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Mon, 27 Apr 2026 00:17:30 -0700 (PDT) X-Received: by 2002:a05:6214:33c3:b0:8ac:b4d5:50db with SMTP id 6a1803df08f44-8b02818ec3fmr627664656d6.28.1777274249811; Mon, 27 Apr 2026 00:17:29 -0700 (PDT) Received: from redhat.com ([2a02:8012:f011:0:f1ce:ac56:d2cd:7c2e]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8b02ae97d89sm256466316d6.42.2026.04.27.00.17.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Apr 2026 00:17:29 -0700 (PDT) Date: Mon, 27 Apr 2026 08:17:25 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Samuel Zhang Cc: qemu-devel@nongnu.org, peterx@redhat.com, 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 v6] migration/rdma: add x-rdma-chunk-size parameter Message-ID: References: <20260427031401.3895523-1-guoqing.zhang@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260427031401.3895523-1-guoqing.zhang@amd.com> User-Agent: Mutt/2.3.1 (2026-03-20) Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.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, DKIMWL_WL_HIGH=-0.001, 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_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_PASS=-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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Apr 27, 2026 at 11:14:01AM +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 | > | 32M | 17.880 | 2,260 | > | 1024M | 4.368 | 17,529 | What is the downside of setting a larger chunk size ? IOW, why should we keep 1M as the default when it gives such terrible relative performance ? Why not make 1G be the default instead of creating this flag and requiring people to know about setting it ?