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Mon, 30 Mar 2026 09:10:16 -0700 (PDT) X-Received: by 2002:a05:620a:17a4:b0:8ca:105a:298f with SMTP id af79cd13be357-8d01c7fc3b5mr1844375085a.68.1774887016229; Mon, 30 Mar 2026 09:10:16 -0700 (PDT) Received: from x1.local ([142.189.10.167]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8d02e72666fsm539538185a.12.2026.03.30.09.10.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 30 Mar 2026 09:10:15 -0700 (PDT) Date: Mon, 30 Mar 2026 12:10:13 -0400 From: Peter Xu To: Samuel Zhang Cc: qemu-devel@nongnu.org, farosas@suse.de, lizhijian@fujitsu.com, eblake@redhat.com, armbru@redhat.com, Emily.Deng@amd.com, Victor.Zhao@amd.com, PengJu.Zhou@amd.com, Qing.Ma@amd.com Subject: Re: [PATCH v3] migration/rdma: add x-rdma-chunk-size parameter Message-ID: References: <20260327065006.2567463-1-guoqing.zhang@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260327065006.2567463-1-guoqing.zhang@amd.com> Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -6 X-Spam_score: -0.7 X-Spam_bar: / X-Spam_report: (-0.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.54, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.01, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=1, RCVD_IN_VALIDITY_RPBL_BLOCKED=1, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no 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 Hi, Samuel, On Fri, Mar 27, 2026 at 02:50:06PM +0800, Samuel Zhang wrote: > The default 1MB RDMA chunk size causes slow live migration because > each chunk triggers a write_flush (ibv_post_send). For 8GB RAM, > 1MB chunk size produces ~15000 flushes vs ~3700 with 1024MB chunk size. > > Add x-rdma-chunk-size parameter to configure the RDMA chunk size for > faster migration. > Usage: `migrate_set_parameter x-rdma-chunk-size 1024M` > > Performance with RDMA live migration of 8GB RAM VM: > > | x-rdma-chunk-size (B) | time (s) | throughput (MB/s) | > |-----------------------|----------|-------------------| > | 1M (default) | 37.915 | 1,007 | This is the default. It surprised me a bit knowing it can only reach 1GB/s throughput with the current code base. Do you know why? I thought RDMA should be much faster than this on throughput with whatever hardware setup. > | 32M | 17.880 | 2,260 | > | 1024M | 4.368 | 17,529 | > > Signed-off-by: Samuel Zhang One thing to mention is RDMA migration is in odd-fixes stage, actually it doesn't have a real maintainer so it is kind of "orphaned". In this case, I actually won't suggest we add any new knobs for performance reasons. Do you have a strong reason to propose this patch to land upstream? Is it used in production systems and it solves some real problems for you? I also wonder what Zhijian would say on this. Thanks, -- Peter Xu