Add a UMA option to support cross domain frees while preserving locality for first-touch zones.

[jeff]

Summary

This makes an optional cross-domain free bucket for memory freed to a different domain than it was allocated from. This gives us support for precise first-touch domains that won't mix memory. If you have a zone which does mix allocs and frees at a significant rate it can eventually bottleneck while freeing memory. I may address that in a follow-up patch if there is enough interest.

Combined with increased thread locality this can offer significant performance improvements for targeted workloads. It is likely not generally faster so it is hidden behind an option for now.

[alc]

sys/vm/uma_core.c

3083–3084 ↗

(On Diff #59668)

I'm curious why you didn't choose to compute (or lookup) the address of the slab header and use its us_domain field here.

[jeff]

sys/vm/uma_core.c

3083–3084 ↗

(On Diff #59668)

I believe this is cheaper. You can touch a very small amount of read-only global memory to calculate the domain assuming that you're allocating from the direct mapped region.

I actually have a version of vm_phys_domain() which operates only from math, not a table, but found it isn't necessary so far. Roughly, you subtract the pci-e hole and divide the resulting address by the size of each domain.

To get to the slab we need a couple extra steps and a cache line that we don't otherwise need.