AMD's Cool'n'Quiet: Disable it

One interesting phenomenon that we encountered during testing on the AMD/NVIDIA platforms with the Phenom series of processors is the effect of Cool’n’Quiet (CnQ) on our BD playback results. We utilized a Phenom 9950BE in our test results today along with the 780G/GF8200/790GX chipsets. When playing back our H.264 title, The Simpsons Movie, with CnQ disabled we noticed an average CPU utilization of 6%~7% with platform power averaging anywhere from 132W to 140W depending on the chipset and motherboard. Video playback was smooth and stutter free on this title and many others regardless of the encoding format utilized.

One of the prime objectives for our HTPC systems is energy saving, especially during playback operation - the idea being that lower power use produces less heat and should therefore also result in a quieter system. The quickest way to reduce platform power on an AMD based system is to enable CnQ. After enabling CnQ in the BIOS and setting our Vista power management profile to balanced or power saver, we noticed our platform power requirements dropped to 99W~112W. This is a 25% or so improvement in power savings so we were naturally pleased with the results.

We fired up Power DVD 8 Ultra, popped The Simpsons Movie back in and decided to see how well the system operated. Our CPU utilization numbers increased to 16%~23%, but considering our processor and lack of background activities, this increase was perfectly acceptable. At least until we sat through this movie and others. We started to notice slight stutters, pauses, and even some judder at various times during the movie. At times, it was very pronounced but other times it was very subtle, but it was not so subtle that we did not notice.

The most frustrating part is that it never occurred in the same section of a title or at the same time. We would notice stuttering in the first 30 seconds of a title at times and other times it would not occur at all. In every case with CnQ enabled, we eventually noticed this behavior, regardless of chipset on the AMD platform. Turning off CnQ resulted in an immediate improvement in our viewing experience. We are investigating this problem (actually it has occurred in the past on the Athlons but not to this extent), but until then our suggestion is leave CnQ off and suffer the consequences of increased power usage. (Or at least, leave it off if you're putting together an HTPC.)

Hardware Blu-ray Decode Acceleration Hardware Accelerated Blu-ray Playback Comparison
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  • MrMilli - Tuesday, October 14, 2008 - link

    If you multiply it all out that gives Intel a throughput of 8 instructions per clock for G35, 10 for G45, 10 for NVIDIA's GeForce 8200 (where two are transcendental operations) and 40 for AMD. In terms of worst case throughput however, AMD falls down to 8 per clock (assuming the compiler can't feed the hardware 4 shader ops + 1 transcendental per SP) as does NVIDIA. This worst case rarely happens, but it is definitely worth noting.

    10 for nvidia => 8 for nvidia
    AMD falls down to 8 per clock => to 10 per clock

  • a1yet - Tuesday, October 14, 2008 - link

    wow finally a video playback comparison :-) TY

    I have a question one of you may be able to answer ?
    In the "Hardware Accelerated Blu-ray Playback Comparison"
    (CPU usage) the 780 beat the 790 in 3 of the 6 tests!
    With the 790 using up to 9% MORE CPU usage, and in the
    other 3 tests. The 790 beat the 780 by only .3% (well within a margin of error)
    Up to 9% MORE CPU usage is A LOT!
    I want to buy the 790 but this is a disappointment!
    Dose anyone know why the 790 uses so much more CPU then the 780.
    Is it's HD Acceleration sub-par ?
    Heck in the "Crank DB" test all the cards beat the 790.
    Please help TY
  • yknott - Tuesday, October 14, 2008 - link

    Do we know if the Radeon HD4xxx cards support output at 1080p/24fps?

    I did some googling and can't find anyone who can verify this
  • Geraldo8022 - Tuesday, October 14, 2008 - link

    "do we know if the Radeon HD4xxx cards support output at 1080p/24fps?"
    this is exactly what I want to know also.
  • Screammit - Wednesday, October 15, 2008 - link

    I just received a 4670 today to plug into my old PC that i'm slowly converting into an HTPC. In the display modes 1080p/24 is natively listed, but i'll have to get my blu ray drive in before I can truly verify that it works. Sure seems to have support though.
  • Calin - Tuesday, October 14, 2008 - link

    An Intel processor and chipset with an AMD discrete card
  • SkCom - Tuesday, October 14, 2008 - link

    testing amd ddr 2 and intel ddr 3 is not fer test and amd made the 780 790 for usage with cheap cpu SEMPRON so why use phenom and rise the w power when simply can do the chep cpu psu ram and still
    watch HD movies surf and dissant gaming price perfom AMD 1 CHAMPION
  • strikeback03 - Tuesday, October 14, 2008 - link

    Checked twice, can't find any punctuation in this post. I have no idea what you are trying to say.
  • fic2 - Tuesday, October 14, 2008 - link

    Apparently using a Sempron takes away your ability to punctuate, spell check or make much sense.
  • Clauzii - Tuesday, October 14, 2008 - link

    He says that by using a Sempron CPU (lower watt than Phenom), it would still be a nice machine for most people, and still be good for movies.

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