Peer review of AMD64/FreeBSD article

[Jem Matzan]

Brooks Davis wrote:

>On Fri, Mar 12, 2004 at 02:44:14PM -0500, Jem Matzan wrote:
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>>Brooks Davis wrote:
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>>>On Fri, Mar 12, 2004 at 08:14:59AM -0500, Jem Matzan wrote:
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>>>>Hyper-Threading seemed to help with processes that didn't require a 
>>>>heavy CPU load. The OpenSSL tests show it being markedly faster in the 
>>>>smaller algorithms, but lagging way behind the 64-bit Athlon64 when the 
>>>>serious number crunching comes into play. Intel's press kit shows HT 
>>>>(and SSE3) giving an advantage when multitasking with four desktop 
>>>>programs in Windows XP. It's just too hard to show that reliably though. 
>>>>There's a lot of anecdotal evidence to suggest that AMD64 is faster on 
>>>>the desktop (in X) in 64-bit mode than the Prescott is in 32-bit, but 
>>>>I'm having trouble proving it.
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>>>>
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>>>I think it would be a mistake to assume the HT is what accounts for
>>>the performance difference.  There are so many other architectural
>>>differences it's hard to see how you could isolate the effects of
>>>HT.  My suspicition is that better performance on small 
>>>algorithms is due to them being more or less memory bound (and thus
>>>similar to the pure synthetic benchmarks).
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>>By comparing the Pentium4 to the Athlon64 in i386 mode, you can better 
>>see the advantage of HT Technology. This is especially evident in the 
>>OpenSSL tests.
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>
>I strongly disagree.  All you can see is that they differ.  The
>architectural differences between P4 CPUs and current generation amd64
>CPUs are a whole lot more then HT.  You've got a different memory memory
>system, vastly different pipeline lengths, etc.  For that matter,
>on paper at least, I wouldn't expect HT to help much if any in this
>situation since you're not trying to do two things at once.
>
>-- Brooks
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I'm pretty sure I did mention that, but I'll go over the article to make 
sure it's clear that the reason why there is a performance difference 
could be due to a number of factors (which are listed here).

-Jem