Re: Adding basic NUMA awareness

Hi,

On 2025-07-08 14:27:12 +0200, Tomas Vondra wrote:
> On 7/8/25 05:04, Andres Freund wrote:
> > On 2025-07-04 13:05:05 +0200, Jakub Wartak wrote:
> > The reason it would be advantageous to put something like the procarray onto
> > smaller pages is that otherwise the entire procarray (unless particularly
> > large) ends up on a single NUMA node, increasing the latency for backends on
> > every other numa node and increasing memory traffic on that node.
> >
>
> That's why the patch series splits the procarray into multiple pieces,
> so that it can be properly distributed on multiple NUMA nodes even with
> huge pages. It requires adjusting a couple places accessing the entries,
> but it surprised me how limited the impact was.

Sure, you can do that, but it does mean that iterations over the procarray now
have an added level of indirection...

> The thing I'm not sure about is how much this actually helps with the
> traffic between node. Sure, if we pick a PGPROC from the same node, and
> the task does not get moved, it'll be local traffic. But if the task
> moves, there'll be traffic. I don't have any estimates how often this
> happens, e.g. for older tasks.

I think the most important bit is to not put everything onto one numa node,
otherwise the chance of increased latency for *everyone* due to the increased
memory contention is more likely to hurt.

Greetings,

Andres Freund