>  Is GC ConcurrentMarkSweep a Stop-The-World situation? Where the
> JVM cannot do anything else? Hence then node is technically Down?
No; the concurrent mark/sweep phase runs concurrently with your
application. CMS will cause a stop-the-world full pause it it fails to
complete a CMS sweep in time and you hit the maximum heap size, but
unless that happens, CMS will run concurrently (though there are
stop-the-world pauses involved, that are typically very short, the
mark/sweep phase is concurrent).
As jbellis pointed out, you're almost certainly swapping.
/ Peter Schuller