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
