How does a JVM process allocate its memory?

Approach:

You may want to use Java HotSpot Native Memory Tracking (NMT).

This may give you an exact list of memory allocated by the JVM, splitted up into the different areas heap, classes, threads, code, GC, compiler, internal, symbols, memory tracking, pooled free chunks, and unknown.

Usage:

• You can start your application with -XX:NativeMemoryTracking=summary.

• Observations of the current heap can be done with jcmd <pid> VM.native_memory summary.

Where to find jcmd / pid:

On a default OpedJDK installation on Ubuntu this can be found at /usr/bin/jcmd.

By just running jcmd without any parameter, you get a list of running Java applications.

user@pc:~$ /usr/bin/jcmd5169 Main                       <-- 5169 is the pid

Output:

You will then receive a complete overview over your heap, looking something like the following:

Total: reserved=664192KB, committed=253120KB <--- total memory tracked by Native Memory Tracking

• Java Heap (reserved=516096KB, committed=204800KB) <--- Java Heap

  (mmap: reserved=516096KB, committed=204800KB)

• Class (reserved=6568KB, committed=4140KB) <--- class metadata

  (classes #665) <--- number of loaded classes

  (malloc=424KB, #1000) <--- malloc'd memory, #number of malloc

  (mmap: reserved=6144KB, committed=3716KB)

• Thread (reserved=6868KB, committed=6868KB) (thread #15) <--- number of threads

  (stack: reserved=6780KB, committed=6780KB) <--- memory used by thread stacks

  (malloc=27KB, #66)

  (arena=61KB, #30) <--- resource and handle areas

• Code (reserved=102414KB, committed=6314KB)

  (malloc=2574KB, #74316)

  (mmap: reserved=99840KB, committed=3740KB)

• GC (reserved=26154KB, committed=24938KB)

  (malloc=486KB, #110)

  (mmap: reserved=25668KB, committed=24452KB)

• Compiler (reserved=106KB, committed=106KB)

  (malloc=7KB, #90)

  (arena=99KB, #3)

• Internal (reserved=586KB, committed=554KB)

  (malloc=554KB, #1677)

  (mmap: reserved=32KB, committed=0KB)

• Symbol (reserved=906KB, committed=906KB)

  (malloc=514KB, #2736)

  (arena=392KB, #1)

• Memory Tracking (reserved=3184KB, committed=3184KB)

  (malloc=3184KB, #300)

• Pooled Free Chunks (reserved=1276KB, committed=1276KB)

  (malloc=1276KB)

• Unknown (reserved=33KB, committed=33KB)

  (arena=33KB, #1)

This gives a detailed overview of the different memory areas used by the JVM, and also shows the reserved and commited memory.

I don't know of a technique that gives you a more detailed memory consumption list.

Further reading:

You can also use -XX:NativeMemoryTracking=detail in combination with further jcmd commands. A more detailed explaination can be found at Java Platform, Standard Edition Troubleshooting Guide - 2.6 The jcmd Utility. You can check possible commands via "jcmd <pid> help"