Prometheus, how to get the actual Java Garbage Collector memory usage?

Question

Prometheus plugin in Springboot app is sending tons of data, I don't find any highlight to the meaning of what I get from the exporter:

1) What does "jvm_gc_memory_allocated_bytes_total" mean?
2) What does "jvm_gc_memory_promoted_bytes_total" mean?

What I need is the actual memory usage of the Java Garbage Collector, so I'm expecting a value which is always below 2GB (max memory size) but at the moment is 8GB and still raising.

"jvm_gc_memory_allocated_bytes_total"

and

"jvm_gc_memory_promoted_bytes_total"

are the only two Garbage Collector related variables delivered from the exporter.

Answer 1

To answer you questions, there's a help text provided with each exposed metric in Prometheus exposition format:

# HELP jvm_gc_memory_allocated_bytes_total Incremented for an increase in the size of the young generation memory pool after one GC to before the next

# HELP jvm_gc_memory_promoted_bytes_total Count of positive increases in the size of the old generation memory pool before GC to after GC

These metrics accumulate the allocated bytes in young generation and the promoted bytes which survived a garbage collection and thus they are promoted to the old generation. (very simplified)

From your question, I think you actually are not looking for "memory usage of the Java Garbage Collector" but actually for the managed memory usage of the JVM. These managed pieces are divided in "heap" and "non-heap" (the area tag) on a first level and can be further drilled down into by the id tag.

Here's the metrics you are likely looking for:

jvm_memory_used_bytes{area="heap|nonheap" id="<depends-on-gc-and-jvm>"}
jvm_memory_committed_bytes{area="heap|nonheap" id="<depends-on-gc-and-jvm>"}
jvm_memory_max_bytes{area="heap|nonheap" id="<depends-on-gc-and-jvm>"}

So if you want to get ahold of the currently used heap, you need to sum together the heap area metrics with the following PromQL:

sum(jvm_memory_used_bytes{job="myjob", instance="myhost", area="heap"})