IBM User Guide for Java V7 on Windows > IBM SDK for Java > The collector and the allocator > Global garbage collection

Compaction phase

When the garbage has been removed from the heap, the Garbage Collector can consider compacting the resulting set of objects to remove the spaces that are between them. The process of compaction is complicated because, if any object is moved, the GC must change all the references that exist to it. The default is not to compact.

The following analogy might help you understand the compaction process. Think of the heap as a warehouse that is partly full of pieces of furniture of different sizes. The free space is the gaps between the furniture. The free list contains only gaps that are above a particular size. Compaction pushes everything in one direction and closes all the gaps. It starts with the object that is closest to the wall, and puts that object against the wall. Then it takes the second object in line and puts that against the first. Then it takes the third and puts it against the second, and so on. At the end, all the furniture is at one end of the warehouse and all the free space is at the other.

To keep compaction times to a minimum, the helper threads are used again.

Compaction occurs if any one of the following conditions are true and -Xnocompactgc has not been specified:

• -Xcompactgc has been specified.
• Following the sweep phase, not enough free space is available to satisfy the allocation request.
• A System.gc() has been requested and the last allocation failure triggering a global garbage collection did not compact or -Xcompactexplicitgc has been specified.
• At least half the previously available memory has been consumed by TLH allocations (ensuring an accurate sample) and the average TLH size falls below 1024 bytes
• The scavenger is enabled, and the largest object that the scavenger failed to tenure in the most recent scavenge is larger than the largest free entry in tenured space.
• The heap is fully expanded and less than 4% of old space is free.
• Less than 128 KB of the heap is free.

With the Balanced Garbage Collection policy, the -Xcompactgc and -Xnocompactgc options are respected only if a global garbage collection is required. A global garbage collection occurs in rare circumstances, as described in Global garbage collection. All other collection activity for the Balanced policy is subject to possible compaction or object movement.

Parent: Global garbage collection

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