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Szmigiero" Cc: Alex Williamson , Eric Blake , Peter Xu , Fabiano Rosas , Markus Armbruster , =?UTF-8?Q?Daniel_P_=2E_Berrang=C3=A9?= , Joao Martins , qemu-devel@nongnu.org References: <2b2469939198c2f31dba33b284576d2df22697b7.1741344976.git.maciej.szmigiero@oracle.com> <3826d47f-d79b-4db2-9719-35f48f582bf0@maciej.szmigiero.name> <538afa76-7456-44b9-abac-c238dbb0e64c@nvidia.com> <4c5ae74a-3b4f-4b60-a93a-7e9dbaf974a2@redhat.com> Content-Language: en-US From: Avihai Horon In-Reply-To: <4c5ae74a-3b4f-4b60-a93a-7e9dbaf974a2@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: LO2P265CA0426.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:a0::30) To DM6PR12MB5549.namprd12.prod.outlook.com (2603:10b6:5:209::13) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR12MB5549:EE_|SA3PR12MB9091:EE_ X-MS-Office365-Filtering-Correlation-Id: 88d6fa2f-9269-461a-a3fa-08dd6139c33d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|1800799024|366016; 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envelope-from=avihaih@nvidia.com; helo=NAM10-DM6-obe.outbound.protection.outlook.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, 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: 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 11/03/2025 18:05, Cédric Le Goater wrote: > External email: Use caution opening links or attachments > > > On 3/11/25 17:01, Avihai Horon wrote: >> >> On 11/03/2025 17:45, Cédric Le Goater wrote: >>> External email: Use caution opening links or attachments >>> >>> >>> On 3/11/25 15:57, Avihai Horon wrote: >>>> >>>> On 11/03/2025 15:04, Cédric Le Goater wrote: >>>>> External email: Use caution opening links or attachments >>>>> >>>>> >>>>> On 3/7/25 14:45, Maciej S. Szmigiero wrote: >>>>>> On 7.03.2025 13:03, Cédric Le Goater wrote: >>>>>>> On 3/7/25 11:57, Maciej S. Szmigiero wrote: >>>>>>>> From: "Maciej S. Szmigiero" >>>>>>>> >>>>>>>> There's already a max in-flight VFIO device state buffers >>>>>>>> *count* limit, >>>>>>> >>>>>>> no. there isn't. Do we need both ? >>>>>> >>>>>> This is on a top of the remaining patches >>>>>> (x-migration-load-config-after-iter >>>>>> and x-migration-max-queued-buffers) - I thought we were supposed >>>>>> to work >>>>>> on these after the main series was merged as they are relatively >>>>>> non-critical. >>>>> >>>>> yes. we don't need both count and size limits though, a size limit >>>>> is enough. >>>>> >>>>>> I would also give x-migration-load-config-after-iter priority over >>>>>> x-migration-max-queued-buffers{,-size} as the former is >>>>>> correctness fix >>>>>> while the later are just additional functionalities. >>>>> >>>>> ok. I have kept both patches in my tree with the doc updates. >>>>> >>>>>> Also, if some setup is truly worried about these buffers >>>>>> consuming too much >>>>>> memory then roughly the same thing could be achieved by >>>>>> (temporarily) putting >>>>>> the target QEMU process in a memory-limited cgroup. >>>>> >>>>> yes. >>>>> >>>>> That said, >>>>> >>>>> since QEMU exchanges 1MB VFIODeviceStatePackets when using multifd >>>>> and that >>>>> the overall device state is in the order of 100MB : >>>>> >>>>>   /* >>>>>    * This is an arbitrary size based on migration of mlx5 devices, >>>>> where typically >>>>>    * total device migration size is on the order of 100s of MB. >>>>> Testing with >>>>>    * larger values, e.g. 128MB and 1GB, did not show a performance >>>>> improvement. >>>>>    */ >>>>>   #define VFIO_MIG_DEFAULT_DATA_BUFFER_SIZE (1 * MiB) >>>>> >>>>> >>>>> Could we define the limit to 1GB ? >>>>> >>>>> Avihai, would that make sense  ? >>>>> >>>> There can be many use cases, each one with its own requirements and >>>> constraints, so it's hard for me to think of a "good" default value. >>>> >>>> IIUC this limit is mostly relevant for the extreme cases where >>>> devices have big state + writing the buffers to the device is slow. >>>> So IMHO let's set it to unlimited by default and let the users >>>> decide if they want to set such limit and to what value. (Note also >>>> that even when unlimited, it is really limited to 2 * >>>> device_state_size). >>>> >>>> Unless you have other reasons why 1GB or other value is preferable? >>> >>> none but UINT_MAX is not good value either. >> >> You mean UINT_MAX is not a good value to represent "unlimited" or >> that unlimited is not a good default value? > > unlimited is not a good default value. Why not? It basically means "disabled".