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Miller" , Eric Dumazet , Jakub Kicinski , Jan Kara , Jiri Pirko , Johannes Berg , Jonathan Corbet , "open list:DOCUMENTATION" , "open list:FILESYSTEMS (VFS and infrastructure)" , open list , Lorenzo Bianconi , Paolo Abeni , Sebastian Andrzej Siewior References: <20240812125717.413108-1-jdamato@fastly.com> <2bb121dd-3dcd-4142-ab87-02ccf4afd469@uwaterloo.ca> <5e52b556-fe49-4fe0-8bd3-543b3afd89fa@uwaterloo.ca> <6f40b6df-4452-48f6-b552-0eceaa1f0bbc@uwaterloo.ca> Content-Language: en-CA, de-DE From: Martin Karsten In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: YT4PR01CA0030.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:fe::7) To YQBPR0101MB6572.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:4b::13) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YQBPR0101MB6572:EE_|YQXPR01MB6480:EE_ X-MS-Office365-Filtering-Correlation-Id: ddf72fdf-ba55-4660-9c83-08dcbca19cf8 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|7416014|366016; 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If we can mark some napi >>>>> contexts as "explicitly polled by userspace with a larger defer timeout", >>>>> we should be able to do better compared to current NAPI_F_PREFER_BUSY_POLL >>>>> which is more like "this particular napi_poll call is user busy polling". >>>> >>>> Then either the application needs to be polling all the time (wasting cpu >>>> cycles) or latencies will be determined by the timeout. > But if I understand correctly, this means that if the application > thread that is supposed > to do napi busy polling gets busy doing work on the new data/events in > userspace, napi polling > will not be done until the suspend_timeout triggers? Do you dispatch > work to a separate worker > threads, in userspace, from the thread that is doing epoll_wait? Yes, napi polling is suspended while the application is busy between epoll_wait calls. That's where the benefits are coming from. The consequences depend on the nature of the application and overall preferences for the system. If there's a "dominant" application for a number of queues and cores, the resulting latency for other background applications using the same queues might not be a problem at all. One other simple mitigation is limiting the number of events that each epoll_wait call accepts. Note that this batch size also determines the worst-case latency for the application in question, so there is a natural incentive to keep it limited. A more complex application design, like you suggest, might also be an option. >>>> Only when switching back and forth between polling and interrupts is it >>>> possible to get low latencies across a large spectrum of offered loads >>>> without burning cpu cycles at 100%. >>> >>> Ah, I see what you're saying, yes, you're right. In this case ignore my comment >>> about ep_suspend_napi_irqs/napi_resume_irqs. >> >> Thanks for probing and double-checking everything! Feedback is important >> for us to properly document our proposal. >> >>> Let's see how other people feel about per-dev irq_suspend_timeout. Properly >>> disabling napi during busy polling is super useful, but it would still >>> be nice to plumb irq_suspend_timeout via epoll context or have it set on >>> a per-napi basis imho. > I agree, this would allow each napi queue to tune itself based on > heuristics. But I think > doing it through epoll independent interface makes more sense as Stan > suggested earlier. The question is whether to add a useful mechanism (one sysfs parameter and a few lines of code) that is optional, but with demonstrable and significant performance/efficiency improvements for an important class of applications - or wait for an uncertain future? Note that adding our mechanism in no way precludes switching the control parameters from per-device to per-napi as Joe alluded to earlier. In fact, it increases the incentive for doing so. After working on this for quite a while, I am skeptical that anything fundamentally different could be done without re-architecting the entire napi control flow. Thanks, Martin