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5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 7/8/25 4:17 AM, Markus Armbruster wrote: > Jonah Palmer writes: > >> On 7/4/25 11:00 AM, Markus Armbruster wrote: >>> Jonah Palmer writes: > > [...] > >>> So, total time increases: early pinning (before main loop) takes more >>> time than we save pinning (in the main loop). Correct? >> >> Correct. We only save ~0.07s from the pinning that happens in the main loop. But the extra 3s we now need to spend pinning before qemu_main_loop() overshadows it. > > Got it. > >>> We want this trade, because the time spent in the main loop is a >>> problem: guest-visible downtime. Correct? >>> [...] >> >> Correct. Though whether or not we want this trade I suppose is subjective. But the 50-60% reduction in guest-visible downtime is pretty nice if we can stomach the initial startup costs. > > I'll get back to this at the end. > > [...] > >>> Let me circle back to my question: Under what circumstances is QMP >>> responsiveness affected? >>> >>> The answer seems to be "only when we're using a vhost-vDPA device". >>> Correct? >> >> Correct, since using one of these guys causes us to do this memory pinning. If we're not using one, it's business as usual for Qemu. > > Got it. > >>> We're using one exactly when QEMU is running with one of its >>> vhost-vdpa-device-pci* device models. Correct? >> >> Yea, or something like: >> >> -netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa0,... \ >> -device virtio-net-pci,netdev=vhost-vdpa0,id=vdpa0,... \ > > I'll get back to this at the end. > > [...] > >>> Let me recap: >>> >>> * No change at all unless we're pinning memory early, and we're doing >>> that only when we're using a vhost-vDPA device. Correct? >>> >>> * If we are using a vhost-vDPA device: >>> - Total startup time (until we're done pinning) increases. >> >> Correct. >> >>> - QMP becomes available later. >> >> Correct. >> >>> - Main loop behavior improves: less guest-visible downtime, QMP more >>> responsive (once it's available) >> >> Correct. Though the improvement is modest at best if we put aside the guest-visible downtime improvement. >> >>> This is a tradeoff we want always. There is no need to let users pick >>> "faster startup, worse main loop behavior." >>> >> >> "Always" might be subjective here. For example, if there's no desire to perform live migration, then the user kinda just gets stuck with the cons. >> >> Whether or not we want to make this configurable though is another discussion. >> >>> Correct? >>> >>> [...] > > I think I finally know enough to give you constructive feedback. > > Your commit messages should answer the questions I had. Specifically: > > * Why are we doing this? To shorten guest-visible downtime. > > * How are we doing this? We additionally pin memory before entering the > main loop. This speeds up the pinning we still do in the main loop. > > * Drawback: slower startup. In particular, QMP becomes > available later. > > * Secondary benefit: main loop responsiveness improves, in particular > QMP. > > * What uses of QEMU are affected? Only with vhost-vDPA. Spell out all > the ways to get vhost-vDPA, please. > > * There's a tradeoff. Show your numbers. Discuss whether this needs to > be configurable. > > If you can make a case for pinning memory this way always, do so. If > you believe making it configurable would be a good idea, do so. If > you're not sure, say so in the cover letter, and add a suitable TODO > comment. > > Questions? > No questions, understood. As I was writing the responses to your questions I was thinking to myself that this stuff should've been in the cover letter / commit messages in the first place. Definitely a learning moment for me. Thanks for your time on this Markus! Jonah