMc2s9 y5zx36b7cGwjDDPIU1AWw8XobSbXIgAzVzvztzBkdZqcjFJNDQPiWezYsxlg+1EFp+ NAQuCoBvWIlevFLqB2dNFE6pFxImmljumqGA8fl4UQfqVke3xPXcQf6GsSGCmPKXmt V4E5dQmrjeAAtwvTwwEDK9wAdOUwO6AYyEr05M2XZ20V4BpFkVWhICt9JfAkvDB29L FNzhu9oZiB2WhB/N+ewidJlIJbLrRavHs4WbMGzOC0EMfOLieSFtmPS+RU1fyX5p0t 0rIcr3WomJl+Q== Received: by macsyma.thunk.org (Postfix, from userid 15806) id A4E16591BA5C; Thu, 19 Feb 2026 09:32:42 -0500 (EST) Date: Thu, 19 Feb 2026 09:32:42 -0500 From: "Theodore Tso" To: Hannes Reinecke Cc: lsf-pc , "linux-nvme@lists.infradead.org" , "linux-block@vger.kernel.org" , linux-mm@kvack.org Subject: Re: [LSF/MM/BPF TOPIC] Memory fragmentation with large block sizes Message-ID: <20260219143242.GC69183@macsyma-wired.lan> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam09 X-Stat-Signature: h83ykhfcnhcyoj5gkfmg43ugu9p479nz X-Rspamd-Queue-Id: 667A280007 X-Rspam-User: X-HE-Tag: 1771511566-736468 X-HE-Meta: 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 Z2SS2Zpf QPY2ASevZ9TnNSnWwV8iT2bQdRMO7MYSiNqbDP04KznsJLmy4q9c/U/vpnKEM/G2YuRMqMvU5JQEpw15nwKjGVpsBGqir9k3y5qEQLHa0LNjEVYfb125wR9q7xacSDVzljHvMvuOCL5i6LfeVlw+OurdZzPU6gnkl+4jQ6D2Fa/Kni28TZNacrS7ZYNQ/XqN4pkGhQ0hoTJxFL06ta46Ikbn0Ww== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 Thu, Feb 19, 2026 at 10:54:48AM +0100, Hannes Reinecke wrote: > Hi all, > > I (together with the Czech Technical University) did some experiments trying > to measure memory fragmentation with large block sizes. > Testbed used was an nvme setup talking to a nvmet storage over > the network. > > Doing so raised some challenges: > > - How do you _generate_ memory fragmentation? The MM subsystem is > precisely geared up to avoid it, so you would need to come up > with some idea how to defeat it. With the help from Willy I managed > to come up with something, but I really would like to discuss > what would be the best option here. I'm trying to understand the goal of the experiment. I'm guessing that the goal was to see how much memory fragmentation would result from using large block sizes with the control being to use, say, 4k blocks. Is that correct? So I guess the question here is what are realstic workloads that people would have in real world situations, so we can do the A-B experiments to see what using LBS result in? > - What is acceptable memory fragmentation? Are we good enough if the > measured fragmentation does not grow during the test runs? I can think of two possible metrics. The first is whether it results in degradation of performance given certain real world workloads. The second is whether given a particular memory pressure, the memory fragmentation results in more jobs getting OOM killed. - Ted