Re: [RFC] io_uring: remove file batch-get optimisation

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

On 8/10/21 6:04 PM, Jens Axboe wrote:
> On 8/10/21 7:52 AM, Pavel Begunkov wrote:
>> For requests with non-fixed files, instead of grabbing just one
>> reference, we get by the number of left requests, so the following
>> requests using the same file can take it without atomics.
>>
>> However, it's not all win. If there is one request in the middle
>> not using files or having a fixed file, we'll need to put back the left
>> references. Even worse if an application submits requests dealing with
>> different files, it will do a put for each new request, so doubling the
>> number of atomics needed. Also, even if not used, it's still takes some
>> cycles in the submission path.
>>
>> If a file used many times, it rather makes sense to pre-register it, if
>> not, we may fall in the described pitfall. So, this optimisation is a
>> matter of use case. Go with the simpliest code-wise way, remove it.
> 
> I ran this through the peak testing, not using registered files. Doesn't
> seem to make a real difference here, at least in the quick testing.
> Which would seem to indicate we could safely kill it. But that's also
> the best case for non-registered files, would be curious to see if it
> makes a real difference now for workloads where the file is being
> shared.

Do you mean shared between cores so there is contention? Or the worst
case for non-reg with multiple files as described in the patch? 

-- 
Pavel Begunkov