jmap - Does histo & heap operation bring overhead to jvm?

Question

As the title states, how much overhead does jmap -histo and jmap -heap bring to a jvm respectively?

If a memory sensitive Java process is at the edge of OutOfMemory (e.g around 96% of heap is full, and can't be cleared by full gc), is it possible for one of the operations to bring the jvm to OutOfMemory?

Answer 1

jmap -histo and jmap -heap work differently: jmap -histo uses Dynamic Attach Mechanism, and jmap -heap works through HotSpot Serviceability Agent. The difference is described here.

Therefore, jmap -histo is executed by JVM itself, but jmap -heap runs in a tool process while JVM process is suspended. In both cases no new Java objects are created, the tool will not cause OutOfMemoryError.

In both cases application threads are stopped: jmap -histo stops Java threads, and jmap -heap stops the whole JVM process. The duration of the pause can be rather long especially for large heaps. E.g. it may take several seconds to walk through 4GB heap.