--heap, that allows one to specify the size of heap in 1024 word blocks. On a 32-bit machine, the heap size is limited because MIT/GNU Scheme stores the type tag in the upper bits of the word. On a 64-bit machine the word size is wider, so it is possible to specify extremely large heaps without worrying that the address will overflow into the type tag.
For kicks, I decided to vary the value of the heap size and see what effect that had on the task of MIT/GNU Scheme compiling itself.
|Heap||GC Count||Cumulative GC Time|
|8192 (64 MiB)||764||19120|
|16384 (128 MiB)||377||10980|
|32768 (256 MiB)||188||7350|
|65536 (512 MiB)||94||5000|
|131072 (1 GiB)||48||3910|
|262144 (2 GiB)||25||3050|
|393216 (3 GiB)||18||2740|
Collecting this data was tedious, so I spent a fair amount of time thinking about a model that could explain the results. At first glance, everything looks about right: the more memory, the fewer GCs and the less total time is spent garbage collecting. The problem is that the curve predicted by my model doesn't fit the empirical data at all. Of course this means that my model is either wrong or incomplete.
I have since figured out the problem, but I thought I'd let people puzzle over it themselves for a bit, first.