Heap fragmentation

When the JVM starts, it allocates heap memory from the operating system and then manages that memory. Whenever an application creates a new object, the JVM automatically allocates a block of memory with a size that's big enough to fit the new object on the heap. After sweeping, in most cases, memory becomes fragmented. Memory fragmentation leads to two problems:

• Allocation operations become more time consuming, because it's hard to find the next free block of sufficient size
• The unused space between blocks can become so great that the JVM won't be able to create a new object

The following diagram illustrates a fragmented memory heap:

To avoid these problems after each GC cycle, the JVM executes a compaction step. Compacting moves all reachable objects to one end of the heap and, in this way, closes all holes. The heap after compacting looks as follows:

These diagrams show how blocks are located before and after compacting. The drawback is that an application must also be suspended during this process.