Whippet's bdw collector is backed by a third-party garbage collector,
the Boehm-Demers-Weiser collector.
BDW-GC is a mark-sweep collector with conservative root-finding, conservative heap tracing, and parallel tracing.
Whereas the other Whippet collectors which rely on mutators to
periodically check if they need to
stop,
bdw will stop mutators with a POSIX signal. Also, it doesn't really
support ephemerons (the Whippet bdw collector simulates them using
finalizers), and both ephemerons and finalizers only approximate the
Whippet behavior, because they are implemented in terms of what BDW-GC
provides.
bdw supports the fixed and growable heap-sizing policies, but not
adaptive, as BDW-GC can't reliably return memory to the OS. Also,
growable has an effective limit of a 3x heap
multiplier.
Oh well!
It's a bit of an oddball from a Whippet perspective, but useful as a migration path if you have an embedder that is already using BDW-GC. And, it is a useful performance comparison.