<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <HTML> <HEAD> <META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9"> <TITLE>Brief Introduction to Alpha Systems and Processors: 21064 performance vs 21066 performance</TITLE> <LINK HREF="Alpha-HOWTO-5.html" REL=next> <LINK HREF="Alpha-HOWTO-3.html" REL=previous> <LINK HREF="Alpha-HOWTO.html#toc4" REL=contents> </HEAD> <BODY> <A HREF="Alpha-HOWTO-5.html">Next</A> <A HREF="Alpha-HOWTO-3.html">Previous</A> <A HREF="Alpha-HOWTO.html#toc4">Contents</A> <HR> <H2><A NAME="s4">4. 21064 performance vs 21066 performance</A></H2> <P> 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 <EM>internal</EM> caches will perform the same. There are 2 big performance killers: <P> <OL> <LI> 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. </LI> <LI> 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.) </LI> </OL> <P> <P> 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. <P> <P> Now, the 21164 solves both these problems: it achieve <EM>much</EM> higher system bus bandwidths (despite having the same number of signal pins - yes, I <EM>know</EM> 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?" <P> <P> <P> <HR> <A HREF="Alpha-HOWTO-5.html">Next</A> <A HREF="Alpha-HOWTO-3.html">Previous</A> <A HREF="Alpha-HOWTO.html#toc4">Contents</A> </BODY> </HTML>
