Just as a quick comment the GC configuration on those Cassandra nodes is kinda crazy. I don't think anyone is recommending anyone to run 30G maxheap CMS. Somewhere around 16G you really should switch to G1GC.

Also I highly doubt you want to have 30G heap with 60G RAM available. Having a larger heap most likely doesn't help you at all. It will just eat away memory you could use for mmapping or off heap things and increase GC pause length.

(This is the bane of running JVM any apps. You really need to know what you are doing with the GC. Also with apps like Cassandra there is no silver bullet. Just some general guidelines to follow and slowly adjust your values and monitoring of the changes as the best possible settings change with the load/hardware setup)