EIST and Turbo Boost but without frequency lower than nominal

EIST and Turbo Boost but without frequency lower than nominal


While this forums about power efficiency on mobile platforms I would like to ask a question about Sandy Bridge EP. If this forum is a wrong place could you point to the right one.

SNB-EP is configured in BIOS that EIST and TurboBoost are enabled. It is Ok, as turbo provide extra performance virtually for free. Usually system is highly utilized, but not always. While OS's power setting is set to performance, CPU drops its frequency to lowest possible p-state (1200MHz) at idle. So here comes to play another factor, latency. There is a piece of code that is really sensitive to that. And as system warming up after idle period it could run code for a while with frequency lower than a reference one. This really hurts latency of code snippet and eventually all system performance. 

The question: Is it possible either via BIOS or via MSRs to have EIST and TurboBoost enabled, but at the same time eliminate any unhalted cycles at frequency lower than the reference one? Or in other words to allow TurboBoost, but forbid p-states other than p0. 







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1.OS power setting at performance : The CPU will operate at P0 state irrespective of processing load.
2. Whenever OS executes a program ( specially code like FFT etc) OS automatically elevates the P state to P0.
You can use tools like Speccy to monitor cpu frequency. Since OS is the decisions maker, you may have to select appropriate power plane setting whose details you can find here ( http://msdn.microsoft.com/en-us/library/windows/hardware/dn613985%28v=vs... )
Also there is some latency during P state transition, though it is minimal but unavoidable.

Hope that Helps,
Naresh Solanki

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