> If I've understood that correctly you can also ensure that they are
> also on the same socket, which apparently improves memory access.
I think this is what is meant by NUMA (Non-Uniform Memory access).
AFAIK, NUMA is dead for everything except a few research systems.
Parallel/multi-processor systems these days are all "symmetric" (all processors have symmetrical access to all memory).
NUMA is really, really, really hard to get right. Why? Cache invalidation. Several companies, organizations, etc. have tried. Last time I looked (and it has been a while, but I was quite involved with this stuff in the mid 1990s), everybody failed.