>  Is GC ConcurrentMarkSweep a Stop-The-World situation? Where the > JVM cannot do anything else? Hence then node is technically Down? > Correct?
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