TDP Power Cap

What makes these new Opterons truly intriguing is the fact that they will offer user-configurable TDP, which AMD calls TDP Power Cap. This means you can buy pretty much any CPU and then downscale the TDP to fit within your server’s power requirements. In the server market, the performance isn’t necessarily the number one concern like it is when building a gaming rig. As all the readers of our data center section are aware, what really counts is the performance per watt ratio. Servers need to be as energy efficient as possible while still providing excellent performance. 

John Fruehe (AMD) states, "With the new TDP Power Cap for AMD Opteron processors based on the upcoming 'Bulldozer' core, customers will be able to set TDP power limits in 1 watt increments." It gets even better: "Best of all, if your workload does not exceed the new modulated power limit, you can still get top speed because you aren’t locking out the top P-state just to reach a power level."

That sounds too good to be true: we can still get the best performance from our server while we limit the TDP of the CPU. Let's delve a little deeper.

Power Capping

Power capping is nothing new. The idea is not to save energy (kWh), but to limit the amount of power (Watt) that a server or a cluster of servers can use. That may sound contradictory, but it is not. If your CPU processes a task at maximum speed, it can return to idle very quickly and save power. If you cap your CPU, the task will take longer and your server will have used about the same amount of energy as the CPU spends less time in idle, where it can save power in a lower p-state or even go to sleep (C-states). So power capping does not make any sense in a gaming rig: it would reduce your fps and not save you any energy at all. Buying CPUs with lower maximum TDP is similar: our own measurements have shown that low power CPUs do not necessarily save energy compared to their siblings with higher TDP specs. 

In a data center, you have lots of servers connected to the same power lines that can only deliver a certain amount of current at a certain voltage (48, 115, 230 V...), e.g. amps. You are also limited by the heat density of your servers. So the administrator wants to make sure that the cluster of servers never exceeds the cooling capacity and the amps limitations of the power lines. Power capping makes sure that the power usage and the cooling requirements of your servers become predictable.

The current power capping techniques limit the processor P-states. Even under heavy utilization, the CPU never reaches the top frequency.  This is a rather crude and pretty poor way of keeping the maximum power under control, especially from a performance point of view. The thing to remember here is that high frequencies always improve processing performance, while extra cores only improve performance in ideal circumstances (no lock contention, enough threads, etc.). Limiting frequency in order to reduce power often results in a server running far below where it could in terms of performance and power use, just to be "safe".

Overview of Bulldozer Lineup Bulldozer's Power Management


View All Comments

  • mino - Friday, July 15, 2011 - link

    "single-threaded performance is still a sore spot for Bobcat compared to other architectures"

    What "other architectures" ??? To my knowledge the is exactly ZERO other architectures with faster single-threaded performance at the power level Bobcat plays at.

    The faster "competitors" are either running at their lowes-feasible power levels (SB, C2D) or are vastly slower (Atom, A15 etc.).
  • JarredWalton - Saturday, July 16, 2011 - link

    We never said "at the same power level". Bobcat is much faster than Atom, but Core 2 beats Bobcat silly, and Core 2010/2011 are even faster. Bobcat is fine for low power, low performance, decent multimedia; that's not the same as being good for general use. Reply
  • GaMEChld - Saturday, July 16, 2011 - link

    Why even go to Intel for the comparison? Bobcat loses to the old STARS cores too doesn't it? Athlon II, Phenom II, Llano? Generally it's assumed that comparisons are done between competing products for a given market or price point.

    What sense would there be in reviewing an Intel Atom chip, and then taking the time to say, well, sadly the Intel Atom does not have as good single-threaded performance as the Core i7 990X Super Jeebus Edition. Or that the Radeon 5450 does not offer superior graphics performance to the GTX 590? Well, duh!
  • 529th - Friday, July 15, 2011 - link

    AMD seems to be highlighted alot around the word "server"

    .. just not my market.. what a let down for the pursuit in competitive CPUs for Intel desktop

    fee nom - whutever
  • shmmy - Friday, July 15, 2011 - link

    Wow really? Do you people really need to nitpick the details on stuff thats not even out yet? 8 cores 10 cores who the heck cares get back to work slackers! :) Reply
  • JarredWalton - Sunday, July 17, 2011 - link

    Core 2 ULV (all the CULV stuff from early 2010) already offered us power levels similar to Bobcat, with better per-core performance. What it didn't offer was the GPU side of things, which is why Optimus was useful. As the article here states, "single-threaded performance is still a sore spot for Bobcat compared to other architectures", it seemed fairly obvious that we're discussing Bobcat in the greater market, not just Bobcat in low-power uses. And yet, Mino went and complained regardless.

    For those interested in a few comparisons:

    Core 2 SU7200 @ 1.3GHz w/GMA4500
    (ASUS UL80Vt:
    PCMark Vantage: 2993
    CB10 1CPU: 1643
    CB10 SMP: 3138
    x264 1st Pass: 18.12 FPS
    x264 2nd Pass: 4.5 FPS
    Idle Power: ~5.94W
    Internet Power: ~8.59W
    H.264 Power: ~13.96W

    Core i3-330UM @ 1.2GHz w/HD Graphics
    (ASUS UL80Jt:
    PCMark Vantage: 3558
    CB10 1CPU: 1724
    CB10 SMP: 3859
    x264 1st Pass: 21.45 FPS
    x264 2nd Pass: 5.67 FPS
    Idle Power: ~7.91W
    Internet Power: ~10.5W
    H.264 Power: ~17.68W

    AMD E-350 @ 1.6GHz w/6310M
    (MSI X370:
    PCMark Vantage: 2511
    CB10 1CPU: 1158
    CB10 SMP: 2175
    x264 1st Pass: 13.96 FPS
    x264 2nd Pass: 3.43 FPS
    Idle Power: ~7.47W
    Internet Power: ~8.81W
    H.264 Power: ~13.57W

    So when Mino says that "to my knowledge the is exactly ZERO other architectures with faster single-threaded performance at the power level Bobcat plays at", he is either uninformed, ignorant, or totally biased. CULV way back in late 2009 offered 42% higher single-threaded performance than Bobcat in early 2011, with lower power requirements. Core 2010 ULV improved performance further at the cost of power requirements: it's 49% faster but uses 6% to 30% more power than Bobcat. Either way, performance per watt both CULV and i3-ULV do better than Bobcat. They also have much worse IGPs, so it's not a complete loss for AMD.

    Even so, architecturally I don't think Bobcat has a lot of legs. Going quad-core does nothing for single-threaded performance, and multi-threaded performance on a low power design is sort of silly to discuss. It's the same problem I have with ARM: sure, they can do low power really well, but what happens when you need more performance? For many tasks, a 2.0GHz dual-core ARM is no worse than a 2.0GHz quad-core ARM, and in raw computer performance even Atom is likely faster than ARM right now.

    Windows 8 running on ARM is going to be interesting; can the chip even handle a full OS like Windows? Will it do so while still offering good battery life? I'd say Bobcat is the bare minimum performance we need for a full Windows OS to work well, and Bobcat is at least twice as fast as Atom. Will ARM manage to equal Bobcat next year? I wouldn't bet on it, but maybe I'll be wrong.
  • zgoodbbb - Wednesday, July 20, 2011 - link

    Women's fashion, men's personality + shoes

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  • morohmoroh - Friday, July 22, 2011 - link

    i have two hand

    5 finger in my left hand and 5 finger in right hand

    i cannot grab a 1 rock with 1 finger , i still can grab it with boths hand with each 2 finger but still hard then decided grab a rock with 3 or 5 finger

    now i have 8 hand and 40 finger how s bout that?

    if i can grab a rock with invisible hand and finger it look like a magic

    my question is core = brain or core = hand with finger?

  • Cyberius - Tuesday, September 20, 2011 - link

    I hope bulldozer for desktop has tdp power cap included in the amd overdrive utility like the radeon 6900 catalyst utility. That would be a great option for us. Reply

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