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Showing content from https://docs.oracle.com/en/java/javase/21/gctuning/garbage-collector-implementation.html below:

Garbage Collector Implementation

Throughput and footprint are best measured using metrics particular to the application.

For example, the throughput of a web server may be tested using a client load generator. However, pauses due to garbage collection are easily estimated by inspecting the diagnostic output of the virtual machine itself. The command-line option -verbose:gc prints information about the heap and garbage collection at each collection. Here is an example:

[15,651s][info ][gc] GC(36) Pause Young (G1 Evacuation Pause) 239M->57M(307M) (15,646s, 15,651s) 5,048ms
[16,162s][info ][gc] GC(37) Pause Young (G1 Evacuation Pause) 238M->57M(307M) (16,146s, 16,162s) 16,565ms
[16,367s][info ][gc] GC(38) Pause Full (System.gc()) 69M->31M(104M) (16,202s, 16,367s) 164,581ms

The output shows two young collections followed by a full collection that was initiated by the application with a call to System.gc(). The lines start with a time stamp indicating the time from when the application was started. Next comes information about the log level (info) and tag (gc) for this line. This is followed by a GC identification number. In this case, there are three GCs with the numbers 36, 37, and 38. Then the type of GC and the cause for stating the GC is logged. After this, some information about the memory consumption is logged. That log uses the format "used before GC" -> "used after GC" ("heap size").

In the first line of the example this is 239M->57M(307M), which means that 239 MB were used before the GC and the GC cleared up most of that memory, but 57 MB survived. The heap size is 307 MB. Note in this example that the full GC shrinks the heap from 307 MB to 104 MB. After the memory usage information, the start and end times for the GC are logged as well as the duration (end - start).

The -verbose:gc command is an alias for -Xlog:gc. -Xlog is the general logging configuration option for logging in the HotSpot JVM. It's a tag-based system where gc is one of the tags. To get more information about what a GC is doing, you can configure logging to print any message that has the gc tag and any other tag. The command line option for this is -Xlog:gc*.

Here's an example of one G1 young collection logged with -Xlog:gc* :

[10.178s][info][gc,start     ] GC(36) Pause Young (G1 Evacuation Pause) 
[10.178s][info][gc,task      ] GC(36) Using 28 workers of 28 for evacuation 
[10.191s][info][gc,phases    ] GC(36) Pre Evacuate Collection Set: 0.0ms
[10.191s][info][gc,phases    ] GC(36) Evacuate Collection Set: 6.9ms 
[10.191s][info][gc,phases    ] GC(36) Post Evacuate Collection Set: 5.9ms 
[10.191s][info][gc,phases    ] GC(36) Other: 0.2ms 
[10.191s][info][gc,heap      ] GC(36) Eden regions: 286->0(276) 
[10.191s][info][gc,heap      ] GC(36) Survivor regions: 15->26(38)
[10.191s][info][gc,heap      ] GC(36) Old regions: 88->88 
[10.191s][info][gc,heap      ] GC(36) Humongous regions: 3->1 
[10.191s][info][gc,metaspace ] GC(36) Metaspace: 8152K->8152K(1056768K)
[10.191s][info][gc           ] GC(36) Pause Young (G1 Evacuation Pause) 391M->114M(508M) 13.075ms 
[10.191s][info][gc,cpu       ] GC(36) User=0.20s Sys=0.00s Real=0.01s

Note:

The format of the output produced by

is subject to change in future releases.

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