IBM User Guide for Java V7 on Windows > IBM SDK for Java > The collector and the allocator
Global garbage collection
Garbage collection is performed when an allocation failure occurs in heap lock allocation, or if a specific call to System.gc() occurs. The thread that has the allocation failure or the System.gc() call takes control and performs the garbage collection.The first step in garbage collection is to acquire exclusive control on the Virtual machine to prevent any further Java operations. Garbage collection then goes through the three phases:
- mark
- sweep
- compaction (if required)
The IBM Garbage Collector (GC) is a stop-the-world (STW) operation, because all application threads are stopped while the garbage is collected.
A global garbage collection occurs only in exceptional circumstances when using the Balanced Garbage Collection policy. Circumstances that might cause this rare event include:
- A System.gc() call.
- A request by tooling.
- A combination of heap size, occupied heap memory, and collection rates that cannot keep up with demand.
See
- Mark phase
- Sweep phase
- Compaction phase
- Reference objects
- Final reference processing
- JNI weak reference
- Heap expansion
- Heap shrinkage
Parent: The collector and the allocator
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