G1 (and other collectors) do not provide a way to retrieve promotion rates (the number of total promoted bytes) during garbage collection in an easily obtainable way.

Currently tools try to infer this from existing measurements like before/after young/survivor gen/heap size, which is very coarse.

Further it would be nice to have this value split up by source and destination (survivor/old), particularly in G1.
I.e. what amount of promotions from eden is copied into survivor or old, what amount of data from survivor to survivor or old, and what amount from old->old.