Tuning Garbage Collection


Applications that use Ehcache can be expected to use large heaps. Some Ehcache applications have heap sizes greater than 6GB. Ehcache works well at this scale. However, large heaps or long held objects, which is what a cache is composed of, can place strain on the default Garbage Collector. Now with Ehcache 2.3 and higher, this problem can be solved with BigMemory in-memory data management, which provides an additional store outside of the heap.

Note: The following documentation relates to Oracle JDK 1.5, which is no longer supported.

Detecting Garbage Collection Problems

A full garbage collection event pauses all threads in the JVM. Nothing happens during the pause. If this pause takes more than a few seconds, it will become noticeable.

The clearest way to see if this is happening is to run jstat. The following command will produce a log of garbage collection statistics, updated every ten seconds.

         jstat -gcutil <pid> 10 1000000


The thing to watch for is the Full Garbage Collection Time. The difference between the total time for each reading is the amount of time the system was paused. A jump of more than a few seconds will not be acceptable in most application contexts.

Garbage Collection Tuning

The Sun core garbage collection team has offered the following tuning suggestion for virtual machines with large heaps using caching:

         java ... -XX:+DisableExplicitGC -XX:+UseConcMarkSweepGC
-XX:NewSize=<1/4 of total heap size> -XX:SurvivorRatio=16


Note that it is better to use -XX:+DisableExplicitGC, instead of calling System.gc(). It also helps to use the low pause collector -XX:+UseConcMarkSweepGC.

Distributed Caching Garbage Collection Tuning

Some users have reported that enabling distributed caching causes a full GC each minute. This is an issue with RMI generally, which can be worked around by increasing the interval for garbage collection. The effect RMI has is similar to a user application calling System.gc() each minute. The setting above disables explicit GC calls, but it does not disable the full GC initiated by RMI. The default in JDK6 was increased to 1 hour. The following system properties control the interval.



Increase the interval as required in your application.