How to take a heap dump in windows with minimum downtime?

Question

I want to figure out why JVM heap usage on Elasticsearch node is staying consistently above 80%. In order to do this, I take a heap dump by running

jmap.exe -heap:format=b 5348

(5348 is the Process ID). Then I can analyze the dump with VisualVM.

The problem is that jmap pauses the JVM while taking the dump, so the node is basically offline for around 5 minutes.

This article suggests a faster approach that relies on taking coredump with gdb on Linux. I already tried WinDbg, which created a core dump, but I couldn't use it in VisualVM.

Is there a similar approach for Windows? How one can take heap dumps in seconds, not minutes?

Answer 1

After you've taken the coredump by WinDbg, you need to extract the heap dump from it by running

jmap -heap:format=b "C:\Program Files\Java\...\bin\java.exe" core.mdmp

This could be done offline; no interaction with running Java process needed. Then you will be able to open the generated heap.bin in VisualVM.

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Alternatively you may take the class histogram. It is produced a way faster than full heap dump.

jmap -histo <PID>

It shows you the list of classes whose instances occupy the most space in the heap. This information is often enough to get the idea of where's the memory lost.