jmap tool works only as root and output columns are not clear

Question

Using jmap on Ubuntu Mate 18.04-64bits with Oracle JDK 10.0.1-64bits, the tool works only when running both the target and the tool as root, but using the same normal user for running both gives the following error:

Exception in thread "main" com.sun.tools.attach.AttachNotSupportedException: Unable to open socket file /proc/13538/cwd/.attach_pid13538: target process 13538 doesn't respond within 10500ms or HotSpot VM not loaded
at jdk.attach/sun.tools.attach.VirtualMachineImpl.<init>(VirtualMachineImpl.java:103)
at jdk.attach/sun.tools.attach.AttachProviderImpl.attachVirtualMachine(AttachProviderImpl.java:58)
at jdk.attach/com.sun.tools.attach.VirtualMachine.attach(VirtualMachine.java:207)
at jdk.jcmd/sun.tools.jmap.JMap.executeCommandForPid(JMap.java:124)
at jdk.jcmd/sun.tools.jmap.JMap.main(JMap.java:114)

When using root user to run the following command

jmap -clstats <pid>

everything works fine but I found some difficulties to understand the meaning of the output columns: Is there any official documentation that explains the meaning of each column ?

Hainan

This is the only info available: -clstats Prints class loader wise statistics of Java heap. For each class loader, its name, how active it is, address, parent class loader, and the number and size of classes it has loaded are printed. – Yettie

Answer 1

By running this command, one could expect output related to ClassLoaders, but it was modified in JDK8 to print result of jcmd {pid} GC.class_stats command. Some details can be found in JDK-8010507 and JDK-8195682 issues.

As for the output - there is no more documentation than this one. Some description can be found in OpenJDK VM source code, heapInspection.cpp file. I don't find this output too useful, but here some explanation (based on description from this header, and Java class format description):

• Index: index of this class.
• Super: index of super class. If -1, then there is no super class (happens for example for array types)
• InstBytes: number of bytes occupied by all instances of the class (in bytes).
• KlassBytes: number of bytes occupied by the class itself (in bytes). (Size of the InstanceKlass or ArrayKlass for this class.)
• annotations: Size of all annotations (in bytes)
• CpAll: Size of all parts of Constants Pool (Sum of Cp + CpTags + CpCache + CpOperands + CpRefMap)
• MethodCount: number methods in this class (including constructors)
• Bytecodes: size occupied by bytecode commands in the class
• MethodAll: Sum of all space occupied by method and its metadata (MethodBytes + Constmethod + Stackmap + Methoddata)
• ROAll: Size of all class meta data that could (potentially) be placed in read-only memory. (This could change with CDS design)
• RWAll: Size of all class meta data that must be placed in read/write memory. (This could change with CDS design)
• Total: ROAll + RWAll. Note that this does NOT include InstBytes (so no space occupied by instances)
• ClassName: fully qualified class name.

Hope it helps.

Erythroblast

Thanks really for your answer, it's very useful. – Hainan

What do Cp, CpTags, CpCache, CpOperands, CpRefMap all mean? – Scheming