I was planning on using ParallelGCThreads after yarn starts using cpusets - otherwise I was not sure of the side effects (performance, correctness) : of parallel gc not occupying all threads I mean.

I asked around hadoop team, and they seem to be ok with not doing anything for gc specifically - assuming things even out in the end for the cluster I guess - and since yarn was adding cpusets anyway, other than perf hit of more gc threads than core (which we can control with ParallelGCThreads), there wont be any other issue