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Thu, 14 May 2020 03:08:04 -0700 (PDT) To: Dmitry Sychov Cc: Sergiy Yevtushenko , Mark Papadakis , "H. de Vries" , io-uring References: <7692E70C-A0EA-423B-883F-6BF91B0DB359@icloud.com> <2F012CBD-7DB6-4E88-BFFE-63427B0DD18D@icloud.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: Any performance gains from using per thread(thread local) urings? Message-ID: Date: Thu, 14 May 2020 13:06:53 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 13/05/2020 22:23, Dmitry Sychov wrote: >> E.g. 100+ cores hammering on a spinlock/mutex protecting an SQ wouldn't do any good. > > Its possible to mitigate the hammering by using proxy buffer - instead > of spinning, the particular thread > could add the next entry into the buffer through XADD instead, and > another thread currently holding an exclusive > lock could in turn check this buffer and batch-submit all pending > entries to SQ before leasing SQ mutex. Sure there are many ways, but I think my point is clear. FWIW, atomics/wait-free will fail to scale good enough after some point. >> will be offloaded to an internal thread pool (aka io-wq), which is per io_uring by default, but can be shared if specified. > > Well, thats sounds like mumbo jumbo to me, does this mean that the > kernel holds and internal pool of threads to > perform uring tasks independent to the number of user urings? If I parsed the question correctly, again, it creates a separate thread pool per each new io_uring, if wasn't specified otherwise. > > If there are multiple kernel work flows bound to corresponding uring > setups the issue with threads starvation could exist if they do not > actively steal from each other SQs. The threads can go to sleep or be dynamically created/destroyed. Not sure what kind of starvation you meant, but feel free to rephrase your questions if any of them weren't understood well. > And starvation costs could be greater than allowing for multiple > threads to dig into one uring queue, even under the exclusive lock. Thread pools can be shared. > >> And there a lot of details, probably worth of a separate write-up. > > I've reread io_uring.pdf and there are not much tech details on the > inner implementation of uring to try to apply best practices and to > avoid noob questions like mine. > > > > On Wed, May 13, 2020 at 7:03 PM Pavel Begunkov wrote: >> >> On 13/05/2020 17:22, Dmitry Sychov wrote: >>> Anyone could shed some light on the inner implementation of uring please? :) >> >> It really depends on the workload, hardware, etc. >> >> io_uring instances are intended to be independent, and each have one CQ and SQ. >> The main user's concern should be synchronisation (in userspace) on CQ+SQ. E.g. >> 100+ cores hammering on a spinlock/mutex protecting an SQ wouldn't do any good. >> >> Everything that can't be inline completed\submitted during io_urng_enter(), will >> be offloaded to an internal thread pool (aka io-wq), which is per io_uring by >> default, but can be shared if specified. There are pros and cons, but I'd >> recommend first to share a single io-wq, and then experiment and tune. >> >> Also, in-kernel submission is not instantaneous and done by only thread at any >> moment. Single io_uring may bottleneck you there or add high latency in some cases. >> >> And there a lot of details, probably worth of a separate write-up. >> >>> >>> Specifically how well kernel scales with the increased number of user >>> created urings? >> >> Should scale well, especially for rw. Just don't overthrow the kernel with >> threads from dozens of io-wqs. >> >>> >>>> If kernel implementation will change from single to multiple queues, >>>> user space is already prepared for this change. >>> >>> Thats +1 for per-thread urings. An expectation for the kernel to >>> become better and better in multiple urings scaling in the future. >>> >>> On Wed, May 13, 2020 at 4:52 PM Sergiy Yevtushenko >>> wrote: >>>> >>>> Completely agree. Sharing state should be avoided as much as possible. >>>> Returning to original question: I believe that uring-per-thread scheme is better regardless from how queue is managed inside the kernel. >>>> - If there is only one queue inside the kernel, then it's more efficient to perform multiplexing/demultiplexing requests in kernel space >>>> - If there are several queues inside the kernel, then user space code better matches kernel-space code. >>>> - If kernel implementation will change from single to multiple queues, user space is already prepared for this change. >>>> >>>> >>>> On Wed, May 13, 2020 at 3:30 PM Mark Papadakis wrote: >>>>> >>>>> >>>>> >>>>>> On 13 May 2020, at 4:15 PM, Dmitry Sychov wrote: >>>>>> >>>>>> Hey Mark, >>>>>> >>>>>> Or we could share one SQ and one CQ between multiple threads(bound by >>>>>> the max number of CPU cores) for direct read/write access using very >>>>>> light mutex to sync. >>>>>> >>>>>> This also solves threads starvation issue - thread A submits the job >>>>>> into shared SQ while thread B both collects and _processes_ the result >>>>>> from the shared CQ instead of waiting on his own unique CQ for next >>>>>> completion event. >>>>>> >>>>> >>>>> >>>>> Well, if the SQ submitted by A and its matching CQ is consumed by B, and A will need access to that CQ because it is tightly coupled to state it owns exclusively(for example), or other reasons, then you’d still need to move that CQ from B back to A, or share it somehow, which seems expensive-is. >>>>> >>>>> It depends on what kind of roles your threads have though; I am personally very much against sharing state between threads unless there a really good reason for it. >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>>>> On Wed, May 13, 2020 at 2:56 PM Mark Papadakis >>>>>> wrote: >>>>>>> >>>>>>> For what it’s worth, I am (also) using using multiple “reactor” (i.e event driven) cores, each associated with one OS thread, and each reactor core manages its own io_uring context/queues. >>>>>>> >>>>>>> Even if scheduling all SQEs through a single io_uring SQ — by e.g collecting all such SQEs in every OS thread and then somehow “moving” them to the one OS thread that manages the SQ so that it can enqueue them all -- is very cheap, you ‘d still need to drain the CQ from that thread and presumably process those CQEs in a single OS thread, which will definitely be more work than having each reactor/OS thread dequeue CQEs for SQEs that itself submitted. >>>>>>> You could have a single OS thread just for I/O and all other threads could do something else but you’d presumably need to serialize access/share state between them and the one OS thread for I/O which maybe a scalability bottleneck. >>>>>>> >>>>>>> ( if you are curious, you can read about it here https://medium.com/@markpapadakis/building-high-performance-services-in-2020-e2dea272f6f6 ) >>>>>>> >>>>>>> If you experiment with the various possible designs though, I’d love it if you were to share your findings. >>>>>>> >>>>>>> — >>>>>>> @markpapapdakis >>>>>>> >>>>>>> >>>>>>>> On 13 May 2020, at 2:01 PM, Dmitry Sychov wrote: >>>>>>>> >>>>>>>> Hi Hielke, >>>>>>>> >>>>>>>>> If you want max performance, what you generally will see in non-blocking servers is one event loop per core/thread. >>>>>>>>> This means one ring per core/thread. Of course there is no simple answer to this. >>>>>>>>> See how thread-based servers work vs non-blocking servers. E.g. Apache vs Nginx or Tomcat vs Netty. >>>>>>>> >>>>>>>> I think a lot depends on the internal uring implementation. To what >>>>>>>> degree the kernel is able to handle multiple urings independently, >>>>>>>> without much congestion points(like updates of the same memory >>>>>>>> locations from multiple threads), thus taking advantage of one ring >>>>>>>> per CPU core. >>>>>>>> >>>>>>>> For example, if the tasks from multiple rings are later combined into >>>>>>>> single input kernel queue (effectively forming a congestion point) I >>>>>>>> see >>>>>>>> no reason to use exclusive ring per core in user space. >>>>>>>> >>>>>>>> [BTW in Windows IOCP is always one input+output queue for all(active) threads]. >>>>>>>> >>>>>>>> Also we could pop out multiple completion events from a single CQ at >>>>>>>> once to spread the handling to cores-bound threads . >>>>>>>> >>>>>>>> I thought about one uring per core at first, but now I'am not sure - >>>>>>>> maybe the kernel devs have something to add to the discussion? >>>>>>>> >>>>>>>> P.S. uring is the main reason I'am switching from windows to linux dev >>>>>>>> for client-sever app so I want to extract the max performance possible >>>>>>>> out of this new exciting uring stuff. :) >>>>>>>> >>>>>>>> Thanks, Dmitry >>>>>>> >>>>> >> >> -- >> Pavel Begunkov -- Pavel Begunkov