Could someone calrify reserving and collecting memory via the Garbabe Collector ?

[wjoe via Digitalmars-d-learn]

There's core.memory.GC.reserve which requests memory from the OS. 
Basically pre-allocating memory for the GC heap.

Is the GC heap shared among all threads ?
E.g what happens if I GC.reserve(4.MiB) ? Is it 4 MiB in total or 
per thread ?

And is it correct that even if I call GC.disable, the GC may 
still start a collection run if, for instance, there's an 
allocation but no free memory on the GC heap ?