Understanding Nehalem’s Turbo Mode

Modern day CPUs and GPUs are more power constrained than anything else. They could run faster, if they could get around pesky problems like power density. Intel and AMD have both figured out that the maximum power consumption for a single processor falls into one of the following ranges depending on the platform:

System Processor TDP Number of Cores
High End Desktop 80 - 130W 4
Mainstream Desktop 65W 2 - 4
Notebook 20 - 45W 2
Ultra Portable Notebook 10 - 20W 1 - 2
Netbook 2 - 5W 1

 

If we look at the bottom of the table we see that our limits to performance aren’t technology, but rather power; netbooks could be as fast as desktops if we could stick 130W processors in them.

Pay attention to the third column however. A high end desktop processor is designed to dissipate up to 130W of heat; you reach that value by running all four cores at full load. But what happens if you only have two active cores? The total power consumption and thermal dissipation of your processor is no longer 130W, it’s noticeably less.

I just finished saying that power was our fundamental limit to faster microprocessors, but if half of a 130W chip is idle - shouldn’t the working half be able to run faster? The answer is yes, but only with some clever technology.

The Nehalem CPU includes a fairly complex hardware monitoring microprocessor on-die. This processor is called the Power Control Unit (engineers r awesome). It monitors the temperature, current and power consumption of each core independently. The PCU also the part of the chip that handles OS requests to drop the cores down to lower power states. Now get this; if there’s room in the power envelope, and the OS requests a high performance state, the PCU will actually increase the clock speed of the active cores beyond their shipping frequency.

It all boils down to the TDP of the chip, or its Thermal Design Point. The more TDP constrained a platform is, the more you stand to gain from Intel’s Turbo mode. Let me put it another way; in order to fit four cores into a 130W TDP, each core has to run at a lower clock speed than if we only had one core at that same TDP.

At higher TDPs, there’s usually enough thermal headroom to run the individual cores pretty high. At lower TDPs, CPU manufacturers have to make a tradeoff between the number of cores and their clock speeds - that’s where we can have some fun.

The Other Difference Between the Quad and Eight Core Models

Apple sells two versions of the new Mac Pro, a quad-core and an eight-core system. The motherboard is the same in both machines, but the processor board is different. The quad-core processor board has a single LGA-1366 socket and four DIMM slots, while the eight-core processor board has two sockets and eight DIMM slots. They also use significantly different CPUs, although Apple doesn’t tell you this.

Below you’ll find the standard and upgraded options for each system:

Apple Mac Pro (2009) Quad Core Model Eight Core Model
Default CPU Xeon W3520 (2.66GHz) Xeon E5520 (2.26GHz)
CPU Upgrade Options Xeon W3540 (2.93GHz) Xeon X5550 (2.66GHz)
Xeon X5570 (2.93GHz)

 

Although Apple offers a 2.93GHz CPU in both systems, it’s actually a different chip that’s used in each model. The clock speeds, core counts and cache sizes are the same, the difference is in the TDP.

The quad-core Mac Pro uses 130W TDP Xeon uniprocessor workstation processors, the eight core Mac Pro however uses an 80W (2.26GHz) or 95W chip (2.66/2.93GHz). There are more CPUs in the eight-core model, so Intel offers chips with lower TDPs to keep total platform power under control. While the eight-core Mac Pro uses more power than the quad-core Mac Pro, each chip individually should use less power. And remember what we discussed earlier: lower TDPs mean higher turbo frequencies.

The table below shows the maximum turbo frequency available for each chip depending on the number of cores currently in use:

System (Processor) Default Clock Max Turbo w/ 4-cores active Max Turbo w/ 3-cores active Max Turbo w/ 2-cores active Max Turbo w/ 1-core active
8-core Mac Pro (Xeon X5570) 2.93GHz 3.20GHz 3.20GHz 3.33GHz 3.33GHz
8-core Mac Pro (Xeon X5550) 2.66GHz 2.93GHz 2.93GHz 3.06GHz 3.06GHz
8-core Mac Pro (Xeon E5520) 2.26GHz 2.40GHz 2.40GHz 2.53GHz 2.53GHz
4-core Mac Pro (Xeon W3540) 2.93GHz 3.06GHz 3.06GHz 3.06GHz 3.20GHz
4-core Mac Pro (Xeon W3520) 2.66GHz 2.80GHz 2.80GHz 2.80GHz 2.93GHz

 

What the table above tells us is that while the quad-core Mac Pro can turbo up by 133MHz if more than one core is active, and 266MHz if only one core is active, the processors in the eight-core Mac Pro can do better. The Xeons in the eight-core Mac Pro can turbo up by 266MHz or 333MHz, depending on the number of cores active. The 333MHz turbo mode is available even if two cores are active.

Apple isn’t big on specs like these so we don’t see any mention of them in Apple’s Mac Pro sales literature, the only clue you get is in the form of the model numbers Apple lists on its spec sheets:

Although it’s a pricey upgrade, you do get better processors with the eight-core Mac Pro than you do with the quad-core version. If you don’t need more than four cores however, you’ll still be better off with a 2.66GHz quad-core Mac Pro than a 2.26GHz eight-core model.

The Crossroads of Simplicity and Sophistication Performance
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  • TonkaTuff - Tuesday, July 14, 2009 - link

    Hello Mr Anandtech,

    My name is Alfredo and I am from Denmarksland.
    I have recently started a computer company called Lemon computers.
    I wish to send you my top of the line LemonPro for review.

    LemonPro Specs.

    1xLemonPro superwhizzbanger professional motherboard (series 2)
    Supports up to superfast DDR3 1066mhz ram. Has 2! Yes you read it right the first time 2! PCIE slots for the true computing professional.

    1xLemonPro Core i7 920 cpu at 2.66ghz
    Custom built by intel for lemon computers, you maybe able to get this cpu in other computers but ours are very "special" i7 920 cpu's.

    3 Gig of Lemon DDR3 1066mhz ram
    Super high performance memory hand picked from corsairs finest value select batchs.

    1x Lemon 640gig 5400rpm High Performance Hard drive
    Theres not enough porn on the net to fill this puppy up. Ultimate storage for the true professional.

    1x Lemon/Nvidea 9500GT Graphics
    Experience awesome crysis in ultra high detail at blazingly fast frame rates of up to 3 (a minute). It just doesnt get better than this.

    1x LemonRay 18x Read 1x Write DVD player
    For the true computing professional

    Lemon computers, for the true computing professional, professional's need only apply. P.S. PROFESSIONAL!

    *LemonPro pc's are fitted with the trademark LemonDP display connection, Unfortunately there is only one Monitor on the planet that has this style of connection (where that far ahead of the competition its SCARY!) and is available through Lemon for RRP $1200 (20inch display). Or a special custom built adapter can be used, available through Lemon RRP $100.

    Hopefully you will supply my system with as good a review as those MacPro's. Also My system is 50 dollers cheaper than the Apple system, only $2449 RRP

    Yours Professionally, Alfredo

    Ok, I am not going to go down the biased track here, I am not a fanboi and never have been, I judge something on what it delivers at its given price.

    But Honestly if this bloke from Lemon computers gave you the above system with a vista o/s for review with a $2500 price tag we all know what the resulting article would have to say about it.

    Do I think your biased? No I don't. Do I think there are very large double standards at play? Absolutely.

    I mean honestly what apple is providing is the equivalent of an $1000 PC. FOR $2500. This is a hardware analysis site is it not?
    How can you possibly justify paying that sort of coin over and above an equivalent PC? Where is that extra $1500 worth of justification?
    Answer me that question and I will eat my words.


    Reply
  • gorbag - Tuesday, July 14, 2009 - link

    There's a difference between tossing a bunch of parts in a bag and calling it a $1000 'system', and the kind of components and design you get from Apple (or other prime tier vendors for that matter). So let's not compare what you buy from the showroom floor with what you can do with the back room sweepings OK? Reply
  • JimmiG - Tuesday, July 14, 2009 - link

    I wouldn't exactly define an i7 920 and an Asus or Gigabyte mobo "back room sweepings" though.

    You must be talking about the housing, because the actual components (you know, those boards and chips that make up the computer) are the same (minus the heat spreader).

    So yeah, for your own home built i7 system to match the Mac Pro in terms of "design", you might have to add a couple of hundred to your budget for a quality case and a quiet aftermarket CPU cooler, then spend 30 minutes doing some cable management after the built is finished. But a $1000 PC will *do* all the things the Mac Pro does, except look shiny and "professional" on the outside, which is not worth a $1000 - $1500 premium + no easy upgrade path, IMO.
    Reply
  • erple2 - Wednesday, July 15, 2009 - link

    The problem is that you can't actually buy a Core i7 computer from apple. You MUST buy the Xeon equivalent. Which drives the cost of the home built computer up significantly. That means that the Apple tax is no longer 1000+ dollars, and closer to about 750 dollars.

    The other problem is that you're probably also the person who will complain that Alienware computers are overpriced. Or Falcon Northwest. Of COURSE they cost more - they're for a particular niche of the population that wants to run a fast computer and wants someone else to put them together and support them. In my experience, the support you get from Falcon Northwest or Alienware IS worth the cost if you aren't the type that likes to crack open your case.

    The other problem is that the Apple hardware is also the only hardware that you can "easily" run OSX (after fighting trying to get OSX running with an i7 920, I can't say that it's easy at all).

    You are essentially arguing that all high end workstations are the same. This is, unfortunately, not entirely true - the integrated package does matter to a significant number of businesses that want to run hardware. The initial cost of hardware is insignificant relatively to the support costs. Maybe Apple is cheaper than a home built computer that you'll have to employ someone to support the home builts. maybe it's cheaper to buy a bunch of Apples, and have a "dumber" tech to administer them with Apple's help.
    Reply
  • Hxx - Tuesday, July 14, 2009 - link

    theres no difference service/quality between "prime tier vendors" and a DIY built except for the warraty, which is useless in most cases. As far as Apple goes, they target a niche market which is why they can afford to charge this much for a box. Nobody in their right mind would pay this kind of money just to have a fast computer for everyday use. Reply
  • BushLin - Tuesday, July 14, 2009 - link

    Yep, the difference is $1500 and no amount of shininess, (almost) proprietary connectors or badges saying "Designed in California" (Made in China) can add enough value to justify the cost of what is relatively cheap hardware. Still, if you've got the money to burn and love Apple regardless...

    I'd like to see if a $1000 Hackintosh would offer such a reduced experience, those who swear by Apple's OS shouldn't have to be so routinely taken from behind by a company they apparently love to the point of promoting the products for free (and ignore all the shortcomings).
    Reply
  • zsdersw - Tuesday, July 14, 2009 - link

    How dare you criticize Apple for not including SSDs! Apple is perfect. Apple is God!

    All must genuflect to Pope Steve Jobs and all must buy his Jesus Phone.

    Those who speak anything negative about Apple and/or those who do not recognize their supreme awesomeness will be excommunicated.
    Reply
  • michal1980 - Tuesday, July 14, 2009 - link

    WTF?

    how is it awesome, for the most part you have to use an adaptor, at a cost of 100 bucks.

    oh and if you need to run a high res, since apple is 'cute' (why a mini display port on a desktop? why?) you need 2 cables just to get it connected? LOL

    and if you need to use the adaptor you still have screws for dvi & vga.


    Finally, do people really hate the screw contections? personally I love them, since there is almost zero chance of them falling out
    Reply
  • MrPIppy - Tuesday, July 14, 2009 - link

    Two questions:

    1) You suggest that for someone wanting higher clocked processors in a Mac Pro, it's a better value to buy the base model, upgrade the processors, and eBay the old ones. But on the 8-core models, the stock CPUs don't have IHSs. Is there any market for these chips on eBay (besides other Nehalem Mac Pro owners who have fried their original processors ;-)?

    2) Can a single socket Mac Pro be upgraded to a dual socket just by replacing the CPU board?
    Starting from the base single-socket ($2500), you could get another W3520 off of eBay cheap ($~350) (possibly one already de-lidded), a dual-socket CPU board ($400), another heatsink ($?) and RAM ($~100), and it would come out far cheaper than Apple charges ($4700) for a Mac Pro with dual X5550s at 2.66 GHz. The total system TDP would be higher, but the already over-specced cooling system would just have to run a little louder.
    The hard part of this plan is convincing a reseller or Apple Store to sell you a 2-socket CPU board and a heatsink, with only a 1-socket CPU board to trade. But, even if some cash had to be discreetly slipped into pockets to make it happen, you still stand to save $1000, which could buy a nice SSD and 24" monitor.
    Reply
  • BoboGO - Tuesday, July 14, 2009 - link

    Two 2.26GHz Quad-Core Intel Xeon "Nehalem" processors
    12GB (6 x 2GB) DDR3 1333 (PC3 10600) memory
    1TB SATA 3.0Gb/s hard drives
    250GB SATA II MLC Internal Solid state disk (SSD)
    22X DVD/CD double-layer writer with LightScribe support
    8X Blu-Ray DVD Burner
    X-Fi Titanium Fatal1ty Champion Series 7.1 Channels PCI-Express Sound Card
    NVIDIA GeForce GTX 285 with 2GB GDDR3 memory
    Thermaltake Xaser VI Black Aluminum Computer Case
    Piano-black 22" 2ms HDMI Widescreen w/LED Backlight LCD Monitor - w/webcam & speakers
    Bonus! Virtual 7.1 Surround Sound Light Weight Circumaural USB Gaming Headset

    Ships: 3 days
    Total Cost: $3,429.92
    Reply

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