Peer review of AMD64/FreeBSD article

[Jem Matzan]

James R. Van Artsalen wrote:

> Jem Matzan wrote:
>
>> I've just finished writing this article comparing performance between 
>> an Athlon64 in 32-bit and 64-bit mode using FreeBSD:
>
>
> Intel would be thrilled were Prescott to "idle" at 60 F anywhere other 
> than outdoors in an Antarctic winter: alas, 60 C sounds more likely 
> (but still seems astoundingly high for a halted processor).
>
> Your Prescott  probably isn't doctored, but it is the case that early 
> steppings of a CPU are always faster than later steppings: bug fixes 
> to the silicon or control store patches by ROM POST rarely speed it up.
>
> The Prescott performance variation may indeed be due to thermal 
> issues.  I think Prescott slows down in response to thermal overload 
> (AMD just enters a non-resumable halt - AMD's is a safety mechanism to 
> protect the motherboard and CPU).  It is not out of the question that 
> Prescotts is regularly bumping up against thermal limits and running 
> slow briefly.  I find this hard to believe, but no harder to believe 
> than a 60 C halted processor...  Test by *lightly* preheating CPU 
> cooler air intake with a well-aimed hairdryer to and see if that hurts 
> performance.
>
> It may be worth mentioning that theoretically the usual win from 
> 64-bit mode comes not from the fact that registers and reg ops are 64 
> bit but rather because more registers are available when in 64-bit 
> mode.  This is a huge win for a compiler which is nearly asphyxiated 
> in register allocation by the i386.
>
> It might be worth mentioning that a powerful differentiator (between 
> i386 and amd64 is maximum memory.  With AMD64 you can keep on adding 
> RAM after 4 GB as long as it wins.  A database-driven web site might 
> win substantially by having an 8 GB resident working set 
> *in-process*.  The max for i386 is around 3 GB; the practical max for 
> amd64 is about 15 GB and growing (Tyan Thunder K8W with 8x 2GB 
> DIMMs).  This is beyond the scope of your tests but might be worth 
> mentioning.
>
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>
Doh -- yes, that's a mistake on my part, should be 60 degrees C, not F. 
More like ~144 degrees F. I wish I could reliably measure the 
temperature under load in FreeBSD. I could do it in Windows but the 
software temp readers are sometimes very inaccurate. Intel included a 
special dashboard utility with the press kit (probably standard issue 
for the Prescott retail processors on the driver CD) but I didn't try it 
out.

I'll add this information to the article -- thanks.

-Jem