Evacuation and Collection
This step is what it's all about -- reclaiming dead objects and shaping the heap for efficient object allocation. The collection set of regions (from the Concurrent Marking process defined above) forms a subset of the heap that is used during this process. When evacuation begins, all mutator threads are paused, and live objects are moved from their respective regions and compacted (moved together) into other regions. Although other garbage collectors might perform compaction concurrently with mutator threads, it's far more efficient to pause them. Since this operation is only performed on a portion of the heap -- it compacts only the collection set of regions -- it's a relatively quick, low-pause, operation. Once this phase completes, the GC cycle is complete.
To help limit the total pause time, much of the evacuation is done in parallel with multiple GC threads. The strategy for parallelization involves the following techniques:
- GC TLABS: The use of thread local allocation buffers (TLAB) for the GC threads eliminates memory-related contention amongst the GC threads. Forwarding pointers are inserted in the GC TLABs for evacuated live objects.
- Work Competition: GC threads compete to perform any of a number of GC-related tasks, such as maintaining remembered sets, root object scanning to determine reachability (dead objects are ignored), and evacuating live objects.
- Work Stealing: Part of mathematical systems theory, the work done by the GC threads is unsynchronized and executed arbitrarily by all of the threads simultaneously. This chaos-based algorithm equates to a group of threads that race to complete the list of GC-related tasks as quickly as they can without regard to one another. The end result, despite the apparent chaos, is a properly collected group of heap regions.
Note: The CMS and parallel collectors, described earlier, also use work competition and work stealing techniques to achieve greater efficiency.
The G1 collector is still considered experimental, but can be enabled in Java SE 6 Update 14 with the following two command-line parameters:
Much of the G1 processing and behavior can be controlled by explicitly setting optional command-line parameters. See the sidebar entitled "Tuning the G1 Collector" to tune G1 behavior.
Tuning the G1 Collector
Let's review some command-line parameters that enable you to tune the behavior of G1. For instance, to suggest a GC pause time goal, use the following parameter:
-XX:MaxGCPauseMillis=50 (for a target of 50 milliseconds)
With G1, a time interval can be specified during which a GC pause applies. In other words, no more than 50 milliseconds out of every second:
-XX:GCPauseIntervalMillis=1000 (for a target of 1000 milliseconds)
Of course, these are only targets and there are no guarantees they will be met in all situations. However, G1 will attempt to meet these targets where it can.
Alternatively, the size of the young generation can be specified explicitly to alter evacuation pause times:
-XX:+G1YoungGenSize=512m (for a 512 megabyte young generation)
To run G1 at its full potential, add the following two parameters:
However, Sun warns that as of this version of G1, the use of these parameters may produce in a race condition and result in an error. However, it's worth a try to see if your application works safely with them set. If so, you'll benefit from the best GC performance that G1 can offer.
In terms of GC pause times, Sun states that G1 is sometimes better and sometimes worse than CMS. As G1 is still under development, the goal is to make G1 perform better than CMS and eventually replace it in a future version of Java SE (the current target is Java SE 7). While the G1 collector is successful at limiting total pause time, it's still only a soft real-time collector. In other words, it cannot guarantee that it will not impact the application threads' ability to meet its deadlines, all of the time. However, it can operate within a well-defined set of bounds that make it ideal for soft real-time systems that need to maintain high-throughput performance.
If your application requires guaranteed real-time behavior even with garbage collection, your only choice is a real-time garbage collector such as those that come with Sun's Java RTS or IBM's WebSphere RT products. However, if low pause times and soft real-time behavior is your goal, the G1 collector should suit it well.
[Bruno09] Bruno, Eric, and Bollella, Greg, Real-Time Java Development with Java RTS, Pearson Publishing, 2009
[Detlefs04] Detlefs, et. al., Garbage-First Garbage Collection, Sun Microsystems Research Laboratories, 2004.
[McCarthy58] McCarthy, John, LISP: A Programming System for Symbolic Manipulations, Communications of the ACM, 1958.