Re: [PATCH 3/3] io_uring: add support for getdents

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

On 7/16/23 19:57, Hao Xu wrote:
> On 7/13/23 23:14, Christian Brauner wrote:
> 
>> Could someone with perf experience try and remove that f_count == 1
>> optimization from __fdget_pos() completely and make it always acquire
>> the mutex? I wonder what the performance impact of that is.
> 
> Hi Christian,
> For your reference, I did a simple test: writed a c program that open a
> directory which has 1000 empty files, then call sync getdents64 on it
> repeatedly until we get all the entries. I run this program 10 times for
> "with f_count==1
> optimization" and "always do the lock" version.
> Got below data:
> with f_count==1:
> 
> time cost: 0.000379
> time cost: 0.000116
> time cost: 0.000090
> time cost: 0.000101
> time cost: 0.000095
> time cost: 0.000092
> time cost: 0.000092
> time cost: 0.000095
> time cost: 0.000092
> time cost: 0.000121
> time cost: 0.000092
> 
> time cost avg: 0.00009859999999999998
> 
> always do the lock:
> time cost: 0.000095
> time cost: 0.000099
> time cost: 0.000123
> time cost: 0.000124
> time cost: 0.000092
> time cost: 0.000099
> time cost: 0.000092
> time cost: 0.000092
> time cost: 0.000093
> time cost: 0.000094
>              time cost avg: 0.00010029999999999997
> 
> So about 1.724% increment
> 
> [1] the first run is not showed here since that try does real IO
>      and diff a lot.
> [2] the time cost calculation is by gettimeofday()
> [3] run it in a VM which has 2 CPUs and 1GB memory.
> 
> Regards,
> Hao

Did another similar test for more times(100 rounds), about 1.4%
increment. How about:
if CONFIG_IO_URING: remove the f_count==1 logic
else: do the old logic.