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

    Komodo is a CPU that replaces Zambezi. It does not have DX11 IGP. So that should be a "No" in that category. It is not an APU.

    In the "Socket" category, both Trinity and Komodo will use some form of Socket FM infrastructure. AMD currently refers them as Socket FMx (where x = 1, 2, 3, etc). It doesn't mean that both will use the same socket.

    See a thread I've created at forums regarding AMD's 2012 lines.
    (I've collected a good number of official and leaked presentation slides.)
  • jjj - Friday, July 15, 2011 - link

    Also about Komodo, it has 10 cores not 8.
  • Kristian Vättö - Friday, July 15, 2011 - link

    All leaked slides suggest 8 cores. If you have something to proof the 10 cores, then please share it with us..
  • TimCh - Friday, July 15, 2011 - link

    Here you go

    No need for leaks.
  • stmok - Friday, July 15, 2011 - link

    The slide you point to is November 9th 2010 for AMD Financial Analyst Day.

    The slide in the thread I've created is dated January 2011 for CES 2011.

    Your info regarding 10-cores is out of date. Its 8-cores for Komodo.
  • inf64 - Friday, July 15, 2011 - link

    No,look closely in the slide. There is a Correction at the bottom.
    It says:
    Correction,MArch 8,2011

    So they corrected the IGP error in Komodo and corrected the core count number.Now it is 6-10 enhanced/NG Bulldozer cores.
    So yes,Komodo will feature up to 10 Bulldozer+ cores.
  • mino - Friday, July 15, 2011 - link

    In other words, Komodo is the C2012 part for desktops.

    However there is one issue - Komodo will go for AM3+ OR FM1, it is VERY unlikely AMD would go for another socket in 2012.

    And since there is no PCIe in AM3+ while also no IGP on FM1 chip ... it is more likely they go for FM1 with Komodo actually having Display controller but not having a GPU - the same as some embedded Brazos parts today.

    Last (sensible) option is for AMD to go FM1 with the same setup as Lynnfield.
  • Topweasel - Thursday, July 21, 2011 - link

    Nothing you said in this makes sense. AM3+ has PCIe, There isn't an IGP on the chipset for FM1 because no CPU in that socket would be missing it, and Brazos has a barely capable IGP, (40SP unit?) but it isn't just some 2d display controller.
  • Kristian Vättö - Saturday, July 16, 2011 - link

    I have updated the article to be up-to-date with the slide you provided.
  • Kristian Vättö - Friday, July 15, 2011 - link

    Even the link you provided suggests that Komodo will feature a DX11 capable IGP. Note that it says GPU for Komodo but nothing for Zambezi.

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