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

[Pavel Begunkov <asml.silence@gmail.com>]

On 13/05/2020 22:23, Dmitry Sychov wrote:
>> E.g. 100+ cores hammering on a spinlock/mutex protecting an SQ wouldn't do any good.
> 
> Its possible to mitigate the hammering by using proxy buffer - instead
> of spinning, the particular thread
> could add the next entry into the buffer through XADD instead, and
> another thread currently holding an exclusive
> lock could in turn check this buffer and batch-submit all pending
> entries to SQ before leasing SQ mutex.

Sure there are many ways, but I think my point is clear.
FWIW, atomics/wait-free will fail to scale good enough after some point.

>> will be offloaded to an internal thread pool (aka io-wq), which is per io_uring by default, but can be shared if specified.
> 
> Well, thats sounds like mumbo jumbo to me, does this mean that the
> kernel holds and internal pool of threads to
> perform uring tasks independent to the number of user urings?

If I parsed the question correctly, again, it creates a separate thread pool per
each new io_uring, if wasn't specified otherwise.

> 
> If there are multiple kernel work flows bound to corresponding uring
> setups the issue with threads starvation could exist if they do not
> actively steal from each other SQs.
The threads can go to sleep or be dynamically created/destroyed.

Not sure what kind of starvation you meant, but feel free to rephrase your
questions if any of them weren't understood well.

> And starvation costs could be greater than allowing for multiple
> threads to dig into one uring queue, even under the exclusive lock.
Thread pools can be shared.

> 
>> And there a lot of details, probably worth of a separate write-up.
> 
> I've reread io_uring.pdf and there are not much tech details on the
> inner implementation of uring to try to apply best practices and to
> avoid noob questions like mine.
> 
> 
> 
> On Wed, May 13, 2020 at 7:03 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>
>> On 13/05/2020 17:22, Dmitry Sychov wrote:
>>> Anyone could shed some light on the inner implementation of uring please? :)
>>
>> It really depends on the workload, hardware, etc.
>>
>> io_uring instances are intended to be independent, and each have one CQ and SQ.
>> The main user's concern should be synchronisation (in userspace) on CQ+SQ. E.g.
>> 100+ cores hammering on a spinlock/mutex protecting an SQ wouldn't do any good.
>>
>> Everything that can't be inline completed\submitted during io_urng_enter(), will
>> be offloaded to an internal thread pool (aka io-wq), which is per io_uring by
>> default, but can be shared if specified. There are pros and cons, but I'd
>> recommend first to share a single io-wq, and then experiment and tune.
>>
>> Also, in-kernel submission is not instantaneous and done by only thread at any
>> moment. Single io_uring may bottleneck you there or add high latency in some cases.
>>
>> And there a lot of details, probably worth of a separate write-up.
>>
>>>
>>> Specifically how well kernel scales with the increased number of user
>>> created urings?
>>
>> Should scale well, especially for rw. Just don't overthrow the kernel with
>> threads from dozens of io-wqs.
>>
>>>
>>>> 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
>>>>>>>
>>>>>
>>
>> --
>> Pavel Begunkov

-- 
Pavel Begunkov