Mill Computing, Inc. Forums The Mill Architecture Scratchpad design decision Reply To: Scratchpad design decision

Ivan Godard
Post count: 627

Tin has only 128 bytes of scratchpad, and Gold has 512. Why so small? I realize that the scratchpad isn’t expected to be used frequently. Then again, maybe the Tin should have more Scratchpad to make up for its lack of Belt.

These sizes are more a matter of marketing than technology. The scratch size is a performance limit, not an architectural limit; on any member a program whose working set runs out of scratch can use the (practically) unbounded DRAM address space via the extended spill/fill ops, they just cost more.

Any given program on any given member will find some architectural limit to increased performance: FU count, cache sizes, retire bandwidth, etc. – and scratchpad size. So for any given configuration there will be customers who will tell us “if you would only increase the size of this feature then my program would run so much faster; pretty please with sugar!”. But as we scale any member’s configuration pretty soon we get to the performance of the next bigger member. So we’ll say “just buy the bigger member”, to which they will say “but that costs more money!”.

Everybody wants Gold performance at a Tin price. We have to set the configurations to provide reasonable coverage of the price/performance spectrum. The currently defined sizes of scratch on the various members are just placeholders, and will certainly be tuned as the members go to market. But the tuning will be driven by market segmentation considerations, not technical ones. Welcome to the high-tech biz.