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From: Dmitry Sychov <dmitry.sychov@gmail.com>
To: Mark Papadakis <markuspapadakis@icloud.com>
Cc: "H. de Vries" <hdevries@fastmail.com>,
	io-uring <io-uring@vger.kernel.org>
Subject: Re: Any performance gains from using per thread(thread local) urings?
Date: Wed, 13 May 2020 16:15:51 +0300	[thread overview]
Message-ID: <CADPKF+eZCE4A2yXnQaZvq1uk3b-zR+-rwQhzA2z=v7+VsTndkQ@mail.gmail.com> (raw)
In-Reply-To: <7692E70C-A0EA-423B-883F-6BF91B0DB359@icloud.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
>

  reply	other threads:[~2020-05-13 13:16 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-12 20:20 Dmitry Sychov
2020-05-13  6:07 ` H. de Vries
2020-05-13 11:01   ` Dmitry Sychov
2020-05-13 11:56     ` Mark Papadakis
2020-05-13 13:15       ` Dmitry Sychov [this message]
2020-05-13 13:27         ` Mark Papadakis
2020-05-13 13:48           ` Dmitry Sychov
2020-05-13 14:12           ` Sergiy Yevtushenko
     [not found]           ` <CAO5MNut+nD-OqsKgae=eibWYuPim1f8-NuwqVpD87eZQnrwscA@mail.gmail.com>
2020-05-13 14:22             ` Dmitry Sychov
2020-05-13 14:31               ` Dmitry Sychov
2020-05-13 16:02               ` Pavel Begunkov
2020-05-13 19:23                 ` Dmitry Sychov
2020-05-14 10:06                   ` Pavel Begunkov
2020-05-14 11:35                     ` Dmitry Sychov

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