SRC Research Report 64
SRC Research Report 64
Garbage collection is an integral component of Modula-2+, the principal systems programming language at SRC. The initial Modula-2+ collector was a concurrent reference-counting collector; it did not reclaim cyclic structures, and the cost of assigning references was relatively high.
I implemented three experimental collectors for Modula-2+ and tested them to explore alternatives to the initial collector: first a simple concurrent mark-and-sweep collector; then a modified concurrent mark-and-sweep collector that used VM synchronization between the mutator and the collector; and then a concurrent mostly-copying collector that also used VM synchronization.
These collectors had advantages and disadvantages compared to the initial Modula-2+ collector. They reclaimed cyclic structures and tended to reduce the cost of assignments, but they provoked VM thrashing far more readily and sometimes produced noticeable interruptions of service. For this reason, we adopted a combined reference-counting and mark-and-sweep collector for Modula-2+ at SRC, in which the reference-counting collector reclaims most garbage and the mark-and-sweep collector executes infrequently to reclaim cyclic garbage.
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