I'm not sure whether pybench is the right tool for this.

Note that pybench disables GC per default for exactly the reasons
causing #4074 :-)

pybench already has a --with-gc switch, so it's possible to benchmark
with or without GC and see the differences.

Regarding memory management: The exact amount of used memory is hard to
determine from within a process due to the way e.g. Linux or other
modern OSes manage memory. We'd have to use special low-level system
APIs to access the true amount of allocated memory on each platform.