Re: [PATCH -next] io_uring: add support for fchmod

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/26/24 15:07, lizetao wrote:
>>>>> On 11/19/24 1:12 AM, lizetao wrote:
>>>>> Adds support for doing chmod through io_uring. IORING_OP_FCHMOD
>>>>> behaves like fchmod(2) and takes the same arguments.
>>>
>>>> Looks pretty straight forward. The only downside is the forced use of REQ_F_FORCE_ASYNC - did you look into how feasible it would be to allow non-blocking issue of this? Would imagine the majority of fchmod calls end up not blocking in the first place.
>>>
>>> Yes, I considered fchmod to allow asynchronous execution and wrote a test case to test it, the results are as follows:
>>>

FYI, this email got into spam.

>>> fchmod:
>>> real	0m1.413s
>>> user	0m0.253s
>>> sys	0m1.079s
>>>
>>> io_uring + fchmod:
>>> real	0m1.268s
>>> user	0m0.015s
>>> sys	0m5.739s
>>>
>>> There is about a 10% improvement.
> 
>> And that makes sense if you're keeping some fchmod inflight, as you'd generally just have one io-wq processing them and running things in parallel with submission. But what you you keep an indepth count of 1, eg do sync fchmod? Then it'd be considerably slower than the syscall.
> Indeed, When performing REQ_F_FORCE_ASYNC operations at depth 1, performance is degraded. The results are as follows:
> 
> fchmod:
> real	0m2.285s
> user	0m0.050s
> sys	0m1.996s
> 
> io_uring + fchmod:
> real	0m2.541s
> user	0m0.013s
> sys	0m2.379s
> 
>> This isn't necessarily something to worry about, but fact is that if you can do a nonblock issue and have it succeed most of the time, that'll be more efficient (and faster for low/sync fchmod) than something that just offloads to io-wq. You can see that from your results too, comparing the sys number netween the two.
> However, when I remove REQ_F_FORCE_ASYNC and use IO_URING_F_NONBLOCK, the performance is not improved. The measured results are as follows:
> fchmod:
> real	0m2.132s
> user	0m0.048s
> sys	0m1.845s
> 
> io_uring + fchmod:
> real	0m2.196s
> user	0m0.005s
> sys	0m2.097s
> 
>> Hence why I'm asking if you looked into doing a nonblocking issue at all. This won't necessarily gate the inclusion of the patch, and it is something that can be changed down the line, I'm mostly just curious.
> Does this result meet expectations? Or maybe I missed something, please let me know

-- 
Pavel Begunkov