Eliot Miranda gave a great explanation about the MMU metric for GC.
An important part of Urs Hölzle's PhD thesis about his Self 3 implementation was the development of a similar metric for evaluating pauses caused by adaptive compilation. Craig Chambers had previously done a simple implementation (Self 1) and then a much more aggressive one (Self 2) that had very impressive benchmarks (half the speed of optimized C on numeric code while offering lots os safe features, like overflow handling, that C doesn't have). Unfortunately Self 2 was not as usable interactively as Self 1. When you pressed the mouse button you would have to hold it for a few seconds before the menu popped up. But the response to the second mouse click was extremely fast.
http://hoelzle.org/publications/urs-thesis.pdf
See chapter 9.
-- Jecel