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
![Java GC, HotSpot's CMS and heap fragmentation](https://cdn-ak-scissors.b.st-hatena.com/image/square/d0670f0b0156855427e9b3f017c0aec46f7d6fac/height=288;version=1;width=512/http%3A%2F%2F1.bp.blogspot.com%2F-qBDgpdgvPTU%2FTpYNRG64CFI%2FAAAAAAAAKHE%2Fo-3dOt87MMY%2Fw1200-h630-p-k-no-nu%2Fblog-10.png)