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Turbo overclocking on High TDP CPUs; the 8-core Intel Sandy Bridge EP example

Both Intel and AMD now include automatic overclocking-style 'Turbo' performance enhancements in their CPUs. Once you reach the very high TDP territory, like what it is with the Bulldozer or will be with the upcoming 8 core Sandy Bridge EP, what would make best sense?

Turbo, Overdrive, call it whatever you want – Intel and AMD alike figured out the potential of that built in 'headroom' that is available in most of their CPUs well beyond the stock clock. The overclocking enthusiast community used it for years now to achieve some incredible results.

However, we should distinguish between 'suicidal overclocking' including some LN2 and other exotic deeds, where the dies become pretty much unusable after sustained exposure to such stresses, and so called 'productive overclocking' where a measurable performance enhancement is achieved by upping the clocks and tightening other parameters without major voltage increases or other stresses on the CPU or system, leading to expected long term productive usage at that higher speed. Obviously, the vendors are interested in the latter one.
Of course, they have to be far more careful with Turbo, since it, unlike overclocking, falls within the warranty specs. Its automatic adjustments also vary depending on the cooling efficiency, power delivery from the mainboard, and of course the compute load on the CPU. What happens once your TDP at default state reaches 150W, like the upcoming high end of the Xeon E5 2600 'Sandy Bridge EP' line?
At that point, the default speed is already 'hot' in every sense of the word. Sandy Bridge already implements power spike control in its Turbo mode management to avoid breaching the Icc_max value, and ensures that everything is ran within thermal capacity of the CPU die, packaging and voltage regulation circuitry – all in 1 millisecond sampling intervals. It can also use transient thermal headroom to exceed the performance further for a limited time.
With all these enhancements, Intel is expected to provide substantial Turbo power and thermal headroom limits for its upcoming flagship processors of the Sandy Bridge EP line. If we look at the 150W top end part, the one that enthusiasts would like to see unlocked in mainboards such as EVGA SR-X, it by default allows for constant usage at 180W TDP, and maximum usage at 230W TDP – not to say that, with the right mainboard and cooling, it couldn't run consistently at 230W as well.
If we assume the starting frequency is, say, 3.1 GHz , and we can go all the way to 3.5 GHz when using all the cores if the system power and cooling allow, this by itself is not bad improvement over the base line. However, three categories of uses – enthusiasts including gamers; workstation 3-D & HPC application users; and high-frequency traders – will want even more. After all, let's not forget who bought most of the EVGA SR2 mainboards over the past year or so.
When we have such high TDP figures at default, is it better to just leave things as they are and rely on Turbo, or get an unlocked version and reach into the overclocking realms with TDP going beyond 250W per socket? Remember, here you'll also have a 4-socket version, the E5 4600, which would then mean a kilowatt just for the four overclocked CPUs…
Based on what could be done with the Core i7 3960X, which after all is just a reduced 6-cores-out-of-8 version of this same die, and assuming progressively better cooling and power delivery to be provided for the full 8-core version, there is no reason why there can't be 4 GHz 'productive overclocking' dual-socket Sandy Bridge-EP systems at very reasonable minimal voltage and system tweaks, providing 25% performance above the baseline. With further cooling and tweaks, even 4.3 GHz should be OK for medium to long term use. Anything beyond that, we need to wait for D-stepping, I guess.
In summary, Turbo will provide a decent performance hop even on super high TDP CPU like the Xeon E5 top of the range in the 150W range, however, the cooling and mainboard design choices have progressed so far now, together with the overclocking experience, that it would be a pity not to open up the headroom to those willing to make the best use of it – whether to reach new records in suicidal overclocking of 16 full cores at one go, or 'productive enhancement' to monetize the extra GHz for those who need it – engineers, designers, gamers or traders alike.

Nebojsa Novakovic
In the spare time over the past two decades, editor and writer of high-end computer hardware and design features and analysis for European and US media and analyst houses. Reviews of high end hardware are my specialty for 28 years already.

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