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

Sweep phase

On completion of the mark phase the mark bit vector identifies the location of all the live objects in the heap. The sweep phase uses this to identify those chunks of heap storage that can be reclaimed for future allocations; these chunks are added to the pool of free space.

A free chunk is identified by examining the mark bit vector looking for sequences of zeros, which identify possible free space. GC ignores any sequences of zeros that correspond to a length less than the minimum free size. When a sequence of sufficient length is found, the GC checks the length of the object at the start of the sequence to determine the actual amount of free space that can be reclaimed. If this amount is greater than or equal to the minimum size for a free chunk, it is reclaimed and added to the free space pool. The minimum size for a free chunk is currently defined as 512 bytes on 32-bit platforms, and 768 bytes on 64-bit platforms.

The small areas of storage that are not on the freelist are known as "dark matter", and they are recovered when the objects that are next to them become free, or when the heap is compacted. It is not necessary to free the individual objects in the free chunk, because it is known that the whole chunk is free storage. When a chunk is freed, the GC has no knowledge of the objects that were in it.

• Parallel bitwise sweep
  Parallel bitwise sweep improves the sweep time by using available processors. In parallel bitwise sweep, the Garbage Collector uses the same helper threads that are used in parallel mark, so the default number of helper threads is also the same and can be changed with the -Xgcthreads option.
• Concurrent sweep
  Like concurrent mark, concurrent sweep gives reduced garbage collection pause times when heap sizes increase. Concurrent sweep starts immediately after a stop-the-world (STW) collection, and must at least finish a certain subset of its work before concurrent mark is allowed to kick off, because the mark map used for concurrent mark is also used for sweeping.

Parent: Global garbage collection

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