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Fri, 20 Feb 2026 07:44:02 +0000 Message-ID: <29a7ef25-27a8-49cd-a2f2-8db693ada39c@suse.de> Date: Fri, 20 Feb 2026 08:44:02 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Memory fragmentation with large block sizes To: Theodore Tso Cc: lsf-pc , "linux-nvme@lists.infradead.org" , "linux-block@vger.kernel.org" , linux-mm@kvack.org References: <20260219143242.GC69183@macsyma-wired.lan> Content-Language: en-US From: Hannes Reinecke In-Reply-To: <20260219143242.GC69183@macsyma-wired.lan> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; TO_DN_EQ_ADDR_SOME(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_DN_SOME(0.00)[]; FROM_HAS_DN(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; RCPT_COUNT_FIVE(0.00)[5]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; DKIM_TRACE(0.00)[suse.de:+]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:mid,suse.de:dkim,suse.de:email,imap1.dmz-prg2.suse.org:helo,imap1.dmz-prg2.suse.org:rdns] X-Rspamd-Action: no action X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Queue-Id: 3DC085BCC2 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260219_234406_742346_F4D4547D X-CRM114-Status: GOOD ( 22.52 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On 2/19/26 15:32, Theodore Tso wrote: > 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? > The main goal was to figure out if we have increased memory fragmentation when using LBS. Clearly, most (internal) allocations still work on page-sized objects, so one can argue that using LBS might increase fragmentation. On the other hand, all _filesystem_ objects will be in LBS sizes, so we won't increase fragmentation if we only allocate in LBS sizes. So which is it? > 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? > Yes. >> - 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. > That would be ideal, but we first need to have a program exerting memory pressure... Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@suse.de +49 911 74053 688 SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich