On 8/10/21 11:11 AM, Pavel Begunkov wrote: > 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? The former. The latter is also a concern, but less so to me. That said, I do think we should just do this. It's less code, and that's always good. To justify it, numbers would have to back it up. -- Jens Axboe
