Which GC Algorithm Is Being Used by a Specific JVM Instance?

Using the monitoring tools provided by the JDK, we can gain insights into the JVM itself. One such insight that may come useful in some situations is the GC algorithm used by a specific JVM instance. First off, we should somehow identify that JVM instance. In order to do that, we can use the jps utility to find all the running JVMs in the current machine or a machine specified with a host-id. 

For example, running the following command in my local machine:

jps -l

Would produce the following result:

2912 org.jetbrains.jps.cmdline.Launcher
1985
2913 me.alidg.Application
2915 sun.tools.jps.Jps
2175 org.jetbrains.idea.maven.server.RemoteMavenServer

As you can spot from the output, the Process Id for my trivial Spring Boot app running with the me.alidg.Application main entry is 2913. You also can find this process id using the typical ps (or whatever OS-specific tool you have at your disposal) but it needs more filtering to find your desired JVM instance.

After finding the process id of the JVM instance, you can use the jmap utility. Basically, jmapwill provide heap dumps and other information about JVM memory usage:

jmap [option] <pid>

So knowing that our pid is 2913, we can use the following command to print a Java heap summary:

jmap -heap 2913

The output of this command would be:

Attaching to process ID 2913, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.121-b13

using parallel threads in the new generation.
using thread-local object allocation.
Concurrent Mark-Sweep GC
// truncated

As you can spot from the output, this JVM instance is using the Concurrent Mark-Sweep GC or CMS GC algorithm. If I run my Spring Boot app with the -XX:+UseG1GC flag, the output for the same command would be:

Attaching to process ID 2961, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.121-b13

using thread-local object allocation.
Garbage-First (G1) GC with 8 thread(s)
// truncated

jmap -heap also will provide information about the JIT compiler, heap configuration, Interned strings statistics and Heap usage.