Re: Any performance gains from using per thread(thread local) urings?

[Dmitry Sychov <dmitry.sychov@gmail.com>]

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.

On Wed, May 13, 2020 at 2:56 PM Mark Papadakis
<markuspapadakis@icloud.com> 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@gmail.com> 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
>