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Tue, 18 Aug 2026 06:57:17 -0700 (PDT) Received: from [10.3.188.167] ([61.213.176.8]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-cc13c03320esm2152832a12.25.2026.08.18.06.57.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Aug 2026 06:57:16 -0700 (PDT) Message-ID: <4a0b20e5-947c-45a4-8fcf-89c77e0a4cfb@gmail.com> Date: Tue, 18 Aug 2026 21:57:12 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 02/10] migration/rdma: Remove unregister code To: Peter Xu Cc: qemu-devel@nongnu.org, Li Zhijian , Samuel Zhang , Fabiano Rosas , Jack Wang , Juraj Marcin , Yanfei Xu References: <20260817202424.2901438-1-peterx@redhat.com> <20260817202424.2901438-3-peterx@redhat.com> <4d0d08d3-ad6e-4de3-85b8-587b70286b04@gmail.com> From: Yanfei Xu In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::535; envelope-from=isyanfei.xu@gmail.com; helo=mail-pg1-x535.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, 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: qemu development 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 2026/8/18 21:01, Peter Xu wrote: > On Tue, Aug 18, 2026 at 07:57:06PM +0800, Yanfei Xu wrote: >> Hi Peter, > Hi, Yanfei, > >> No objection to removing the dead code — it clearly never worked >> >> I do have one question about the direction, though. The removed logic >> was the only in-tree attempt at MR unregistration for the non-pin-all >> path. Without it, registered MRs grow monotonically over a migration, >> and with large, widely-spread dirty memory over chunks the accumulated >> MR metadata (user + kernel) can cost more than pin-all and even perform >> worse — which rather defeats the purpose of not pinning everything. >> >> do we still intend to keep and improve the non-pin-all path going >> forward? If so, some form of dynamic MR unregistration will eventually >> be needed and it might be worth keeping this code，or at least leaving >> a TODO to mark the gap? > Thanks for taking a look. This is a valid question to ask. > > Though it was there for 13 years without being "enhanced", it means the > possibility we leverage it in the next couple of years is low. > > You also discussed the other side of things: I am not a frequent RDMA user, > but my understanding is frequent MR reg operations already slow down > migration quite a bit. It means dynamic management including unregisters > will be even worse. AFAICT, it'll be a challenging task if we want to keep > the performance in bar and add a hard throttle to pinned memory. One advantage of non-pin-all is that it neither sends the all-zero chunk nor registers the corresponding MRs. For guests with a low dirty-page workload and a large number of zero pages, this lets it migrate faster than pin-all and pin less guest memory during the migration. > > Obviously, RDMA migration users care a bunch on performance. > > So I see no good reason to not drop it. Even if someone will work out that > problem, it's still not much code to add, and it'll likely be easier we > design it from scratch with the demand, and without worry of breaking > anyone. Got it, thanks for the explanation. Regards, Yanfei > > When that happens (if it ever will..), someone should also send an update > to MAINTAINER file on RDMA migration to change it out of Odd Fixes stage.. > > Thanks, >