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Problem :

I had to run the jmap so that I can take the heap dump of my process. But the jvm returned below error :

Unable to open socket file: target process not responding or HotSpot VM not loaded

The -F option can be used when the target process is not responding

So I tried using the -F:

./jmap -F -dump:format=b,file=heap.bin 10330

Attaching to process ID 10331, please wait...

Debugger attached successfully.

Server compiler detected.

JVM version is 24.51-b03

Dumping heap to heap.bin ...

1.      Using -F is correct approach to take the heap dump?

2.      I am waiting for more than 20 minutes and not finished yet. Any ideas why it is taking so long?

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1 Answer

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Solution :

jmap -F OR  jstack -F

While trying to run with -F the tools switch to the special mode that features the HotSpot Serviceability Agent. In current mode the target process is frozen; the tools often read its memory via the OS debugging facilities, namely, ptrace on the Linux.

1.      jmap -F invokes the PTRACE_ATTACH on your target JVM. Your target process is unconditionally suspended in response to the SIGSTOP signal.

2.      The tool reads the JVM memory using PTRACE_PEEKDATA. Here ptrace can read only one word at the time, so too many calls are required to read the large heap of your target process. This is too slow.

3.      The tool reconstructs your JVMs internal structures based on the knowledge of the particular JVMs version. As different versions of JVM have the different memory layout, your -F mode works only if the jmap comes from the same JDK as that of the target Java process.

4.      The tool will create heap dump itself and after that resumes the target process.

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