Athlon II X2: Hardware C1E and Return of the CnQ Bug

I noticed something strange in my initial testing of the Athlon II X2, take a look at these SYSMark results:

Processor SYSMark 2007 Overall
AMD Phenom II X2 550 BE (3.10GHz) 167
AMD Athlon II X2 250 (3.00GHz) 134
AMD Athlon X2 7850 (2.80GHz) 145

 

The Athlon II X2 250 is slower than the Athlon X2 7850 and significantly slower than the Phenom II X2 550. Remembering the Cool’n’Quiet bug from the original Phenom processor I decided to turn CnQ off in the BIOS to see if the scores would go up:

Processor SYSMark 2007 Overall - CnQ On SYSMark 2007 Overall - CnQ Off
AMD Athlon II X2 250 (3.00GHz) 134 148

 

Indeed they did. 

I contacted AMD and was informed that there’s more than meets the eye with the Athlon II X2.  Although the architecture is fundamentally a couple of Phenom II cores with larger L2 caches and no L3, there’s one more change to the die: microcode support for the C1E power state.

When the OS executes a halt instruction on a CPU (during a period of no activity for example) the clock signal is shut off to the CPU for a period of time.  This saves power as no transistors are actively switching during this time.  Voltage supplied to the processor is left unchanged however.  This power state is known as C1.

In the late Pentium 4 era Intel introduced an Enhanced Halt State, called C1E.  Instead of just shutting off the clock to the CPU, when a CPU was in C1E its clock speed and voltage would both be reduced to their lowest possible value.  The reduction in voltage results in a reduction in leakage current, which in turn saves power.

Apparently prior to the Athlon II X2, AMD enabled support for C1E outside of the processor.  Although I tried, I couldn’t get access to anyone at AMD to explain things any further so what I offer is my best guess.  I’m guessing that whenever a halt instruction was executed by the OS, AMD used some combination of its existing C1 support and Cool’n’Quiet to both stop the clock to the CPU and reduce voltage. 

Regardless of how AMD enabled it, motherboard makers were constantly botching it up in their BIOS which would result in different motherboards having very different power consumption levels especially at idle.  It appears that some vendors were properly enabling this software-hack C1E state while others weren’t.

AMD always expressed frustration to me that the motherboard vendors kept screwing things up and I’m guessing they got tired of dealing with it.  The new Athlon II X2 has microcode level support for the C1E state; when the OS executes a halt instruction, the CPU now knows to both shut off its clock and drop its voltage.  No BIOS trickery necessary.

The problem with this, as you can guess, is that not all current motherboards have proper BIOS support for it.  Yep.

But that’s only half of the problem.  Simply not supporting the new hardware C1E in the Athlon II X2 won’t cause the issue I saw above, that has to do with Cool’n’Quiet, not C1E.  So what’s going on?

Late last week AMD finally got back to me with an answer.  The feature that caused the CnQ bug in the original Phenom processor was the processor’s ability to run each core at a different clock speed.  A nasty combination of Windows’ scheduler and the Phenom’s power management could result in cores, under load, running at 50% of their frequency.  AMD fixed the problem by removing the feature; in the Phenom II all cores attempt to run at the same frequency. 

When AMD put out its master BIOS code for all 7xx series reference motherboards, the Athlon II did not exist.  The fix that was applied to the Phenom II would not be applied to any other Phenom II based derivatives, they would simply get treated as original Phenom processors with varying clock speeds between cores.

And that’s what’s going on.  The Athlon II isn’t told to run both of its cores at the same frequency and thus you can have situations where performance is much lower than it should be.

AMD is aware of the issue and are currently working with motherboard vendors to properly enable BIOS support for the Athlon II.  Until then, the best way to run and use the Athlon II is with CnQ disabled.  Unlike the original Phenom, this bug should get fixed in the near future.

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  • Gary Key - Tuesday, June 2, 2009 - link

    The X2 6400+ is in the charts now and you can always use our Bench tool to compare a whole litany of processors against each other. AMD is currently phasing out of 90nm production and even several 65nm products will be phased out this year as they ramp the 45nm production. Reply
  • Spoelie - Thursday, June 4, 2009 - link

    WOW, it really amazes me how little performance has improved. Athlon II X2 750 (3ghz) is barely faster in most benchmarks than a Athlon X2 6400+ (3,2ghz), and loses in 1 or 2.

    So the phenom core redesign buys around 300mhz around 3ghz, or only 10%. Everything else that improved in phenom is uncore.

    And this while the original is at 90nm and the new one is 45nm, what a waste of potential. It seems to me AMD could've tried a little harder with the Athlon II.
    Reply
  • Spoelie - Tuesday, June 2, 2009 - link

    There's a small util/service out there that brings phenom II cnq behaviour from vista over to windows xp (phenom II's cnq behaves like phenom I's cnq under windows xp). It does this by disabling standard cnq (set power management not on "minimal") and implementing pstate changing itself

    http://home.comcast.net/~pmc650/site/?/page/CNQ_Ph...">http://home.comcast.net/~pmc650/site/?/page/CNQ_Ph...

    Maybe it can do the same for the Athlon II X2 on vista...
    Reply
  • mohindar - Tuesday, June 2, 2009 - link

    Hello Anand,

    It will be very nice to provide some benches regarding desktop virtualization, like how windows-xp usage on this chip and so on...
    Reply
  • plonk420 - Tuesday, June 2, 2009 - link

    i'm curious to see this on the bench! Reply
  • mapesdhs - Tuesday, June 2, 2009 - link


    I'm beginning to wonder whether AMD/Intel are making the same mistake
    we saw last year with gfx cards, ie. too many different options. What
    is the target market for the new AMD CPUs? Many retailers seem to
    offer just a small selection.


    Any chance you could add an i7 920 and a 6000+ to the tables please?
    The former for completeness, the latter to show how the newer AMD
    parts stack up against a typical older product. I'd been hoping for
    a suitable replacement for the 6000+ in my ASUS board, but still nothing (no BIOS support).

    Atm it looks like my next system will be an i7 920 setup (core task
    is video encoding). In the past there's been lots of talk about the
    higher cost of an i7 system, but looking around yesterday, I was
    surprised at how small the difference has now become. The i7 920 is
    only 18% more than the Ph2 955 BE. Expecting a larger difference for
    the mbd cost, I found an X58 board from Gigabyte (the GA-EX58-UD3R,
    135 UKP from LambdaTek) right in the middle of the price range of
    typical AM3 boards. They both use DDR3, so that isn't a factor. I
    was going to build a Ph2 955 BE system for my brother as his next
    gaming rig, but with such small price differences now in play, the
    i7 looks more sensible.

    Oh, a typo on the first page perhaps? Surely it should be 2 cores for
    the Athlon64 X2?

    Ian.

    Reply
  • smilingcrow - Tuesday, June 2, 2009 - link

    Judging by other reviews your choice of using the x264 HD Bench Pass 1 for the Power Consumption comparison doesn’t give a true representation of the situation.
    As expected other reviews shows the Athlon II X2 having a noticeably lower power consumption under typical loads.

    The Phenom II X2 has too much extra circuitry to have lower power consumption and I’m surprised that you didn’t deduce that something was amiss.
    Reply
  • Anand Lal Shimpi - Tuesday, June 2, 2009 - link

    As I mentioned on the power consumption page, I'm guessing it has more to do with the current level of BIOS support for the Athlon II's power management. AMD is expecting a much better situation in the coming weeks.

    Take care,
    Anand
    Reply
  • Eeqmcsq - Tuesday, June 2, 2009 - link

    The one comparing various Athlon X2 specs. The table says the Athlon 64 X2 has 4 cores. Reply
  • Anand Lal Shimpi - Tuesday, June 2, 2009 - link

    Woops, thank you :)

    -A
    Reply

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