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
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  • stmok - Friday, July 15, 2011 - link

    No it doesn't. It expressly says CPU for Komodo. Not APU. Look carefully!

    Even the latest leaked slide ( source in my thread) refers Komodo as a CPU with "Next-Gen Discrete Graphics" as part of the Corona enthusiast/performance desktop platform for 2012.
  • Kristian Vättö - Friday, July 15, 2011 - link

    I have added a (?) to suggest that it's not certain whether Komodo features an IGP or not.
  • jjj - Friday, July 15, 2011 - link

    I don't see any GPU on the slide TimCh provided (and do read the fine print).Anyway there is only 1 botched slide with GPU and 8 cores and if you think about it, it makes no sense.
    Why have Sepang with 10 and Komodo with 8,it would be a waste of time and money and there would be no reason to have a different name for it,instead of Trinity.Also how big would the chip be with 8 cores and GPU,even if they drop the L2 cache and why add a GPU to a chip adressing the high end where most don't need it (and make the same mistake as Intel using the HD 3000 for K chips).
  • jjj - Friday, July 15, 2011 - link

    edit to prev post: obviously i ment L3
  • rnssr71 - Saturday, July 16, 2011 - link

    'Also how big would the chip be with 8 cores and GPU,even if they drop the L2 cache and why add a GPU to a chip adressing the high end where most don't need it (and make the same mistake as Intel using the HD 3000 for K chips).'

    well, 8 core bulldozer(4 module) is going to be over 300sq. mm. larger than llano(on 32nm) but smaller than thuban(45nm).
    i would guess that 8 meg of L3 cache that the 8 core bulldozer will have would take up 2/3 as much room as the current gpu in llano. so, quite a lot of space.
    so you're right, it would be a mistake to have a full sized gpu......maybe ANY gpu until gpu computing really takes off.
  • Casper42 - Friday, July 15, 2011 - link

    Just an FYI, while it is true that Insight Control is required for power capping a cluster of Rackmount (DL) servers, you get Dynamic Power Capping withing a single c7000 chassis today for free.

    So for Clusters of 16 servers in the same chassis, you don't need IC licenses to see a pretty big benefit. When one server needs more power, the other servers can all be throttled back slightly to let the one burst and still get the workload done, like you mentioned.

    DPC in blade chassis also has 3 different settings.
    One is Average Load for Thermals
    Two is Average Power Draw (80% rule on individual circuits)
    Three is Maximum Power Draw (Do Not Exceed - Circuit protection)
    So you can not only unlock excess capacity with the Max Draw setting being lower than faceplate value, but you can also tune the other values to hit the appropriate thermal values for the DC.

    I've heard there will be an Insight Control powered Multi-chassis DPC coming next year as well. And for those familiar with HP gear, Insight Control licenses, when purchased in 8/16 packs along side a new chassis are only like $50 more per server than iLO Advanced which alot of people already purchase. With normal Enterprise discounts the gap can be even smaller.

    Johan/Kristian - where are you guys located? (roughly)
  • Kristian Vättö - Friday, July 15, 2011 - link

    I'm from Finland (GMT +3 now) but I don't know were Johan lives. FYI, Johan covered the power capping section so you have to wait for him if you think there are any changes necessary.

    I would suggest shooting him an email (click his name on the top) if you want to contact him. Not all of us read the comments, even though we are supposed to :)
  • JohanAnandtech - Friday, July 15, 2011 - link

    I am located in Belgium. You know the land without a government but with the best beer in the world. Paradise thus ;-).

    Thanks for the valuable feedback. Are those 3 settings the only choices you have to tune your power draw?
  • StevoLincolnite - Friday, July 15, 2011 - link

    Spelling error, Johan!
    "It will aslo be compatible with AMD's current San Marino and Adeleide platforms (Opteron 4000 Series) for socket C32."

    It should be Adelaide, not Adeleide. :)

    Other than that, good job.
    Hanging out badly for Zambezi's release, my rig is ready to drop a new 8-core chip into it. :)
  • Kristian Vättö - Friday, July 15, 2011 - link

    Actually, I wrote that part. I have fixed it now along with a few other minor edits. I added that the latest word appears to be October release (just saw it in my RSS today) and I also added a (?) to the Komodo's IGP as it seems to be uncertain.

    Thanks for the feedback :)

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