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

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

Anyone could shed some light on the inner implementation of uring please? :)

Specifically how well kernel scales with the increased number of user
created urings?

> If kernel implementation will change from single to multiple queues,
> user space is already prepared for this change.

Thats +1 for per-thread urings. An expectation for the kernel to
become better and better in multiple urings scaling in the future.

On Wed, May 13, 2020 at 4:52 PM Sergiy Yevtushenko
<sergiy.yevtushenko@gmail.com> wrote:
>
> 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:
>>
>>
>>
>> > 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.
>>
>> 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@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
>> >>
>>