Thread: What is garbage collection log? How to enable & analyze?

Garbage collection has more profound impact on the application in contrary to what most engineers think. In order to optimize memory and garbage collection settings and to troubleshoot memory-related problems, one has to analyze Garbage Collection logs.

Enabling GC logs
GC Logging can be enabled by passing below-mentioned system properties during application startup

Until Java 8:

Below is the system property that is supported by all version of Java until JDK 8.

-XX:+PrintGCDetails -Xloggc:<gc-log-file-path>
 
Example:
 
-XX:+PrintGCDetails -Xloggc:/opt/tmp/myapp-gc.log

From Java 9:

Below is the system property that is supported by all version of Java starting from JDK 9.

-Xlog:gc*:file=<gc-log-file-path>
 
Example:
 
-Xlog:gc*:file=/opt/tmp/myapp-gc.log

How to analyze GC logs?

Here is a sample GC log generated when above system properties were passed:

what-is-gc.jpg

GC log has rich information, however, understanding GC log is not easy. There isn’t sufficient documentation to explain GC log format. On top of it, GC log format is not standardized. It varies by JVM vendor (Oracle, IBM, HP, Azul, …), Java version (1.4, 5, 6, 7, 8, 9), GC algorithm (Serial, Parallel, CMS, G1, Shenandoah), GC system properties that you pass (-XX:+PrintGC, -XX:+PrintGCDetails, -XX:+PrintGCDateStamps, -XX:+PrintHeapAtGC …). Based on this permutation and combination, there are easily 60+ different GC log formats.

Thus, to analyze GC logs, it’s highly recommended to use GC log analysis tools such as GCeasy, HPJmeter. These tools parse GC logs and generate great graphical visualizations of data, reports Key Performance Indicators and several other useful metrics.