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

[Sergiy Yevtushenko <sergiy.yevtushenko@gmail.com>]

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@icloud.com> wrote:
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> > On 13 May 2020, at 4:15 PM, Dmitry Sychov <dmitry.sychov@gmail.com> 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.
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> 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.
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> > 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
> >>
>