Reading a Java GC log: every number explained
Turning on the GC log is easy. Reading it is harder: one G1 line holds about ten pieces of information, and the words change from one Java version to the next. This post reads real lines, written by three small programs of the public repository java-gc-lab: a normal load, very big objects, and a heap that is almost full. Turn on the log java -Xlog:gc*:file=gc.log:time,uptime,level,tags -jar app.jar gc*: all the GC messages (gc alone gives one line per GC); file=gc.log: in a file, with rotation built in (5 files of about 20 MB by default); time,uptime,level,tags: the “decorations” at the start of each line: the date and time, the time since the JVM started, the level and the tags. If you set nothing, the JVM uses uptime, level and tags. A pause line, with these decorations: ...