Brief Introduction to Alpha Systems and Processors: 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:
- 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.
- 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?"
