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

Balanced Garbage Collection policy

The Balanced Garbage Collection policy uses a region-based layout for the Java™ heap. These regions are individually managed to reduce the maximum pause time on large heaps, and also benefit from Non-Uniform Memory Architecture (NUMA) characteristics on modern server hardware.

The Balanced Garbage Collection policy is intended for environments where heap sizes are greater than 4 GB. The policy is available only on 64-bit platforms. You activate this policy by specifying -Xgcpolicy:balanced on the command line.

The Java heap is split into potentially thousands of equal sized areas called "regions". Each region can be collected independently, which allows the collector to focus only on the regions which offer the best return on investment.

Objects are allocated into a set of empty regions that are selected by the collector. This area is known as an eden space. When the eden space is full, the collector stops the application to perform a Partial Garbage Collection (PGC). The collection might also include regions other than the eden space, if the collector determines that these regions are worth collecting. When the collection is complete, the application threads can proceed, allocating from a new eden space, until this area is full. This process continues for the life of the application.

From time to time, the collector starts a Global Mark Phase (GMP) to look for more opportunities to reclaim memory. Because PGC operations see only subsets of the heap during each collection, abandoned objects might remain in the heap. This issue is like the "floating garbage" problem seen by concurrent collectors. However, the GMP runs on the entire Java heap and can identify object cycles that are inactive for a long period. These objects are reclaimed.

• Region age
  Age is tracked for each region in the Java heap, with 24 possible generations.
• NUMA awareness
  The Balanced Garbage Collection policy can increase application performance on large systems that have Non-Uniform Memory Architecture (NUMA) characteristics.
• Partial Garbage Collection
  A Partial Garbage Collection (PGC) reclaims memory by using either a Copy-Forward or Mark-Compact operation on the Java heap.
• Global Mark Phase
  A Global Mark Phase (GMP) takes place on the entire Java heap, finding, and marking abandoned objects for garbage collection.
• When to use the Balanced garbage collection policy
  There are a number of situations when you should consider using the Balanced garbage collection policy. Generally, if you are currently using the Gencon policy, and the performance is good but the application still experiences large global collection (including compaction) pause times frequently enough to be disruptive, consider using the Balanced policy.

Parent: The collector and the allocator

Error 404 - Not Found

The document you are looking for may have been removed or re-named. Please contact the web site owner for further assistance.