Java GC, HotSpot's CMS and heap fragmentation

Concurrent Mark Sweep (CMS) is one of garbage collection algorithms implemented in HotSpot JVM. CMS is designed to be mostly concurrent, requiring just two quick stop-the-world pauses per old space garbage collection cycle. But if CMS cannot keep up with allocation of memory by application, it may fallback to stop-the-world whole heap collection - infamous Full GC pause. Full GC is very noticeable

[miyaga]

ConcurrentGC　fragmentation promotion-failed

[msykt]

"My experiments have shown drastic reduction of heap fragmentation in few application with known promotion failure problems."