[gclist] Name that hypothesis

William D Clinger will@ccs.neu.edu
Thu, 05 Dec 1996 13:22:20 +0000

Henry Baker wrote, in excerpts from two messages:

> If the 'null hypothesis' is that the likelihood of
> dying is completely independent of how long you have lived already,
> then the 'generational hypothesis' is one that says that young objects
> die even more often than would be expected by this null hypothesis.
> Now second order effects -- e.g., cache locality -- might allow you
> to take advantage of even the 'null hypothesis', but to win better than
> this you will need the 'generational hypothesis'.

> I didn't mean to imply that the curve had to have discontinuities, but
> that it had to have a different shape/slope in some places than the
> curve for the 'null hypothesis'.

The claims Henry is making here are commonly believed, but
they are not true unless you take a very narrow view of
generational garbage collection.  I have designed, simulated,
and analyzed a generational collector that derives a clear
first order advantage over non-generational collectors for
the radioactive decay model Henry proposed in SIGPLAN Notices,
April 1993.  This advantage can be demonstrated with a
perfectly uniform memory system.

Henry's claims are probably true of all generational collectors
that are being used for production work in 1996.  I just want to
warn that Henry is talking about current practice, not theory.

William D Clinger