Anyone could shed some light on the inner implementation of uring please? :) Specifically how well kernel scales with the increased number of user created urings? > 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 <sergiy.yevtushenko@xxxxxxxxx> 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 <markuspapadakis@xxxxxxxxxx> wrote: >> >> >> >> > On 13 May 2020, at 4:15 PM, Dmitry Sychov <dmitry.sychov@xxxxxxxxx> 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 >> > <markuspapadakis@xxxxxxxxxx> 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 <dmitry.sychov@xxxxxxxxx> 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 >> >> >>
