Re: io_uring_prep_openat_direct() and link/drain

[Jens Axboe <axboe@kernel.dk>]

On 4/21/22 6:31 AM, Miklos Szeredi wrote:
> On Tue, 5 Apr 2022 at 16:44, Jens Axboe <axboe@kernel.dk> wrote:
>>
>> On 4/5/22 1:45 AM, Miklos Szeredi wrote:
>>> On Sat, 2 Apr 2022 at 03:17, Jens Axboe <axboe@kernel.dk> wrote:
>>>>
>>>> On 4/1/22 10:21 AM, Jens Axboe wrote:
>>>>> On 4/1/22 10:02 AM, Miklos Szeredi wrote:
>>>>>> On Fri, 1 Apr 2022 at 17:36, Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>
>>>>>>> I take it you're continually reusing those slots?
>>>>>>
>>>>>> Yes.
>>>>>>
>>>>>>>  If you have a test
>>>>>>> case that'd be ideal. Agree that it sounds like we just need an
>>>>>>> appropriate breather to allow fput/task_work to run. Or it could be the
>>>>>>> deferral free of the fixed slot.
>>>>>>
>>>>>> Adding a breather could make the worst case latency be large.  I think
>>>>>> doing the fput synchronously would be better in general.
>>>>>
>>>>> fput() isn't sync, it'll just offload to task_work. There are some
>>>>> dependencies there that would need to be checked. But we'll find a way
>>>>> to deal with it.
>>>>>
>>>>>> I test this on an VM with 8G of memory and run the following:
>>>>>>
>>>>>> ./forkbomb 14 &
>>>>>> # wait till 16k processes are forked
>>>>>> for i in `seq 1 100`; do ./procreads u; done
>>>>>>
>>>>>> You can compare performance with plain reads (./procreads p), the
>>>>>> other tests don't work on public kernels.
>>>>>
>>>>> OK, I'll check up on this, but probably won't have time to do so before
>>>>> early next week.
>>>>
>>>> Can you try with this patch? It's not complete yet, there's actually a
>>>> bunch of things we can do to improve the direct descriptor case. But
>>>> this one is easy enough to pull off, and I think it'll fix your OOM
>>>> case. Not a proposed patch, but it'll prove the theory.
>>>
>>> Sorry for the delay..
>>>
>>> Patch works like charm.
>>
>> OK good, then it is the issue I suspected. Thanks for testing!
> 
> Tested with v5.18-rc3 and performance seems significantly worse than
> with the test patch:
> 
> test patch:
>         avg     min     max     stdev
> real    0.205   0.190   0.266   0.011
> user    0.017   0.007   0.029   0.004
> sys     0.374   0.336   0.503   0.022
> 
> 5.18.0-rc3-00016-gb253435746d9:
>         avg     min     max     stdev
> real    0.725   0.200   18.090  2.279
> user    0.019   0.005   0.046   0.006
> sys     0.454   0.241   1.022   0.199

It's been a month and I don't remember details of which patches were
tested, when you say "test patch", which one exactly are you referring
to and what base was it applied on?

-- 
Jens Axboe