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          


        

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 

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.