Yep, I want for all states to be uncoupled from threads - its more about moving unique state from one thread(core) to another for processing, only SQ+CQ are shared between cores-bound threads. > I am personally very much against sharing state between threads unless there a really good reason for it. Yeah, I understand, but for max performance we should start to think about states as independent from threads entities or whats the reason to use uring for max performance at first place - we could as well stuck to very poor Apache model(unbound number of threads with coupled states). On Wed, May 13, 2020 at 4:27 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 > >> >
