I mentioned this in the second paragraph. Like I said, I'd be surprised if that helps mark times that much in practice. For minor collections in a generational GC you're typically doing a Cheney scan, so it's very unlikely to matter as you're copying the whole live region of the semispace anyhow. For major collections on tenured objects, in theory it could help, but again I'm skeptical that it will affect mark performance that much, because compaction does an excellent job of mitigating the cache effects. (It's impossible to accurately measure this stuff right now, as the fact that Go's GC is much more immature than the HotSpot GC will skew the numbers.)