The Story of Phenom's Erratic Performance

A few months ago I called AMD with a problem. In testing for AMD's Phenom re-launch, I encountered a major issue: Phenom performance actually degraded since I first tested it. There were two benchmarks in particular that saw performance go down: SYSMark 2007 and Adobe Photoshop CS3. SYSMark gave me scores that were around 10% lower than what they were when Phenom launched, despite these being B3-stepping parts. Photoshop was far worse, with performance being around half of what it was at the Phenom launch.

I originally attributed the changes to something strange that happened with the move to Vista SP1. I theorized that somehow the TLB fix was being applied to B3 stepping parts and negatively impacting performance, but WinRAR and memory tests didn't support the thought. AMD couldn't figure out what was happening so I chalked it up to a problem with my testbed or testing methodology, something I'd have to revisit at a later time.

The SYSMark issue actually went away on its own; I swapped from my 780G motherboard to a 790FX without reinstalling Windows to see if it was a 780G/integrated graphics issue, the performance problems remained. But upon swapping back, once again without a Vista reinstall, my SYSMark scores magically jumped around 10%. The "fix" lasted long enough for me to finish the benchmarks for the Phenom re-launch review, but sure enough the problem reappeared when I tried to re-run one test after I'd gotten everything I needed for the review. I never did figure out what was causing my Photoshop performance issues however.

The Culprit: Cool'n'Quiet

When I started testing for today's review, I ran into the same issue again. I always start by benchmarking SYSMark first, since the suite takes forever to complete on a single CPU. As soon as I got my first results, I realized my problem was back. Determined to find the cause I tried everything...again. The one thing I didn't change however was the Cool'n'Quiet setting in the BIOS, but I did try it this time.

Cool 'n Quiet is the marketing name for AMD's on-the-fly clock speed/voltage adjustment. Depending on the software load on the CPU, AMD's Cool'n'Quiet will adjust the p-state of the CPU cores - which includes adjusting core voltage and clock speed. If you're running a processor intensive game or application, CnQ will make sure your CPU runs at full speed, but if you're just typing a text document it will underclock/undervolt the CPU.

Phenom's CnQ is a more advanced version of what was in the Athlon 64 X2, it not only allows for individual core clock speed adjustment but is able to transition between states faster than previous versions of CnQ (at least that's what AMD implies).

SYSMark 2007 Preview Overall Score CnQ/EIST Enabled CnQ EIST/Disabled Performance Increase from Disabling CnQ/EIST
AMD Phenom X4 9350e 101 113 11.8%
AMD Athlon X2 6400+ 121 123 1.7%
Intel Core 2 Quad Q9450 153 156 2.0%

 

 

Disabling CnQ increased my SYSMark scores by around 12% and cut my Photoshop CS3 render times in half (58.7s with CnQ enabled, 35.2s with CnQ disabled); enabling CnQ had the opposite effect. Gary ran similar numbers using PCMark Vantage and found a 5% difference. AMD originally insisted that the problem was because SYSMark introduces unrealistic pauses into its benchmark (so called "think" times or periods of time while the system is waiting for user input), but since we found the same issue in other benchmarks (PCM Vantage and our Photoshop test), we believe this is more than just a SYSMark issue.

The SYSMark problem was mostly repeatable, it would consistently produce lower scores with CnQ enabled on the Phenom CPUs. The Photoshop scores were a bit more erratic - the problem went away for a little while but has since returned and won't go away again, even with CnQ disabled. It is worth mentioning that the majority of our benchmarks wasn't impacted by the problem, but that doesn't mean that it won't impact your daily usage.

Note that the same problem doesn't plague the Athlon X2, this appears to be a Phenom/K10 issue only. As a reference we ran some numbers with Intel's SpeedStep enabled vs. disabled and didn't see any similar behavior.

I had found the source of my problems, but I didn't understand why it caused them.

Intel Makes 45nm Affordable The Mystery of the Missing Performance
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  • Regs - Tuesday, July 1, 2008 - link

    Between cool n' quite and flimsy power management, it just seems like AMD overshot their goals. Though to me, it seems like they could easily be fixed in Shanghai, but that's if they can keep all four cores busy instead I have 3 cores at stall, and one pumping at max in threaded or shared instruction instances. This will though cause more power consumption, and I think you guys all ready said that mobo support is just not their to power these suckers. You can have your cake, you just cant eat it.

    What do you goes think about AMD at 2.6 GHz? Looks more competitive stacked up to Intel's finniest at the given price point. Just makes me wonder if the over complicated power management features are keeping AMD from hitting 3.0 GHz or above. What do you think is holding AMD back?
    Reply
  • DigitalFreak - Tuesday, July 1, 2008 - link

    Shitty engineering? Reply
  • Griswold - Wednesday, July 2, 2008 - link

    Well, I dont know for sure. But its definitely not moronic comments from dumbasses such as you. Reply
  • Assimilator1 - Tuesday, July 1, 2008 - link

    Yeah it looks like they've messed up the clock speeds for the lower Phenoms too, lol. Reply
  • Aries1470 - Tuesday, July 1, 2008 - link

    Hi,
    Just found the following strange:
    AMD Phenom X4 9850 $205
    AMD Phenom X4 9750 $215
    The slower one is more expensive, while in the article it has the prices reversed?
    "The new Phenom X4 9950 will occupy the $235 space, which will push the 9850 down to $215. The Phenom 9750 will go away temporarily to make room for the new chips at the high end, leaving the 9650 at $195 and the 9550 at $175."

    I wonder which one is correct ;-) Hmm... I think a proof reader and an eye for detail is needed :-)

    Ok, now for me to read the rest of the article.

    Btw, any update on the new VIA Nano CPU - Codename Isaiah? Will there be a review? It is as fast as a 9150e or faster at the same clock speed? It has much less power usage. Now if someone over here could do a review or get more info that would be great, since it is like there is no other x86 competitor out there...

    That's all from me.
    Reply
  • Gary Key - Tuesday, July 1, 2008 - link

    The 9750 pricing will not be changed by AMD officially and thankfully that model is being phased out in the retail sector and replaced by the 9850BE.

    I have a picture of the VIA Nano PR flag from Computex and a handout explaining how it should perform. That is about as far as VIA is willing to go at this point with information. I did hear from some OEMS that VIA was not even close to getting the CPU out this summer as originally thought, much less advanced reviews. However, we do push them on an almost daily basis for it.
    Reply

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