Right but the problem we had was that when you fired up the debugger (GDB) you could only see the stack traces of the currently active user level thread on each

Right but the problem we had was that when you fired up the debugger (GDB) you could only see the stack traces of the currently active user level thread on each host thread. To understand what was going on in other user threads (e.g. who is holding what locks) you had to manually go into the scheduler data structures to find the descheduled threads etc, which was extremely tedious. If that all happens out of the box for Loom then that is great.