Re: [PATCH 0/8] Various io_uring micro-optimizations (reducing lock contention)

[Jens Axboe <axboe@kernel.dk>]

On 1/28/25 6:39 AM, Max Kellermann wrote:
> While optimizing my io_uring-based web server, I found that the kernel
> spends 35% of the CPU time waiting for `io_wq_acct.lock`.  This patch
> set reduces contention of this lock, though I believe much more should
> be done in order to allow more worker concurrency.
> 
> I measured these patches with my HTTP server (serving static files and
> running a tiny PHP script) and with a micro-benchmark that submits
> millions of `IORING_OP_NOP` entries (with `IOSQE_ASYNC` to force
> offloading the operation to a worker, so this offload overhead can be
> measured).
> 
> Some of the optimizations eliminate memory accesses, e.g. by passing
> values that are already known to (inlined) functions and by caching
> values in local variables.  These are useful optimizations, but they
> are too small to measure them in a benchmark (too much noise).
> 
> Some of the patches have a measurable effect and they contain
> benchmark numbers that I could reproduce in repeated runs, despite the
> noise.
> 
> I'm not confident about the correctness of the last patch ("io_uring:
> skip redundant poll wakeups").  This seemed like low-hanging fruit, so
> low that it seemed suspicious to me.  If this is a useful
> optimization, the idea could probably be ported to other wait_queue
> users, or even into the wait_queue library.  What I'm not confident
> about is whether the optimization is valid or whether it may miss
> wakeups, leading to stalls.  Please advise!

That last patch is the only one that needs a bit more checking, so I'd
suggest we just ignore that one for now. We're in the middle of the
merge window anyway, so all of this would have to wait for the 6.15
merge window anyway - iow, plenty of time.

The other patches look pretty straight forward to me. Only thing that
has me puzzled a bit is why you have so much io-wq activity with your
application, in general I'd expect 0 activity there. But Then I saw the
forced ASYNC flag, and it makes sense. In general, forcing that isn't a
great idea, but for a benchmark for io-wq it certainly makes sense.

I'll apply 1-7 once 6.14-rc1 is out and I can kick off a
for-6.15/io_uring branch. Thanks!

-- 
Jens Axboe