Java HotSpot Garbage Collection

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The Garbage-First Garbage Collector

 

Introduction

The Garbage-First (G1) garbage collector is fully supported in Oracle JDK 7 update 4 and later releases. The G1 collector is a server-style garbage collector, targeted for multi-processor machines with large memories. It meets garbage collection (GC) pause time goals with high probability, while achieving high throughput. Whole-heap operations, such as global marking, are performed concurrently with the application threads. This prevents interruptions proportional to heap or live-data size.

Technical description

The G1 collector achieves high performance and pause time goals through several techniques.

The heap is partitioned into a set of equal-sized heap regions, each a contiguous range of virtual memory. G1 performs a concurrent global marking phase to determine the liveness of objects throughout the heap. After the mark phase completes, G1 knows which regions are mostly empty. It collects in these regions first, which usually yields a large amount of free space. This is why this method of garbage collection is called Garbage-First. As the name suggests, G1 concentrates its collection and compaction activity on the areas of the heap that are likely to be full of reclaimable objects, that is, garbage. G1 uses a pause prediction model to meet a user-defined pause time target and selects the number of regions to collect based on the specified pause time target.

The regions identified by G1 as ripe for reclamation are garbage collected using evacuation. G1 copies objects from one or more regions of the heap to a single region on the heap, and in the process both compacts and frees up memory. This evacuation is performed in parallel on multi-processors, to decrease pause times and increase throughput. Thus, with each garbage collection, G1 continuously works to reduce fragmentation, working within the user defined pause times. This is beyond the capability of both the previous methods. CMS (Concurrent Mark Sweep ) garbage collection does not do compaction. ParallelOld garbage collection performs only whole-heap compaction, which results in considerable pause times.

It is important to note that G1 is not a real-time collector. It meets the set pause time target with high probability but not absolute certainty. Based on data from previous collections, G1 does an estimate of how many regions can be collected within the user specified target time. Thus, the collector has a reasonably accurate model of the cost of collecting the regions, and it uses this model to determine which and how many regions to collect while staying within the pause time target.

For more further information about using and configuring G1 please see the command line options.

Recommended Use Cases for G1

The first focus of G1 is to provide a solution for users running applications that require large heaps with limited GC latency. This means heap sizes of around 6GB or larger, and stable and predictable pause time below 0.5 seconds.

Applications running today with either the CMS or the ParallelOld garbage collector would benefit switching to G1 if the application has one or more of the following traits.

  • More than 50% of the Java heap is occupied with live data.
  • The rate of object allocation rate or promotion varies significantly.
  • Undesired long garbage collection or compaction pauses (longer than 0.5 to 1 second)

Future

G1 is planned as the long term replacement for the Concurrent Mark-Sweep Collector (CMS). Comparing G1 with CMS, there are differences that make G1 a better solution. One difference is that G1 is a compacting collector. G1 compacts sufficiently to completely avoid the use of fine-grained free lists for allocation, and instead relies on regions. This considerably simplifies parts of the collector, and mostly eliminates potential fragmentation issues. Also, G1 offers more predictable garbage collection pauses than the CMS collector, and allows users to specify desired pause targets.

If you are interested in helping improving G1, please try it and give feedback via OpenJDK and the hotspot-gc-use@openjdk.java.net mailing list.

Resources

  • Description of HotSpot GCs: Memory Management in the Java HotSpot Virtual Machine White Paper: link
  • The original CMS paper: Printezis, T. and Detlefs, D. 2000. A generational mostly-concurrent garbage collector. In Proceedings of the 2nd international Symposium on Memory Management (Minneapolis, Minnesota, United States, October 15 - 16, 2000). http://portal.acm.org/citation.cfm?id=362422.362480 (requires access to ACM's portal)
  • The original G1 paper: Detlefs, D., Flood, C., Heller, S., and Printezis, T. 2004. Garbage-first garbage collection. In Proceedings of the 4th international Symposium on Memory Management (Vancouver, BC, Canada, October 24 - 25, 2004). http://portal.acm.org/citation.cfm?id=1029879 (requires access to ACM's portal)
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