Re: postmaster uses more CPU in 18 beta1 with io_method=io_uring

+#if defined(HAVE_LIBURING_QUEUE_INIT_MEM) && defined(IORING_SETUP_NO_MMAP)
&& 1

Is that && 1 intentional?

Nit:
+ "mmap(%zu) to determine io_uring_queue_init_mem() support has failed: %m",
IMHO that would read better without "has".

+ /* FIXME: This should probably not stay at DEBUG1? */
+ elog(DEBUG1,
+ "can use combined memory mapping for io_uring, each ring needs %d bytes",
+ ret);
Assuming my read that this is only executed at postmaster start is correct,
I agree that NOTICE would also be reasonable. Though I'm not sure what a
user could actually do with the info...

+ elog(DEBUG1,
+ "can't use combined memory mapping for io_uring, kernel or liburing too
old");
OTOH this message would definitely be of interest to users; I'd say it
should at least be NOTICE, possibly even WARNING. It'd also be good to have
a HINT either explaining the downside or pointing to the docs.

+ * Memory for rings needs to be allocated to the page boundary,
+ * reserve space. Luckily it does not need to be aligned to hugepage
+ * boundaries, even if huge pages are used.
Is "reserve space" left over from something else?
AFAICT pgaio_uring_ring_shmem_size() isn't even reserving space...

On Mon, Jun 30, 2025 at 11:28 AM Andres Freund <andres(at)anarazel(dot)de> wrote:

> Hi,
>
> On 2025-06-05 14:32:10 -0400, Andres Freund wrote:
> > On 2025-06-05 12:47:52 -0400, Tom Lane wrote:
> > > Andres Freund <andres(at)anarazel(dot)de> writes:
> > > > I think this is a big enough pitfall that it's, obviously assuming
> the patch
> > > > has a sensible complexity, worth fixing this in 18. RMT, anyone,
> what do you
> > > > think?
> > >
> > > Let's see the patch ... but yeah, I'd rather not ship 18 like this.
> >
> > I've attached a first draft.
> >
> > I can't make heads or tails of the ordering in configure.ac, so the
> function
> > test is probably in the wrong place.
>
> Any comments on that patch? I'd hoped for some review comments... Unless
> I'll
> hear otherwise, I'll just do a bit more polish and push..
>
> Greetings,
>
> Andres
>
>
>