Brief Introduction to Alpha Systems and Processors

4. 21064 performance vs 21066 performance

The 21064 and the 21066 have the same (EV4) CPU core. If the same program is run on a 21064 and a 21066, at the same CPU speed, then the difference in performance comes only as a result of system Bcache/memory bandwidth. Any code thread that has a high hit-rate on the internal caches will perform the same. There are 2 big performance killers:

1. Code that is write-intensive. Even though the 21064 and the 21066 have write buffers to swallow some of the delays, code that is write-intensive will be throttled by write bandwidth at the system bus. This arises because the on-chip caches are write-through.
2. Code that wants to treat floats as integers. The Alpha architecture does not allow register-register transfers from integer registers to floating point registers. Such a conversion has to be done via memory (And therefore, because the on-chip caches are write-through, via the Bcache). (Editor's note: it seems that both the EV4 and EV45 can perform the conversion through the primary data cache (Dcache), provided that the memory is cached already. In such a case, the store in the conversion sequence will update the Dcache and the subsequent load is, under certain circumstances, able to read the updated d-cache value, thus avoiding a costly roundtrip to the Bcache. In particular, it seems best to execute the stq/ldt or stt/ldq instructions back-to-back, which is somewhat counter-intuitive.)

If you make the same comparison between a 21064A and a 21066A, there is an additional factor due to the different Icache and Dcache sizes between the two chips.

Now, the 21164 solves both these problems: it achieve much higher system bus bandwidths (despite having the same number of signal pins - yes, I know it's got about twice as many pins as a 21064, but all those extra ones are power and ground! (yes, really!!)) and it has write-back caches. The only remaining problem is the answer to the question "how much does it cost?"