Re: [PATCH RFC for-next 0/8] io_uring: tw contention improvments

[Hao Xu <hao.xu@linux.dev>]

On 6/22/22 17:31, Dylan Yudaken wrote:
> On Tue, 2022-06-21 at 15:34 +0800, Hao Xu wrote:
>> On 6/21/22 15:03, Dylan Yudaken wrote:
>>> On Tue, 2022-06-21 at 13:10 +0800, Hao Xu wrote:
>>>> On 6/21/22 00:18, Dylan Yudaken wrote:
>>>>> Task work currently uses a spin lock to guard task_list and
>>>>> task_running. Some use cases such as networking can trigger
>>>>> task_work_add
>>>>> from multiple threads all at once, which suffers from
>>>>> contention
>>>>> here.
>>>>>
>>>>> This can be changed to use a lockless list which seems to have
>>>>> better
>>>>> performance. Running the micro benchmark in [1] I see 20%
>>>>> improvment in
>>>>> multithreaded task work add. It required removing the priority
>>>>> tw
>>>>> list
>>>>> optimisation, however it isn't clear how important that
>>>>> optimisation is.
>>>>> Additionally it has fairly easy to break semantics.
>>>>>
>>>>> Patch 1-2 remove the priority tw list optimisation
>>>>> Patch 3-5 add lockless lists for task work
>>>>> Patch 6 fixes a bug I noticed in io_uring event tracing
>>>>> Patch 7-8 adds tracing for task_work_run
>>>>>
>>>>
>>>> Compared to the spinlock overhead, the prio task list
>>>> optimization is
>>>> definitely unimportant, so I agree with removing it here.
>>>> Replace the task list with llisy was something I considered but I
>>>> gave
>>>> it up since it changes the list to a stack which means we have to
>>>> handle
>>>> the tasks in a reverse order. This may affect the latency, do you
>>>> have
>>>> some numbers for it, like avg and 99% 95% lat?
>>>>
>>>
>>> Do you have an idea for how to test that? I used a microbenchmark
>>> as
>>> well as a network benchmark [1] to verify that overall throughput
>>> is
>>> higher. TW latency sounds a lot more complicated to measure as it's
>>> difficult to trigger accurately.
>>>
>>> My feeling is that with reasonable batching (say 8-16 items) the
>>> latency will be low as TW is generally very quick, but if you have
>>> an
>>> idea for benchmarking I can take a look
>>>
>>> [1]: https://github.com/DylanZA/netbench
>>
>> It can be normal IO requests I think. We can test the latency by fio
>> with small size IO to a fast block device(like nvme) in SQPOLL
>> mode(since for non-SQPOLL, it doesn't make difference). This way we
>> can
>> see the influence of reverse order handling.
>>
>> Regards,
>> Hao
> 
> I see little difference locally, but there is quite a big stdev so it's
> possible my test setup is a bit wonky
> 
> new:
>      clat (msec): min=2027, max=10544, avg=6347.10, stdev=2458.20
>       lat (nsec): min=1440, max=16719k, avg=119714.72, stdev=153571.49
> old:
>      clat (msec): min=2738, max=10550, avg=6700.68, stdev=2251.77
>       lat (nsec): min=1278, max=16610k, avg=121025.73, stdev=211896.14
> 

Hi Dylan,

Could you post the arguments you use and the 99% 95% latency as well?

Regards,
Hao