On Tue, Dec 15, 2020 at 8:08 AM Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> OK, ran some numbers. The test app benchmarks opening X files, I just
> used /usr on my test box. That's 182677 files. To mimic real worldy
> kind of setups, 33% of the files can be looked up hot, so LOOKUP_NONBLOCK
> will succeed.
Perhaps more interestingly, what's the difference between the patchset
as posted for just io_uring?
IOW, does the synchronous LOOKUP_NONBLOCK actually help?
I'm obviously a big believer in the whole "avoid thread setup costs if
not necessary", so I'd _expect_ it to help, but maybe the possible
extra parallelism is enough to overcome the thread setup and
synchronization costs even for a fast cached RCU lookup.
(I also suspect the reality is often much closer to 100% cached
lookups than just 33%, but who knows - there are things like just
concurrent renames that can cause the RCU lookup to fail even if it
_was_ cached, so it's not purely about whether things are in the
dcache or not).
Linus
