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

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

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