First of all, let's discuss the limitations of this review. The benchmark we used allowed us to control the number of threads very accurately, but it is not a real world benchmark for most IT professionals. The fact that it is an integer dominated benchmark means that it has some relevance, but it's still not ideal. In our next article we will be using MS SQL Server 2008 R2. That will allow us to measure power efficiency at a certain performance level, which is also much more relevant than pure performance/watt. Also, the low power six-core Opteron 2419 EE is missing. This CPU just arrived in the labs as we finished this article, so expect an update soon.

"Academic" Conclusion

The days where dynamic frequency scaling offers significant power savings are over. The reason is that you can only lower voltages if you scale the complete package towards a lower clock. In that case the power savings are considerable (P ~ V²), but we did not encounter that situation very often. No, both AMD and Intel favor the strategy of placing the idle cores in higher C-states. The most important power savings come from fine grained clock gating, from placing cores in a completely clock gated C-state (AMD's Smart Fetch + C1), or even better placing them in a power gated stated (Intel's Power Gating into deep C6 sleep).

Practical Conclusions

Windows 2008 makes you choose between Balanced and Performance power plans. If your application runs at idle most of the time and you are heavily power constrained, Balanced is always the right choice. But in all other cases, we would advise using the "Performance" plan for the Opterons. For some reason, the CPU driver does not deliver the performance that is demanded. With Balanced, when you ask for 25% of the total CPU performance, you'll get something like 15% to 20%. The result is that you get up to 25% less performance than the CPU delivers in "Performance" mode, without significant power savings. That's not good. We can already give away that we saw response time increases in MS SQL Server 2008 due to this phenomenon. It is also worth saying that our new measurements confirm that the performance/watt ratio of the six-core Opterons is significantly better than the quad-core Opterons.

The Xeons are a different story. For the normal 95W Xeons it makes sense to run in Balanced mode. The "base" performance is excellent and Turbo Boost adds a bit of performance but also quite a bit of power. Ideally, it should be possible to run in Balanced mode and use Turbo Boost when your application is performing a single threaded batch operation, but unfortunately this is not possible with the default Windows 2008 settings.

For the low power Xeons, it is different. Those CPUs run closer to their specified TDP power limit and will rarely use Turbo Boost as soon as they are loaded at 25% or more. If your application is limited by regular single threaded batch operations, it makes a lot of sense to choose the Performance plan. Turbo Boost pays off in that case: the clock speed is raised from a meager 1.86GHz to an impressive 3.2GHz. As Xeons based on the "Nehalem" architecture place idle cores in C6 very quickly, the Performance mode hardly consumes more than the Balanced mode. As we have shown, frequency scaling does not save much power, as most of the cores are power gated automatically. This aggressive "go to C6 sleep" policy allows the architecture with the highest IPC in the industry to morph into a high performance server CPU with modest power consumption. There is a huge difference between this CPU inside a machine where it is pushed towards 100% load and inside a server where it hovers between 20 and 70% load most of the time. The latter situation allows the CPU to put cores in C6 mode a significant amount of time. As a result the power savings in a server environment are nothing short of impressive.

Now that we understand the nuts and bolts, we are able to move on to our next question: How can we get the best power efficiency at a certain performance point? We will follow up with a power efficiency case study based on SQL Server 2008.


[1] "Planet Google": One Company's Audacious Plan to Organize Everything, page 82, Randall Stross, Free Press New York.

[2] "AMD Family 10h Server and Workstation Processor Power and Thermal Data Sheet Publication Revision: 3.07, September 2009"

[3]"Power Reduction through RTL Clock Gating," F. Emnett and M. Biegel, SNUG (Synopsis User Group) Conference San Jose, 2000.

[4] "45nm Next Generation Intel Core™ Microarchitecture (Penryn)", Varghese George Principal Engineer Intel Corp, HOT CHIPS 2007

[5] "Analysis of Dynamic Power Management on Multi-Core Processors", W. Lloyd Bircher and Lizy K. John, The University of Texas at Austin. ICS '08 June 2008

[6] "Intel Xeon Processor 3400 series thermal/mechanical specifications and design guidelines, December 2009

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  • UrQuan3 - Thursday, January 21, 2010 - link

    I'm trying to remember for 2008, but wasn't there a way to either force or suggest thread/core affinity? It looks like the scheduler was hopping all over the place on the Opterons.
  • JarredWalton - Thursday, January 21, 2010 - link

    You guys better pay attention and answer this post, or his species will try to enslave and/or wipe out the entire galaxy! ;-)
  • mino - Wednesday, January 20, 2010 - link

    I mean, not, why do you use them for this article.
    They are fine examples of low-power platforms, even if from vastly different markets.

  • IntelUser2000 - Wednesday, January 20, 2010 - link

    By the way, I don't know if you have the settings wrong or that's how it works, the Turbo Boost mode is not affected on the Home PC versions of Windows. Balanced uses Turbo Boost just as well on my Windows 7 Home Premium with Core i5 661.

  • JarredWalton - Wednesday, January 20, 2010 - link

    I was wondering this as well, but I'm not familiar with Windows Server... what I do know is that Power Saver on consumer Windows OSes really limits the CPU frequency scaling features, and it sort of looks like Balanced on the Server OS has aspects of consumer "Power Saver" as well as some elements of "Balanced". Odd to see only two power settings available, where Win7 now has at least 3 and often 5.
  • mino - Wednesday, January 20, 2010 - link

    It seems a classic example of KISS strategy of choosing the most-sensible options and so reducing decision complexity for IT people.

    Modes like "Max battery" have anyway no reason for existence on a server box.
  • RobinBee - Tuesday, January 19, 2010 - link

    If you use your pc as a music server:

    Power saving methods ruin sound quality even if using a good sound card. The problem is »electronic« sound distortion. I do not know why this happens.

    Also: The chosen number of IRQ pr. second in a net card can ruin sound quality too. Why, I do not know.
  • Anato - Tuesday, January 19, 2010 - link

    I'm interested to see results from different operating systems which may be better at controlling processes in different CPU's. Namely no CPU hopping and is their power management as efficient as Windows is.

    Most interested at:
    Linux and Solaris
  • JohanAnandtech - Tuesday, January 19, 2010 - link

    Excellent suggestion :-). Problem is to keep the application the same. We currently tested SQL Server 2008 on Windows 2008 and of course this can not be done on Linux. However, I am not stranger to linux as a server.

    I am no fan of MySQL on Windows, but maybe this has improved. Would MySQL on Windows and Linux makes sense as a comparison?
  • maveric7911 - Tuesday, January 19, 2010 - link

    Why not use oracle ;)

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