From: Sergiy Yevtushenko <firstname.lastname@example.org> To: Mark Papadakis <email@example.com> Cc: Dmitry Sychov <firstname.lastname@example.org>, "H. de Vries" <email@example.com>, io-uring <firstname.lastname@example.org> Subject: Re: Any performance gains from using per thread(thread local) urings? Date: Wed, 13 May 2020 16:12:08 +0200 [thread overview] Message-ID: <CAO5MNusGPbxXw77g4Yf0hSGj2WZepZgOANWf3KunZhR8H06apw@mail.gmail.com> (raw) In-Reply-To: <2F012CBD-7DB6-4E88-BFFE-63427B0DD18D@icloud.com> 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 <email@example.com> wrote: > > > > > On 13 May 2020, at 4:15 PM, Dmitry Sychov <firstname.lastname@example.org> 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 > > <email@example.com> 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 <firstname.lastname@example.org> 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 > >> >
next prev parent reply other threads:[~2020-05-13 14:12 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-05-12 20:20 Dmitry Sychov 2020-05-13 6:07 ` H. de Vries 2020-05-13 11:01 ` Dmitry Sychov 2020-05-13 11:56 ` Mark Papadakis 2020-05-13 13:15 ` Dmitry Sychov 2020-05-13 13:27 ` Mark Papadakis 2020-05-13 13:48 ` Dmitry Sychov 2020-05-13 14:12 ` Sergiy Yevtushenko [this message] [not found] ` <CAO5MNut+nD-OqsKgae=eibWYuPim1f8-NuwqVpD87eZQnrwscA@mail.gmail.com> 2020-05-13 14:22 ` Dmitry Sychov 2020-05-13 14:31 ` Dmitry Sychov 2020-05-13 16:02 ` Pavel Begunkov 2020-05-13 19:23 ` Dmitry Sychov 2020-05-14 10:06 ` Pavel Begunkov 2020-05-14 11:35 ` Dmitry Sychov
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