Re: Remove Instruction Synchronization Barrier in spin_delay() for ARM64 architecture

Hi,

On 2025-08-14 11:29:08 +0200, Álvaro Herrera wrote:
> On 2025-May-01, Tom Lane wrote:
>
> > One other thing that comes to mind is that pg_stat_statements
> > has stretched the intention of "short straight-line code segment"
> > to the point of unrecognizability. Maybe it needs to stop using
> > spinlocks to protect pgssEntry updates. But I'm not sure if that
> > would move the needle on whether ISB is a good idea or not.
>
> Yeah, it looks like pgss_store() is being too generous on the amount of
> code run with that spinlock held.

Indeed. To the point that these days pgss is basically unusable for workloads
with many short queries.

> However, changing that spinlock to an lwlock doesn't look easy, because of
> the way each pgss entry is created as a dynahash entry, and then deallocated
> from there. With spinlocks we can just reinit the spinlock each time, but
> that doesn't work with lwlocks. We have no easy way to associate then
> disassociate each entry from a specific lwlock.

I'm not following? The lwlock can just be inside the struct, just like the
spinlock is? "Association" is just LWLockInitialize() and deassociation is not
needed.

Greetings,

Andres Freund