Full Garbage Collections Manifested as Spikes in Respones Times

While performance testing a web application deployed on Tomcat, we noticed that the average response time was about 0.5 seconds while some requests were getting response times of 2.0 seconds. In JConsole we noticed see-saw pattern in memory usage — more importantly we noticed that there were 20 major garbage collections and garbage collector used in PS MarkSweep.

JConsole - Full Garbage Collection

Average time for a major garbage collection is 1.4 seconds. Since PS MarkSweep garbage collector pauses the application when performing a major garbage collection, the response times for requests arriving when major garbage collection is performed is request time + full GC time – i.e., 0.5 + 1.4 ~ 2.0 seconds

After increasing the heap size (thereby giving more memory for young generation and the two survivor spaces) no full major garbage collections and spikes in response times were noticed.JConsole - No Full Garbage Collection

Rationale: When a minor garbage collection is performed, active objects in Eden and survivor space (holding objects) are copied into the empty survivor space. Any objects that couldn’t be copied into survivor space (because of it getting full) are copied into tenured region. After a few such minor collections, tenured region gets full and a full garbage collection is performed to free up space in tenured region. By increasing the heap space, survivor space is increased and the new size is large enough to hold all active objects when a minor garbage collection is performed and a copy to tenured region is avoided.


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