Re: Should io_method=worker remain the default?

Hi,

On 2025-09-03 17:53:55 +0200, Álvaro Herrera wrote:
> On 2025-Sep-02, Jeff Davis wrote:
>
> > The good news is that io_uring seemed to solve the problem.
> > Unfortunately, that's platform-specific, so it can't be the default.
>
> Why not? We have had wal_sync_method with a platform-dependent default
> for a very long time.

At least for now there are other reasons:

1) io_uring needs a higher ulimit -n, because we need to create an io_uring
instance for each proc entry, in postmaster. In the future, we might want
to increase soft ulimit -n, but we aren't doing that yet.

In older kernels the number of io_uring memory mapping also can pose a
performance issue at high connection rates.

2) Sometimes worker is faster than io_uring, specifically when checksums are
enabled. Workers can offload the checksum computations, io_uring can't.

3) We can't, at build time, know whether io_uring is actually available at
runtime. io_uring can be disabled with a runtime sysctl.

We could add a dynamic fallback to worker, but we don't have that yet.

Anyway, the issue with worker in this case isn't inherent, it's just a bit of
lock contention in an extreme workload...

Greetings,

Andres Freund