256 bytes stores 32 64-bit values (ignoring metadata), but do you need 256 positions? I would (naively) expect the slow belt to have the same number of entries as the main belt.
You need as many positions as the working set you design for has distinct operands, regardless of size. Working set is per algorithm, not per member, although market segmentation will let us downgrade a little in the smaller members. We haven’t paid much attention to tuning this, but seat-of-the-pants suggest that 64 would be good, maybe 127 better.
However, because the slow belt is populated by long-lived values that are going to be referenced many times during their lifetime, a slow belt would have a larger fill/spill (read/write) ratio than the fast belt
I does? I’m afraid I don’t follow.
You need one spill per operand, and one or more fills per operand. You need a new fill for each reference that no longer has the previous fill reachable. Generally, longer lived values are referenced more times than shorter-lived values, and are more likely to need a new fill rather than re-using a previous fill. As the spill count is constant, and the slow belt can be expected to have more references than the fast belt (and hence more fills) the ratio is different, which drives the fill bandwidth provision among other things.