Re: io_uring_prep_openat_direct() and link/drain

[Jens Axboe <axboe@kernel.dk>]

On 4/21/22 6:39 AM, Miklos Szeredi wrote:
> On Thu, 21 Apr 2022 at 14:34, Jens Axboe <axboe@kernel.dk> wrote:
>>
>> 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?
> 
> https://lore.kernel.org/all/47912c4c-ccc2-0678-6c2f-3e3c0dd1f04b@kernel.dk/
> 
> The base is a good question, it was after the basic fixed slot
> assignment issues were fixed.

Gotcha, ok then this makes sense. The ordering issues were sorted out
for 5.18-rc3, but the direct descriptor optimization is only in the 5.19
branch.

-- 
Jens Axboe