What is "Compile Time" in VisualGC?

Question

VisualVM has an official plugin called "VisualGC". In the VisualGC tab, there is a "Compile Time" meter displaying number of compiles and time spent.

The meter shows up on any OS with OpenJDK JVM 6+, Oracle Hotspot JVM 6+ no matter what GC method/parameters are specified.

What does the "Compile Time" mean?

Does it become a concern if the compile "time spent" shows many minutes?

Answer 1

Checking Oracle documentation on VisualGC:

Compile Time This panel plots the amount of time spent compiling Java byte codes into native code. The height of this display is not scaled to any particular value. A non zero value in this graph indicates that compilation activity occurred during the last interval. A narrow pulse indicates a relatively short duration and a wide pulse indicates a long duration. The title bar indicates the number of compilation tasks and the accumulated compilation time since the start of the application.

It makes sense as visualgc monitor the JVM. As you probably know, one of the JVM main tasks is to "translate" the virtual machine (byte code) to your concrete architecture instructions.

In my machine I have observed that this time is almos always zero since my JVM seems to use JIT.

Answer 2

According to the Oracle documentation here compile time shows "...the amount of time spent compiling Java byte codes into native code" therefore the more new code is being compiled the higher it gets.