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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  • rpmurray - Saturday, July 18, 2009 - link

    Awesome!!!

    Please point me to the webpage where I can spec out a system.
    Reply
  • BoboGO - Tuesday, July 14, 2009 - link

    Two 3.2GHz Quad-Core Intel Xeon "Nehalem" processors!
    12GB (6 x 2GB) DDR3 1333 (PC3 10600) memory
    1TB SATA 3.0Gb/s hard drive
    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: $6,073.92
    Reply
  • ddobrigk - Tuesday, July 14, 2009 - link

    Here's my take on this:

    http://www.tomshardware.co.uk/forum/page-262853_10...">http://www.tomshardware.co.uk/forum/page-262853_10...

    Can't remember the exact price in USD, but it was around US$6000 too. And that's with a very-high-end cooling solution.
    Reply
  • Tesselator - Tuesday, July 14, 2009 - link

    I'm not sure what the author or others here are talking about when they say Apple's extremely outrageous prices are due to Intel's price hike. There is no price hike. There was no price hike. Either the author is misinformed or it's a straw-man technique being employed in order to justify and excuse Apple having gone off the deep end with their price scheduling. With so many of the comparisons in this article omitting the 2008 Mac Pro models all together and only comparing the 2009 to the 2006, I'm to believe the later.

    Here are the prices of the respective chips at the time the respective Mac Pro machines were released:

    2008 Models:
    1 x 2.80GHz E5462 Harpertown: $797 ($2299 overall Mac Pro price)
    2 x 2.80GHz E5462 Harpertown: $797 x 2 = $1594 ($2699 overall Mac Pro price)
    2 x 3.00GHz X5472 Harptertown: $958 x 2 = $1914 ($3599 overall Mac Pro price)
    2 x 3.20GHz X5482 Harpertown: $1279 x 2 = $2558 ($4399 overall Mac Pro price)

    2009 Models:
    1 x 2.66GHz W3520 Bloomfield: $284 ($2499 overall Mac Pro price)
    1 x 2.93GHz W3540 Bloomfield: $562 ($2999 overall Mac Pro price)
    2 x 2.26GHz E5520 Gainestown: $373 x 2 = $746 ($3299 overall Mac Pro price)
    2 x 2.66GHz X5550 Gainestown: $958 x 2 = $1916 ($4699 overall Mac Pro price)
    2 x 2.93GHz X5570 Gainestown: $1386 x 2 = $2772 ($5899 overall Mac Pro price)

    As you can see, there is no significant price hike and in fact Intel has provided a massive price REDUCTION in offering the Bloomfield line. This delivers a second more severe slap in the face from Apple to it's user base.

    Just look at this price scheduling:

    2006
    Mac Pro Quad 2.0GHz $2,199 NEW
    Mac Pro Quad 2.66GHz $2,499 NEW
    Mac Pro Quad 3.0GHz $3,299 NEW

    2007
    Mac Pro Quad 2.0GHz $2,199
    Mac Pro Quad 2.66GHz $2,499
    Mac Pro Quad 3.0GHz $3,299
    Mac Pro 8-core 3.0GHz $3,997 NEW

    2008
    Mac Pro Quad 2.8GHz (2008) $2,299 NEW
    Mac Pro 8-core 2.8GHz (2008) $2,799 NEW
    Mac Pro 8-core 3.0GHz (2008) $3,599 NEW
    Mac Pro 8-core 3.2GHz (2008) $4,399 NEW

    2009
    Mac Pro Quad 2.66GHz $2,499 NEW
    Mac Pro Quad 2.93GHz $2,999 NEW
    Mac Pro 8-core 2.26GHz $3,299 NEW
    Mac Pro 8-core 2.66GHz $4,699 NEW
    Mac Pro 8-core 2.93GHz $5,899 NEW

    That's a $1,000 to $2,000 price hike on Apple's part and the first of it's kind in the past 10 as I've researched it. It might be due to poor sales or future economic projections, I have no idea. But it's unprecedented and completely unjustified by looking at part prices and other offerings in the industry.

    I recently priced a DIY build and I could assemble a system with 18 DIMM slots, 6 PCIe slots, 36GB RAM, two quad-core 3.2GHz Nehalem, typical drives, cards, and case go for well under $5k. Apple wants $6K for much much less.

    Something isn't right in Denmark!
    Reply
  • winterspan - Tuesday, July 14, 2009 - link


    Thank you for posting this. I'm a frequent reader of Macrumors.com forums, and I actually made a similar chart showing the very similar launch prices of Xeon 5400/5500 when new and how the massive $1000+ increase in price of the new Mac Pros are totally unjustified and ridiculous.

    It would still be overpriced, but a far better deal if they tossed the 2.26Ghz in favor of the 2.53Ghz or 2.66Ghz as standard, with the 2.8Ghz and 2.93Ghz as the upgrades. (the 3.2Ghz is clearly too hot) This would be far more similar to the Mac Pro configurations in the past, where the base CPU + 2 upgrade levels usually represented the 3 fastest CPUs available (or nearly that).
    To show the huge discrepancy, there are 6!! CPU levels above the 2.26Ghz 5520. (2.4, 2.53, 2.66, 2.8, 2.9, 3.2)

    I was looking forward to maybe picking up a Mac Pro for the office, (software dev) but I'm not sure now. If the HAckintosh crew gets to a certain level of progress that offers no-hassle installation and complete compatibility with mainstream components, I can see many enthusiasts who want to dualboot OSX using high-clock speed Core i7 boxes, with loads of ram, SSDs, RAID'd drive storage, GTX285s, etc, and still spending far less than the single-cpu 2.93Ghz Mac Pro.


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

    Firewire 800 works with 400 and 200. You just need a cable with the appropriate connectors, it will auto-negotiate the connection.

    Firewire rocks.
    Reply
  • mesiah - Tuesday, July 14, 2009 - link

    Too many sites get in mac vs pc battles any time a mac review goes up. I'm not going to bash apple and talk about how overpriced things are. But honestly, I've always been a pc person. In the past apple did have some innovations to tout and some people could justify the premium price. But looking at these 2009 models, it's a disgrace. for 3k+ we are getting 640gb HDDs, video cards with 512mb frame buffers, and 6gb of ram. And in return you get a xeon processor. For a company that tries to cut a niche out by offering the highest performance for a premium, this seems like a joke to me. Its like taking a chevy cobalt and putting a corvette engine in it, then trying to sell it for the price of a Ferrari. It seems to me when it comes to the desktop market these days, people are paying thousands of dollars because they like apples OS, and overlooking the hardware as long as the case looks cool and has a xeon sticker on it. Unfortunately most tech sites these days are in love with apple and just pretend that everything is fine and dandy. I am glad that you pointed out the deficiencies Anand, but I think all apple fans should be more vocal and actually show some disgust for your beloved company for once. If you are going to pay all that money, you should atleast get some innovation to show for it. Reply
  • TheFace - Tuesday, July 14, 2009 - link

    Since when has apple been all about the performance of the hardware? Apple is about the performance of the hardware/software package. These aren't corvettes or ferraris for you to zip around fun-town with. These are peterbilts and mack trucks to get work done with. This is a PRO model. Just because it LOOKS like a regular pc with it's big boxy structure, doesn't mean it's your home PC. I will concede that the author of the article does make some valid points about the lack of SSD and video card options. The fact is though, the mac pro isn't for the l337 who build SLI OC'd gaming rigs.

    If you don't understand that Apple rarely makes an upgrade without having an unveiling party about it, then you just don't understand Apple. I'm not saying that that fact is a good thing, but they certainly generate more press and hype than other computer makers.
    Reply
  • Shadowself - Tuesday, July 14, 2009 - link

    Interesting that you state, " I'm not going to bash apple and talk about how overpriced things are." then you go on to do virtually noting but this.

    Are Apple's Mac Pro systems over priced? Yes. Are they more over priced than historically for Apple? Yes. However, as Anand points out this seems to be a trend with all tier one vendors. The equivalent machines out of HP and Dell are similarly or higher priced. Is this justifiable? No. They are all just gouging their high end users.
    Reply
  • mesiah - Wednesday, July 15, 2009 - link

    No, I did not bash apples price. I did not compare it to comperable PCs. I clearly pointed out that there is no innovation here to demand the premium price. But people like you are so used to having to defend your purchase based on this argument, thats all you've got. This is not a mack truck by any means. Real workstations have more than 3gb of ram standard. Real workstations have more than a measly 640gb HDD. Real workstations come with high speed SSD or SAS system disks. Yes, it is capable of high end graphics work. And yes, you can get a dell for $8000. But you are making that comparison not me. If you want to make it, go see what you get for 4k from dell. And those dells are not marketed at the main stream. If I walk into a dell store a salesman isn't going to try and sell me a workstation because I am a power user. When I walk into an apple store there is no hesitation to push me in the direction of a 3k+ purchase. But none of that was my point and not what was discussed in my comment. It was simply, where is my innovation that apple is supposed to be all about? I'm not talking about some quad sli water cooled system, I'm talking about simple things like 6GB of ram standard. SSD system drives. Atleast a TB of storage. Seriously, this is for power users and it comes standard with 640GB? I will admit apple gets some things right. And they are damned good at marketing. But please people, you don't have to kiss steves ass all the way to the bank. Don't make excuses for products that are clearly lacking. Otherwise your precious macs will just continue the long slide into bland vanilla every day systems and you will still be paying premium prices for everyday merchandise. Reply

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