This week we got a plenty of out of heap memory exceptions in Java. So I started looking on Java Garbage Collection, a revision of System Software Internals theory again. Java has its heap divided into new gen and old generation. Java's garbage collection tries to take advantage of the fact that new object will be deleted sooner(objects will have smaller life time).
So Java's heap is divided into
1)Young Generation(smaller in size)
2)Old Generation (larger chunk)
Young Generation is smaller in size. Traditional Recursive Garbage collection will be faster if the size to be collected is smaller. New Objects are created in Eden part of the Heap. Garbage Collector runs frequently in this space and marks objects that are referenced. Objects that are not not referenced are removed in the second pass and the live objects are moved to Survivor Space.
Survivor Space has two regions Survivor 0 and Survivor 1 which uses an algorithm similar to Copying Garbage Collector. So Garbage Collector runs on Survivor(0/1) depending on live region and moves the live objects to the other region and the former region will have only death objects that will be freed. Objects that survive more than two/three rounds of garbage collection will be moved to Old Generation.
Old Generation has objects which are alive for longer time. Since Java's garbage collection is based on the fact that objects tend to die sooner. Old Generation is collected very rarely and it is very huge in size. The old generation runs two algorithms
1)CMS-Concurrent Mark Sweep
This marks all objects that are not alive and flushes them but there will be defragmentation across the heap
2)Stop the World Collector
Stop the world collector kicks in when heap is almost full, it compacts the live objects. This will cause the java process to hang for few seconds and cause performance impact especially when dealing with heap space as high as 20/30 GB as in our case.
Java 7 has come up with G1 collector which optimizes by combining CMS and STW collector.
To see the garbage collection live, there is a good tool jstat
jstat -gc <pid> <interval to monitor>