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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  • analog1 - Thursday, July 16, 2009 - link

    Why not run the same benchmarks, like Premier pro cs 4 and Photoshop on a Core i7 PC?
    I think the results will surprise a low of people. I have sen comparisons done with Protools - a highly threaded proffessional DAW ( Digital Audio Workstation) 8 core nehalem Mac Pro (16 thread) vs an OCed i7 920 (3.6 ghz) and the 920 beat the mac pro by roughly 20%. This is probably due to much higher clock speeds, much higher memmory speeds and bandwidth, and much more efficient OS! (yes Windows XP). All this for about $2000 less.

    If display cards had anything to do with this test the price/performance difference would be even higher. This is testing audio processing only.

    I think if Anand could take the time and bench an OCed or stock i7 920 vs 8 core mac pro on the apps used with the same content we will all be finally able to throw the mac 'CREATIVITY' marketing slogan out the window for good.

    I use macs every other day and am writing this on a mac (dual G5, but still). I also use PCs on a daily basis. all for work no play. I have been doing this for years, and honestly I don't understand why people like macs. they are SIMPLER not BETTER.

    OSX has nice animation, and the iPhone is cool. Bravo Apple. Now can we please get a REAL bench for these apps PC vs Mac so we can all go to sleep knowing we actually do have the best system for $1000, even if it was designed by Antec+Seasonic+Corsair+Termalright+Gigabyte and so on... (and not buy Apple)
    Reply
  • tstm - Wednesday, July 15, 2009 - link

    What this article deserves, is a slap to Apple because of their memory configurations.

    The low end Mac Pro comes with four (4) memory slots. That's utterly ridiculous, considering that Nehalem supports triple channel memory. Adding 8GB (4x2GB) of memory will bring the memory bandwidth down somewhat, which is unacceptable for a $2500 machine.

    The 8 core version, on the other hand, comes with 8 slots, again not a multiple of 3. With DDR3 SDRAM 4GB sticks being so ridiculously expensive, this makes any larger memory configs for the Mac Pro extremely expensive.

    There's one more gripe: The server CPUs Apple uses could use RDIMM or UDIMM memory, which is _a LOT_ cheaper than normal DDR3 SDRAM mostly because it's being used in server configurations where it's not unheard of to have 192GB of ram in a machine, no one wants to pay gazillions for the 8GB DDR3 SDRAM sticks that would require.

    Buying a server board with 18 RDIMM/UDIMM memory slots for building a similar workstation as the Mac Pro would be an insanely much better solution for anything that requires memory to operate (running multiple test VMs for instance). I think it's pretty sad that apple is not even trying to cater professionals with this "Mac Pro" toy they've built. I really would like to use an apple computer, but these drawbacks made me use a Hackintosh, which has its own drawbacks.. but for workstation use none are so bad when you compare it to the Mac Pro.
    Reply
  • fmaste - Tuesday, July 14, 2009 - link

    So, this is a good computer after changing the CPU and buying a second video card. How could someone say that this is OK? I don't think that Apple notebooks are expensive, you really get a premium notebook with all premium components, but this seems very overpriced.
    I like Apple and OSX a lot, but the case and interior design don't cost that much, you can build your own computer for much less, and with one of the awesome cases reviewed here at anandtech.

    Also, what about GPU performance and comparison?
    Third party GPUs? How? Expansion slot available?
    Power, Noise, heat?
    What about Boot Camp?
    Type of memory?
    And I would really appreciate a price and performance comparison with other workstations and what you can build for that money.
    Reply
  • Tutor - Thursday, July 16, 2009 - link

    This is my dream machine at post #10. I call it MyHackedUpMac. Reply
  • Tutor - Thursday, July 16, 2009 - link

    http://forums.macrumors.com/showthread.php?t=71393...">http://forums.macrumors.com/showthread.php?t=71393... 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
    NVIDIA GeForce GTX 285 with 2GB GDDR3 memory
    8X Blu-Ray Writer
    250GB Vertex SATA II MLC Internal Solid state disk (SSD)
    1TB SATA 3.0Gb/s hard drive
    22X DVD/CD double-layer writer with LightScribe support
    X-Fi Titanium Fatal1ty Champion Series 7.1 Channels PCI-Express Sound Card
    Thermaltake Xaser VI Black Aluminum Computer Case

    Sorry, no monitor included.

    Ships: 3 days
    Total Cost: $5833.00
    Reply
  • vailr - Tuesday, July 14, 2009 - link

    Re:
    "I couldn’t wait any longer and I ended up building a Hackintosh based on Intel’s Core i7. Literally a day after I got it up and running, Apple announced the new Nehalem-EP based Mac Pro."
    More details, please.
    List of parts used & cost, the method used for installing OSX, & a "bang for buck" comparison with Apple's equivalent machine.
    TIA
    Reply
  • erple2 - Wednesday, July 15, 2009 - link

    That's a good question. Also, how much time did you spend getting the Hackintosh up and running?

    And for the final question, how much do you believe your time to be worth? I know how much I am paid per hour at my job. It doesn't take that long of hassling with a Hackintosh to make it worth my while to just buy the Mac instead. Include time spent when I go to update the OS and have to research which updates will work with my particular Hackintosh, plus those times that I accidentally do an update that I didn't fully research and hosed my setup. Oh, and make sure that I factor in some time for when my soundcard just didn't quite work right after a reboot...

    Ultimately, it boils down to the triplet: Time, Money, Productivity - pick two.
    Reply
  • Baked - Tuesday, July 14, 2009 - link

    Wait, people actually upgrade their Mac? I thought they just buy a new one when it gets real slow. At least that's what happens at the places I worked at. Buy a brand new Mac, use it for a few years, buy a new one and send the old one to surplus. We do buy memory from micron instead of doing it through Apple though. Reply
  • xz4gb8 - Tuesday, July 14, 2009 - link

    You said, "Between the high cost of the adapter and the high likelihood of problems, I’d suggest simply getting another video card if you want to have multiple 30” displays connected to your Mac Pro. Apple sells the GeForce GT 120 for $150 as an upgrade option, and at least with it each 30” display will be driven by its own frame buffer, which should make for smoother Exposé and Dashboard operation."

    If you allow non-Apple displays, many higher-end displays have DisplayPort connectors - see the Dell 3008WFP, for example. A mini-DP to DP cable is under $20 including shipping.

    The mini-DP to DVI adapter is too clunky and tends to unplug itself. The mini-DP to HDMI adapter is under $20 and is slightly less clunky. Neither are as elegant as the mini-DP to DP cable.

    I make no claims for the Video card performance.
    Reply

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