How can I get the max heap size of a Java process?

Question

I want to check whether my application gets -Xmx option well.

The java process is run in a docker container.

In order to do that, I am to find the max heap size of the created process.

At first, I tried jmap command, but it occurs an error:

bash-4.2$ jmap -heap 128
Attaching to process ID 128, please wait...
Error attaching to process: sun.jvm.hotspot.debugger.DebuggerException: Can't attach to the process: ptrace(PTRACE_ATTACH, ..) failed for 128: Operation not permitted
sun.jvm.hotspot.debugger.DebuggerException: sun.jvm.hotspot.debugger.DebuggerException: Can't attach to the process: ptrace(PTRACE_ATTACH, ..) failed for 128: Operation not permitted
        at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.execute(LinuxDebuggerLocal.java:163)
        at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.attach(LinuxDebuggerLocal.java:278)
        at sun.jvm.hotspot.HotSpotAgent.attachDebugger(HotSpotAgent.java:671)
        at sun.jvm.hotspot.HotSpotAgent.setupDebuggerLinux(HotSpotAgent.java:611)
        at sun.jvm.hotspot.HotSpotAgent.setupDebugger(HotSpotAgent.java:337)
        at sun.jvm.hotspot.HotSpotAgent.go(HotSpotAgent.java:304)
        at sun.jvm.hotspot.HotSpotAgent.attach(HotSpotAgent.java:140)
        at sun.jvm.hotspot.tools.Tool.start(Tool.java:185)
        at sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)
        at sun.jvm.hotspot.tools.HeapSummary.main(HeapSummary.java:49)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at sun.tools.jmap.JMap.runTool(JMap.java:201)
        at sun.tools.jmap.JMap.main(JMap.java:130)
Caused by: sun.jvm.hotspot.debugger.DebuggerException: Can't attach to the process: ptrace(PTRACE_ATTACH, ..) failed for 128: Operation not permitted
        at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.attach0(Native Method)
        at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.access$100(LinuxDebuggerLocal.java:62)
        at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$1AttachTask.doit(LinuxDebuggerLocal.java:269)
        at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.run(LinuxDebuggerLocal.java:138)

I have no idea why jmap command fails. I searched for the same issues, but none of them helped me.

• What I tried:
  • run the command with the user who created the process
  • run a command outside docker container docker exec container_id jcmd PID GC.heap_dump /tmp/docker.hprof

And I cannot modify the existing java code, so I need to do this with CLI.

jstat command works fine, so is there a way to check jvm max heap size with jstat command?

Is it okay to assume (NGCMX+OGCMX) = ( -Xmx value )?

https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstat.html

Answer 1

You could use the jcmd tool:

 jcmd <you_vm_process_id> VM.flags | grep MaxHeapSize

And that will show the maximum heap (even if you do not set it yourself)