After OOME happens inside jmap it hangs completly:

Attaching to process ID 5698, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 1.5.0-b64
finding class loader instances ..Unknown oop at 0x4592a520
Oop's klass is 0x4666bef0
Unknown oop at 0x459ec648
Oop's klass is 0x4e883334
done.
computing per loader stat ..done.
please wait.. computing liveness............................Exception in thread "Thread-1" java.lang.OutOfMemoryError: Java heap space

Moreover, after killing jmap debugee process becomes paused forever.