On 1/29/25 19:11, Max Kellermann wrote:
On Wed, Jan 29, 2025 at 7:56 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
What architecture are you running? I don't get why the reads
are expensive while it's relaxed and there shouldn't even be
any contention. It doesn't even need to be atomics, we still
should be able to convert int back to plain ints.
I measured on an AMD Epyc 9654P.
As you see in my numbers, around 40% of the CPU time was wasted on
spinlock contention. Dozens of io-wq threads are trampling on each
other's feet all the time.
I don't think this is about memory accesses being exceptionally
expensive; it's just about wringing every cycle from the code section
that's under the heavy-contention spinlock.
Ok, then it's an architectural problem and needs more serious
reengineering, e.g. of how work items are stored and grabbed, and it
might even get some more use cases for io_uring. FWIW, I'm not saying
smaller optimisations shouldn't have place especially when they're
clean.
--
Pavel Begunkov
