IBM User Guide for Java V7 on Windows > IBM SDK for Java > The collector and the allocator > Balanced Garbage Collection policy

Global Mark Phase

A Global Mark Phase (GMP) takes place on the entire Java™ heap, finding, and marking abandoned objects for garbage collection.

A GMP runs independently between Partial Garbage Collections (PGCs). Although the GMP runs incrementally, like the PGC, the GMP runs only a mark operation. However, this mark operation takes place on the entire Java heap, and does not make any decisions at the region level. By looking at the entire Java heap, the GMP can see more abandoned objects than the PGC might be aware of. The GMP does not start and finish in the same "stop-the-world" operation, which might lead to some objects being kept alive as "floating garbage". However, this waste is bounded by the set of objects that died after a given GMP started.

GMP also performs some work concurrently with the application threads. This concurrent mark operation is based purely on background threads, which allows idle processors to complete work, no matter how quickly the application is allocating memory. This concurrent mark operation is unlike the concurrent mark operations that are specified with -Xgcpolicy:gencon or -Xgcpolicy:optavgpause. For more information about the use of concurrent mark with these options, see Concurrent mark.

When the GMP completes, the data that the PGC process is maintaining is replaced. The next PGC acts on the latest data in the Java heap.

This diagram shows that the GMP live object set is a subset of the PGC live object set when the GMP completes:

When the GMP replaces the data for use by the PGC operation, the next PGC uses this smaller live set for more aggressive collection. This process enables the GMP to clear all live objects in the GMP set, ready for the next global mark:

Parent: Balanced Garbage Collection policy

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