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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe