D runtime Garbage Collector details

[Vadim]

I am looking for the details on D Garbage Collection 
implementation. I will much appreciate if someone suggests the 
answers or give some links to existing documentation clarifying 
the following points:

1. Is it Mark-n-Sweep or copy or generational collector or simple 
reference counting? Or anything else? Any documentation on this 
would be very helpful

2. Sun/Oracle JVM publishes a number of counters in shared memory 
so that user may easily monitor the memory usage and the GC 
statistics real-time without affecting the application (and 
without modifying the application). Is there anything similar for 
D GC?

3. Is there any performance tests of D GC efficiency? Is it 
possible to write GC-predictable code (without manual 
allocate/free) or at least do something like System.gc()?