Multiple Clock Domains

Functionally there are some basic differences between Nehalem and previous Intel architectures. The Front Side Bus is gone and replaced with Intel's Quick Path Interconnect, similar to AMD's Hyper Transport. The QPI implementation on the first Nehalem is a 25.6GB/s interface which matches up perfectly to the 25.6GB/s of memory bandwidth Nehalem has.

The CPU operates on a multiplier of the QPI source clock, which in this case is 133MHz. The top bin Nehalem runs at 3.2GHz or 133MHz x 24. The L3 cache and memory controller operate on a separate clock frequency called the un-core clock. This frequency is currently 20x the BCLK, or 2.66GHz.

This is all very similar to AMD's Phenom, but where the two differ is in how they handle power management. While AMD will allow individual cores to request different clock speeds, Nehalem attempts to run all of its cores at the same frequency; if one core is idle then it's simply power gated and the core is effectively turned off. I explain this in greater detail here but the end result is that we don't have the strange performance issues that sometimes appear with AMD's Cool'n'Quiet enabled. While we have to turn off CnQ to get repeatable results in some of our benchmarks (in some cases we'll see a 50% performance hit with CnQ enabled), Intel's EIST seems to be fine when turned on and does not concern us.

My Concern

Looking at Nehalem's microarchitecture one thing becomes very clear: this is a CPU designed to address Intel's shortcomings in the server space. There's nothing inherently wrong about that, but it's a different approach than what Intel did with Conroe. With Conroe Intel took a mobile architecture and using the philosophy that what was good for mobile, in terms of power efficiency and performance per watt, would also be good for the desktop, it created its current microarchitecture.

This was in stark contrast to how microprocessor development used to work; chips would be designed for the server/workstation/high end desktop market and trickle down to mainstream users and the mobile space. But Conroe changed all of that, it's a good part of why Intel's Core 2 architecture makes such a great desktop and mobile processor.

Power obviously also matters in servers but not to the same extent as notebooks, needless to say Conroe did well in the server market but it lacked some key features that allowed AMD to hang onto market share.

Nehalem started out as an architecture that addressed these enterprise shortcomings head on. The on-die memory controller, Hyper Threading, larger TLBs, improved virtualization performance, restructured cache hierarchy, the new 2nd level branch predictor, all of these features will be very important to making Intel more competitive in the enterprise space, but at what cost to desktop power consumption and performance?


Intel promises better energy efficiency for the desktop, we'll be the judge of that...

I'm stating the concern up front because when I approached today's Nehalem review that's what I had in mind. Everyone has high expectations for Nehalem, but it hasn't been that long since Intel dropped Prescott on us - what I want to find out is whether Intel has stayed true to its mission on keeping power in check or if we've simply regressed with Nehalem.

The only hope I had for Nehalem was that it was the first high performance desktop core that implemented Intel's new 2:1 performance:power ratio rule. Also used by the Atom's design team, every feature that made its way into Nehalem had to increase performance by 2% for every 1% increase in power consumption otherwise it wasn't allowed in the design. In the past Intel used a general 1:1 ratio between power and performance, but with Nehalem the standards were much higher. We'll find out if Intel was all talk in a moment, but let's take a look at Nehalem's biggest weakness first.

Index The Chips
POST A COMMENT

74 Comments

View All Comments

  • Jingato - Monday, November 03, 2008 - link

    If the 920 can easily be overclocked to 3.8Ghz on air, what intensive is there to purchase the 965 for more that triple the price? Reply
  • TantrumusMaximus - Monday, November 03, 2008 - link

    I don't understand why the tests were on such low resolutions... most gamers are running higher res than 1280x1024 etc etc....

    What gives?
    Reply
  • daniyarm - Monday, November 03, 2008 - link

    Because if they ran gaming benchmarks at higher res, the difference in FPS would be hardly visible and you wouldn't go out and buy a new CPU.
    If they are going to show differences between Intel and AMD CPUs, show Nehalem at 3.2 GHz vs 9950 OC to 3.2 GHz so we can see clock for clock differences in performance and power.
    Reply
  • npp - Monday, November 03, 2008 - link

    9950 consumes about 30W more at idle than the 965XE, and 30W less under load. I guess that OC'ing it to 3,2Ghz will need more than 30W... Given that the 965 can process 4 more threads, I think the result should be more or less clear. Reply
  • tim851 - Monday, November 03, 2008 - link

    Higher resolutions stress the GPU more and it will become a bottleneck. Since the article was focussing on CPU power and not GPU power they were lowering the resolution enough to effectively take the GPU out of the picture. Reply
  • Caveman - Monday, November 03, 2008 - link

    It would be nice to see these CPU reviews use relevant "gaming" benchmarks. It would be good to see the results with something like MS flight simulator FSX or DCS Black Shark, etc... The flight simulators these days are BOTH graphically and calculation intensive, but really stress the CPU. Reply
  • AssBall - Monday, November 03, 2008 - link

    No, they don't, actually. Reply
  • philosofool - Monday, November 03, 2008 - link

    It would have been nice to see a proper comparison of power consumption. Given all of Intel's boast about being able to shut off cores to save power, I'd like to see some figures about exact savings. Reply
  • nowayout99 - Monday, November 03, 2008 - link

    Ditto, I was wondering about power too. Reply
  • Anand Lal Shimpi - Monday, November 03, 2008 - link

    Soon, soon my friend :)

    -A
    Reply

Log in

Don't have an account? Sign up now