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
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  • Th3Eagle - Monday, November 03, 2008 - link

    I wonder how close you came to those temperatures while overclocking these processors.

    The 920 to 3.6/3.8 is a nice overclock but I wonder what you mean by proper cooling and how close you came to crossing the 80C "boundary"?
    Reply
  • Gary Key - Monday, November 03, 2008 - link

    "The 920 to 3.6/3.8 is a nice overclock but I wonder what you mean by proper cooling and how close you came to crossing the 80C "boundary"?"

    It was actually quite easy to do with the retail cooler, in fact in our multi-task test playing back a BD title while encoding a BD title, the core temps hit 98C. Cinebench multi-core test and OCCT both had the core temps hit 100C at various points. Our tests were in a closed case loaded out with a couple of HD4870 cards, two optical drives, three hard drives, and two case fans.

    Proper cooling (something we will cover shortly) consisted of the Thermalright Xtreme120, Vigor Monsoon II, and Cooler Master V8 along with the Freezone Elite. We were able to keep temps under 70C with a full load on air and around 45C with the Freezone unit.
    Reply
  • Th3Eagle - Tuesday, November 04, 2008 - link

    Wow, thats interesting. Can't wait to see the new article. Always nice to see an article about coolers.

    Thanks for the reply.
    Reply
  • Anand Lal Shimpi - Monday, November 03, 2008 - link

    Gary did the i7-920 tests so I'll let him chime in there, we're also working on an overclocking guide that should help address some of these concerns.

    -A
    Reply
  • whatthehey - Monday, November 03, 2008 - link

    Tom's? You might as well reference HardOCP....

    Okay, THG sometimes gets things right, but I've seen far too many "expose" articles where they talk about the end of the world to take them seriously. Ever since the i820 chipset fiasco, they seem to think everything is a big deal that needs a whistle blower.

    Anandtech got 3.8GHz with an i7-920, and I would assume due diligence in performance testing (i.e. it's not just POSTing, but actually running benchmarks and showing a performance improvement). I'm still running an overclocked Q6600, though, and the 3.6GHz I've hit is really far more than I need most of the time. I should probalby run at 3.0GHz and shave 50-100W from my power use instead. But it's winter now, and with snow outside it's nice to have a little space heater by my feet!
    Reply
  • The0ne - Monday, November 03, 2008 - link

    TomHardware and Anandtech were the one websites I visited 13 years ago during my college years. Tom's has since been pushed far down the list of "to visit sites" mainly due to their poor articles and their ad littered, poorly designed website. If you have any type of no-script enable there's quite a bit to enable to have the website working. The video commentary is a joke as they're not professionals to get the job done professionally...visually anyhow.

    Anandtech has stayed true to it's root and although I find some articles a bit confusing I don't mind them at all. Example of this are camera reviews :)
    Reply
  • GaryJohnson - Monday, November 03, 2008 - link

    Geez, calling a core 2 a space heater. How soon we forget prescott... Reply
  • JarredWalton - Monday, November 03, 2008 - link

    I think overclocked Core 2 Quad is still very capable of rating as a space heater. The chips can easily use upwards of 150W when overclocked, which if memory serves is far more than any of the Prescott chips did. After all, we didn't see 1000W PSUs back in the Prescott era, and in fact I had a 350W PSU running a Pentium D 920 at 3.4 GHz without any trouble. :-) Reply
  • Griswold - Tuesday, November 04, 2008 - link

    Funny comparison. If it was just for the space heater arguments sake (well, 150W is by far not enough to qualify as a real space heater to be honest), I could follow you but saying the 150W of a 4 core, more-IPC-than-any-P4-can-ever-dream-of, processor should or could be compared to the wattage of the infamous thermonuclear furnace AKA prescott, is a bit of a long stretch, dont you think? :p Reply
  • Ryan Smith - Monday, November 03, 2008 - link

    Intel can call it supercalifragilisticexpialidocious until they're blue in the face, but take it from a local, it's Neh-Hay-Lem. Just see how it's pronounced in this news segment:

    http://www.katu.com/outdoors/3902731.html?video=YH...">http://www.katu.com/outdoors/3902731.html?video=YH...
    Reply

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