D - Garbage Collection
- Jim Starkey <jas netfrastructure.com> Mar 21 2002
Once again, I just ran across D. Please pardon me if I walk on toes already trodden upon. A couple of ideas on garbage collection. First, many of the ideas of "modern" garbage collectors were formed when physical memory was much smaller than virtual memory. At the moment, however, max memory on a x86 < $1000. While a compacting garbage collector may save page faults on the 486 in your basement, it doesn't make much sense on a 2 GB machine. Slopping crud around wastes processor cycles that can be usedelsewhere. Probably. Second, it isn't necessary to stop program execution to do garbage collection. I havea very JVM garbage collector (OK, it's a retro-grade mark-and-sweep [see above]) that asks each thread to mark itself when convenient (it will mark threads known to be stalled), then waits for all threads to report completion. In the meantime, objects are created marked, and assignments into objects are marked. The net result is that no thread stalls waiting on garbage collection. Furthermore, on a multi-processor, marking of thread can take place in parallel on a multi-processor. All in all, the scheme takes a great deal of the pain out of garbage collection. I'm not going to insist that this technique is appropriate for D. I just want to remind folks that most thinking about garbage collection was done for Lisp umpteen generations ago. Before anything gets cast in concrete, fresh thinking would be a very good thing.
Mar 21 2002