MPS question: small object allocation

[Daniel Colascione <dancol@dancol.org>]

I'm catching up on the MPS branch.  I'm super excited to see Emacs get a
modern GC.  (I regret that I never found time to finish my
attempt from a few years ago.) It looks like MPS supports both manual
memory management by pool and automatic garbage collection.  Is there
anything that might help us deal with the problem of having tons of
small objects?  In a traditional JVM-style GC, each object begins with a
type-word of some sort (sometimes inflated for locking, etc.)  This
approach is inefficient for small objects: a cons cell, for example,
would have 50% overhead, because it'd have to have one machine word for
the type, then two machine words for car and cdr.  Does MPS have a way
to eliminate this per-object overhead for objects like cons cells,
markers, and so on?