> 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 >>
