AMD Quantispeed ratings becoming fanciful?

time

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I'm concerned that AMD's latest Performance Rating claims originated solely in the marketing department rather than having any basis in engineering.

As background, here are the actual clock speeds of various Athlons:

Code:
Model     Cache     FSB       Clock  Multiplier  %
XP1600    256MB    133MHz    1400MHz    10.5    87.5
XP1700    256MB    133MHz    1467MHz    11.0    86.3
XP1800    256MB    133MHz    1533MHz    11.5    85.2
XP1900    256MB    133MHz    1600MHz    12.0    84.2

XP2000    256MB    133MHz    1667MHz    12.5    83.4
XP2100    256MB    133MHz    1733MHz    13.0    82.5
XP2200    256MB    133MHz    1800MHz    13.5    81.8
XP2400    256MB    133MHz    2000MHz    15.0    83.3

XP2500    512MB    166MHz    1833MHz    11.0    73.3
XP2600    256MB    133MHz    2133MHz    15.5    82.0
XP2600    256MB    166MHz    2083MHz    12.5    80.1
XP2700    256MB    166MHz    2167MHz    13.0    80.3
XP2800    256MB    166MHz    2250MHz    13.5    80.4

XP2800    512MB    166MHz    2083MHz    12.5    74.4
XP3000    512MB    166MHz    2167MHz    13.0    72.2
XP3200    512MB    200MHz    2200MHz    11.0    68.8
The "Quantispeed" rating system is not linear, nor does it have an origin of zero. So the XP1600 was clocked a mere 12.5% slower than the equivalent P4, whereas the XP3200 cops a whopping 31.2% handicap.

Huh?

I can see that as the clock speed of each generation of P4 rises, the IPC drops as the CPU starts to run out of memory bandwidth. So you could argue that it's easier for Athlon to keep up. The problem is, the Athlon suffers from exactly the same effect! It may not have the bandwidth requirements of the P4 (although Northwood's larger cache must have reduced that slightly anyway), but higher clock speeds still need associated higher bandwidth.

I'm going to propose a rule of thumb: Athlon performance is significantly impaired when the multiplier exceeds 12. I suggest that 20 could be the equivalent number for Pentium 4. Conversely, decreasing the multiplier is unlikely to have a noticeable effect beyond a certain point.

We already know that the 1GHz Pentium III didn't appreciably benefit from increasing memory bandwidth beyond the theoretical 1066MHz of PC133. Whereas the Athlon 1GHz did, suggesting it preferred a multiplier of 7 or less with SDRAM. So, the Athlon 2600 (256kB Thoroughbred), with 82% of the P4 clock speed, may have the additional impediment of at least 20% less bandwidth than it needs. Not good. It's now quite some time since people discovered that you could significantly improve Athlon performance by increasing the FSB from 133 to 166MHz.

Of course, this is how AMD's marketing department has managed to prop up their outrageous claims for Barton CPUs: they run faster than the strangulated FSB models that they replace. They're ignoring the fact that the previous models were already underperforming. Intel, on the other hand, has simply introduced faster FSB as needed to support faster clock speeds. I'm betting the latest '800FSB' (200MHz QDR) is designed to carry them through to 4GHz, with a tangible benefit for CPUs from 2.8GHz up.

If you look at the table, the CPUs on which many of us based our perception that Athlons were fast, do not have either problem. The XP1600 and XP1700 have tons of bandwidth and run nearly as fast as the equivalent P4 anyway. I put it to you that these were in fact under-rated. Here's another rule of thumb: given an adequate FSB, an Athlon Palomino/Throughbred doesn't need any more than 5/6 of the clock speed of a Pentium 4 to be broadly competitive (faster in some things, slower in others). Or put another way, the PR rating should be at least 20% higher than the clock speed.

Things are probably still okay at XP2000, but going off the rails at XP2400 (of course, a P4 with 100MHz FSB will be even worse). By the time we move up to the XP2800 Thoroughbred, it's hard to see how there could be much less than a 5% variance between AMD's PR and what we actually see (referenced to the slower Athlons). That would make it more comparable to a Pentium 4 2667 (although I guess one of these is on the edge with the 533FSB, and the Athlon would still be better for many applications).

Then along comes Barton. Based on the tests at Aces Hardware, applications that benefit seem to average a boost of about 9% from the bigger 512kB cache. AMD reckons it needs 8% less clock speed for the Barton version of the XP2800, so let's round it off at 10%. That would change the rule of thumb from 83% (5/6) to 75% (3/4).

So if they were both run at a high enough FSB, I'd theorize that the Barton 2500 is about the same as the Thoroughbred 2400 (1833 vs 2000MHz). In practise, the Barton multiplier is a trim 11 compared to a stodgy 15 for its older cousin. You'd expect it to be faster, and it is. But I suspect it's not quite comparable to a P4 2533. After all, the Pentium 4 already whipped the old Athlon in apps that are dependent mainly on raw clock speed. The bigger L2 cache may not help here at all.

The Barton XP2800 looks a little better except for its slight bandwidth restriction.

Applying the 75% rule to the XP3000 places it as almost a 2900. Indeed, Aces' conclusion was that XP2900 would have been a more realistic rating. It also breaks the multiplier limit of 12, so perhaps it should best be compared with a P4 2800 with 800FSB?

Then there's the XP3200. :p It's actually identical to the XP2500 except for the FSB - same multiplier of 11. So how the hell did it pick up a further 7% performance?

Based on AMD's claimed rating for the XP2500, the XP3200 should be called an XP3000! Comparing it to the XP2800 (and allowing for slightly less bandwidth restriction) brings the same conclusion. The 75% rule of thumb suggests XP2900+. When comparing apples to oranges, or Athlon to Pentium 4 800FSB, I reckon this might be a better bet.

Here endeth the treatise. Thank you for reading this far. Now you can throw the brickbats!
 

blakerwry

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The reason it's non-linear is because you are using different cores, different amounts of L2 cache, and different FSB speeds. The rating is only suposed to show you how the AthlonXP performs compared to the Athlon Thunderbird core.

I agree tht current ratings of AthlonXPs make them seem like they are going to be as fast as a p4 w/ comparable clock speed to the AMD rating. And this may or may not be the case. Personally, I think it's pretty obvious that a Athlon XP 3500 is not equivalent of a p4 northwood running at 3.5GHz.

However. if AMD removed the speed rating entirely it would show consumers how much lower their clockspeed is compared to that of pentium4, making an Athlon seem much slower to most "joe sixpacks". This is not a good solution because AthlonXPs clearly perform better per clock cycle than the current intel chips. Making the MHz comparison obsolete.

The easiest comparison would be to simply have AMD rate their CPUs based on intel's performance... however I have heard that this would be illegal in the US due to marketting laws.

So we're left with what we have, a product rating where we cannot directly compare one CPU with another without ateast a little guesswork involved.
 

Jan Kivar

Learning Storage Performance
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Tom's HW charts are useful for something like this (if for anything). Yes, there is more air in the ratings over 2800+. It varies from app to app.

Jan
 

Jan Kivar

Learning Storage Performance
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e_dawg said:
Yet another insightful post from you, time -- and rightfully so to mark your 1,000 th post at SF. Congrats!
Bugger... Why didn't I spot that...

Congrats Time!

Jan
 

timwhit

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e_dawg said:
Yet another insightful post from you, time -- and rightfully so to mark your 1,000 th post at SF. Congrats!
Did Time disappear like all those other people who finally reached 1000 posts?
 
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