Re: [RFC] single cqe per link

Jens Axboe
2020-02-25T03:13:19Z

On 2/24/20 7:36 PM, Jens Axboe wrote:
> On 2/24/20 7:14 PM, Carter Li 李通洲 wrote:
>>> 2020年2月25日 上午8:39，Pavel Begunkov <asml.silence@gmail.com> 写道：
>>>
>>> I've got curious about performance of the idea of having only 1 CQE per link
>>> (for the failed or last one). Tested it with a quick dirty patch doing
>>> submit-and-reap of a nops-link (patched for inline execution).
>>>
>>> 1) link size: 100
>>> old: 206 ns per nop
>>> new: 144 ns per nop
>>>
>>> 2) link size: 10
>>> old: 234 ns per nop
>>> new: 181 ns per nop
>>>
>>> 3) link size: 10, FORCE_ASYNC
>>> old: 667 ns per nop
>>> new: 569 ns per nop
>>>
>>>
>>> The patch below breaks sequences, linked_timeout and who knows what else.
>>> The first one requires synchronisation/atomic, so it's a bit in the way. I've
>>> been wondering, whether IOSQE_IO_DRAIN is popular and how much it's used. We can
>>> try to find tradeoff or even disable it with this feature.
>>
>> Hello Pavel,
>>
>> I still think flags tagged on sqes could be a better choice, which
>> gives users an ability to deside if they want to ignore the cqes, not
>> only for links, but also for normal sqes.
>>
>> In addition, boxed cqes couldn’t resolve the issue of
>> IORING_IO_TIMEOUT.
>
> I would tend to agree, and it'd be trivial to just set the flag on
> whatever SQEs in the chain you don't care about. Or even an individual
> SQE, though that's probably a bit more of a reach in terms of use case.
> Maybe nop with drain + ignore?
>
> In any case it's definitely more flexible.

In the interest of taking this to the extreme, I tried a nop benchmark
on my laptop (qemu/kvm). Granted, this setup is particularly sensitive
to spinlocks, they are a lot more expensive there than on a real host.

Anyway, regular nops run at about 9.5M/sec with a single thread.
Flagging all SQEs with IOSQE_NO_CQE nets me about 14M/sec. So a handy
improvement. Looking at the top of profiles:

cqe-per-sqe:

+ 28.45% io_uring [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
+ 14.38% io_uring [kernel.kallsyms] [k] io_submit_sqes
+ 9.38% io_uring [kernel.kallsyms] [k] io_put_req
+ 7.25% io_uring libc-2.31.so [.] syscall
+ 6.12% io_uring [kernel.kallsyms] [k] kmem_cache_free

no-cqes:

+ 19.72% io_uring [kernel.kallsyms] [k] io_put_req
+ 11.93% io_uring [kernel.kallsyms] [k] io_submit_sqes
+ 10.14% io_uring [kernel.kallsyms] [k] kmem_cache_free
+ 9.55% io_uring libc-2.31.so [.] syscall
+ 7.48% io_uring [kernel.kallsyms] [k] __io_queue_sqe

I'll try the real disk IO tomorrow, using polled IO.

--
Jens Axboe