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 <asml.silence@xxxxxxxxx> 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 >>> <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 -- Pavel Begunkov
