Difference between "on-heap" and "off-heap"

java memory heap ehcache
Synesso picture Synesso · May 23, 2011 · Viewed 97k times · Source

Ehcache talks about on-heap and off-heap memory. What is the difference? What JVM args are used to configure them?

Answer

Vineet Reynolds picture Vineet Reynolds · May 23, 2011

The on-heap store refers to objects that will be present in the Java heap (and also subject to GC). On the other hand, the off-heap store refers to (serialized) objects that are managed by EHCache, but stored outside the heap (and also not subject to GC). As the off-heap store continues to be managed in memory, it is slightly slower than the on-heap store, but still faster than the disk store.

The internal details involved in management and usage of the off-heap store aren't very evident in the link posted in the question, so it would be wise to check out the details of Terracotta BigMemory, which is used to manage the off-disk store. BigMemory (the off-heap store) is to be used to avoid the overhead of GC on a heap that is several Megabytes or Gigabytes large. BigMemory uses the memory address space of the JVM process, via direct ByteBuffers that are not subject to GC unlike other native Java objects.

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