We noticed high GC(Mark Sweep Compact) time on this WebSphere server but no suspension time corresponding to these GC's. Post this GC, we can see increase in committed memory, drop in used memory etc. so this GC is not false +ve.
Wondering what could be the reasons for Zero/No suspension times here??
Java : java_1.7_64
From your screenhots I don't see any direct evidence that the GC time is due to CMS compact phase. In the second screenshot the GC times seem to be due to the GC Copy phase which I would think runs concurrently with the app.
FYI: if you have enabled GC logging on your app any JVM suspensions due to the CMS GC will produce "Concurrent mode failure" messages.
Yes, that's the whole point of the CMS GC.
Under normal circumstances it will only (briefly) stop for Mark, Remark and Sweep phases (the latter only if tenured space runs full before all garbage can be collected).
The most common scenario I've seen leading up to a full GC (which will stop all application threads and attempt to compact/defrag tenured space) is failed promotion - which can happen even if there is still "enough" free heap space available due to excess memory fragmentation).