HotSpot JVM Collectors

I was curious about how different JVM garbage collector command line options mapped to actual selections in the virtual machine at runtime. Note: all of my testing was using Oracle JDK 1.8.0_60, but the update number (60 in this case) shouldn't matter.

According to Oracle's GC Tuning Guide, these are the available GC selection command line options:

• -XX:+UseConcMarkSweepGC
• -XX:+UseG1GC
• -XX:+UseParallelGC
• -XX:+UseParallelOldGC
• -XX:+UseParNewGC and -XX:-UseParNewGC
• -XX:+UseSerialGC

For example, you'd use the following options with the java executable for selecting the CMS collector:

java -XX:+UseConcMarkSweepGC -XX:+UseParNewGC

And the JVM would select the ParNew young and ConcurrentMarkSweep old generation collectors, as named by the GarbageCollectorMXBean:

ParNew, ConcurrentMarkSweep

GraphCollectors is a short program that outputs a directed graph using the DOT graph language. It creates combinations of GC options, and uses them to construct a command line for starting an external JVM process that prints actual collector settings to stdout.

Running the program

Use the following Maven Wrapper command line to run the program, which outputs a graph to stdout:

$ mvnw compile exec:exec

Rendering the graph

You'll need to download and install Graphviz here.

Render the graph using the following command line:

$ dot -Tpng collectors.gv > collectors.png

Results

There are 7 combinations of GC command line options, mapping to 6 young and old collectors at runtime. This means, for example, that it's not strictly necessary to pass -XX:+UseConcMarkSweepGC -XX:+UseParNewGC to select the ParNew collector: the JVM will do that by default!

References

1. HotSpot JVM GC Tuning Guide

2. MXBean Javadoc

3. GarbageCollectorMXBean Javadoc

4. A great blog post by Jon Masamitsu