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 > <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 >>>>> >>> -- Pavel Begunkov
