Inside the Nehalem: Intel’s New Core i7 Microarchitecture

Perhaps the most interesting bit of news out of Intel’s Nehalem was something called Turbo Mode – a feature directly enabled by the PCU we discussed on the previous page.  With modern processors, the debate has raged whether users are better off getting a quad-core CPU at a lower frequency or a dual-core CPU at a higher frequency.  Intel is hoping that with Turbo Mode users will get the best of both worlds. 

The idea is pretty straight forward: if you have four cores that run at combined power consumption (and heat dissipation) of X, then if you only have two cores loaded (with the other two at idle) then you have additional power headroom to overclock the working cores to a higher frequency. 

For enthusiasts and gamers this should been an exciting turn of events.  While Intel wasn’t very specific at this point I imagine we’ll see ranges of 200-300 MHz going from the full quad-core clock rate to the a dual-core or single-core (based on idle cores at the time.  This means if you purchase a 3.2 GHz Core i7 Nehalem based processor, you will likely see clock rates as high as 3.5 GHz when running single threaded or just dual threaded applications.  Gamers should also take note of this!

This functionality can be built on even further if a single core is more heavily loaded than another – it could get another “notch” of performance boost from the PCU.

Intel claims that with the power of the PCU inside the chip the Nehalem core is aware of its surroundings and conditions.  If your system is running very cool, say you have water cooling for example, the chip will recognize that it is well under its own TDP and push the clocks even faster.  This is possible even while loading all four cores as the above diagram shows.  The on-board micro-controller tunes voltages based around a given frequency, operating conditions and specific silicon characteristics.  In some ways it appears that the Nehalem core will be able self award enough to find out how far it can be pushed without burning up.

This new Turbo Mode technology is treated just like another P-state to the operating system and is completely transparent to any software or games.  The PCU regulates the frequency and power within it’s own specs so there is no need to adapt or tune software for it. 

We did learn that users will be able to disable this feature if they want (say for our own testing purposes) and that Extreme Edition processors will be allowed to toggle Turbo Mode “notches” on their own to tweak the system manually.  For those that want full control Intel’s performance team again reiterated that would be an option for the Extreme Edition CPUs – be prepared to open up that wallet!  But for general users of Intel’s Core i7 processor, Turbo Mode technology is a great attempt at balancing the quad-core vs dual-core debate.

Final Thoughts on Nehalem Deep Dive

We are now fully in Nehalem anticipation mode – the CPU technology world has been floundering since AMD’s less than spectacular Barcelona/Phenom processor release and we are really looking forward to newer, faster and more efficient parts.  Performance numbers on Nehalem are apparently still under embargo though we can say we saw ranges from single-digit percentages in gaming to 40%+ in encoding and rendering.  We now have all/most of the details on the CPU design and I know we will have LOTS of scenarios to test with all of Intel’s new features and technologies. 

Be prepared for a busy fall and winter.