I would expect that low pause times are easy enough (or relatively easy considering the difficult domain of GC optimization), but keeping the allocations cheap is (one of) the key constraints, no?
In multicore's case it's about keeping low pause times while also keeping allocation cheap _and_ maintaining throughput.
I don't really understand why not. Can you expand on it? Doesn't the JVM for example do thread-local allocations in a conventional shared-memory parallel environment?
To avoid frequent synchronisation we could have gone with a (potentially generational) non-moving collector for the minor GC which would still have preserved the C API, probably allowed for very low pauses but would have made allocation more expensive.