© 2004 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice Minimally Conservative Garbage Collection Hans-J. Boehm HP Labs

The problem Type accurate collectors −Less space overhead. −Better chance of space guarantees. −But require either Per instruction pointer information, or Safe point overheads Conservative collectors −No safe points. −No type information, or −Less type information (e.g. Barabash et al.) −Typically some space overhead (Hirzel, Diwan, Henkel). −Unbounded overhead possible, though unlikely. With limited conservativism requires an extra testing step.

A solution? Maintain precise pointer location info for −Heap objects. −Static Data. −Stack frames and registers, but only At call sites. For locations that always contain non-pointers or dead data during innermost loop. Type information doesn’t track rapidly changing information  much less of it. Can hope to guarantee that the probability of unbounded long term retention is zero. −Only pointer misidentification is for locations rapidly changing between pointer and non-pointer values. −With random scheduling, the probability of continuously observing the same false pointer should be zero.

Remaining issues What about rarely executed code in inner loops? −Locations may almost always contain non-pointers, but static descriptors are not constant in loop. −Treat as branches out of, and back into loop? But requires profile information? Cliff Click observed that we may need safe points for other reasons. −But for many systems we don’t. Nobody has actually implemented this. −Barabash et al., “Mostly Accurate Stack Scanning”, JVM 01 came closest.