Re: [PATCH 5.11 2/2] io_uring: don't take percpu_ref operations for registered files in IOPOLL mode

[Jens Axboe <axboe@kernel.dk>]

On 11/17/20 9:58 AM, Pavel Begunkov wrote:
> On 17/11/2020 16:30, Jens Axboe wrote:
>> On 11/17/20 3:43 AM, Pavel Begunkov wrote:
>>> On 17/11/2020 06:17, Xiaoguang Wang wrote:
>>>> In io_file_get() and io_put_file(), currently we use percpu_ref_get() and
>>>> percpu_ref_put() for registered files, but it's hard to say they're very
>>>> light-weight synchronization primitives. In one our x86 machine, I get below
>>>> perf data(registered files enabled):
>>>> Samples: 480K of event 'cycles', Event count (approx.): 298552867297
>>>> Overhead  Comman  Shared Object     Symbol
>>>>    0.45%  :53243  [kernel.vmlinux]  [k] io_file_get
>>>
>>> Do you have throughput/latency numbers? In my experience for polling for
>>> such small overheads all CPU cycles you win earlier in the stack will be
>>> just burned on polling, because it would still wait for the same fixed*
>>> time for the next response by device. fixed* here means post-factum but
>>> still mostly independent of how your host machine behaves. 
>>
>> That's only true if you can max out the device with a single core.
>> Freeing any cycles directly translate into a performance win otherwise,
>> if your device isn't the bottleneck. For the high performance testing
> 
> Agree, that's what happens if a host can't keep up with a device, or e.g.

Right, and it's a direct measure of the efficiency. Moving cycles _to_
polling is a good thing! It means that the rest of the stack got more
efficient. And if the device is fast enough, then that'll directly
result in higher peak IOPS and lower latencies.

> in case 2. of my other reply. Why don't you mention throwing many-cores
> into a single many (poll) queue SSD?

Not really relevant imho, you can obviously always increase performance
if you are core limited by utilizing multiple cores. 

I haven't tested these patches yet, will try and see if I get some time
to do so tomorrow.

-- 
Jens Axboe