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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  • mfago - Monday, July 13, 2009 - link

    Apple lists the Mac Pro as supporting DDR3 1066 only, yet the higher speed CPUs should support 1333. Have you tested this? I have an application that would love faster memory.

    Secondly, while Apple's prices are much higher than previous models (due to Intel's CPU prices, as you note), have you compared similar machines from Dell/HP? _Significantly_ more expensive: a T5500 with 2x2.93 GHz and 12 GB ram is $8000 (before discounts). Otherwise, the only way to get decent prices is through second/third tier vendors, or building one yourself. Sure CPU/RAM upgrades have always been this way, but Intel's extreme prices on Nehalem make this very obvious.
    Reply
  • austin512 - Saturday, May 1, 2010 - link

    Hi Anand,

    Any word on if the W3680 will work in the 2009 Mac Pro?
    Reply
  • rominator - Thursday, July 22, 2010 - link

    "While you could stick Clovertown into the first generation Mac Pros, you couldn’t upgrade them to Harpertown without hardware modifications to the system (don’t ask me what they are :)..)."

    AFAIK, nobody has ever discussed being able to do this. ANyone have an idea what he's referring to?
    Reply
  • mrob27 - Thursday, September 23, 2010 - link

    Upgrading CPUs:
    I have the 2.26 GHz 8-core model. I cannot afford to take the risk, but I'd love to upgrade it someday if it was less risky.

    I think you should carefully measure the thickness of Xeon processors with and without the heat spreader (using an outside micrometer or something), and add washers to compensate for the needed spacing where the screws mate the heatsink to the processor board. This would properly relieve strain from the CPU package and allow you to tighten the hex nuts to the proper (standard) tension. Based on your photos it looks like that should work.

    Turbo mode:
    I was able to get valid convincing single-threaded benchmark results (demonstrating Turbo mode in action) by using the "Processor.prefPane" from Apple's Developer Tools. This is a System Preferences module that is placed in /Developer/Extras/PreferencePanes when you install the Dev Tools, and you copy it into /Library/PreferencePanes to enable it. Then use System Preferences to access it. You can turn off hyperthreading and can selectively disable any combination of cores 2-8 (#1 is always on). See for example goo.gl/7MZm or goo.gl/OH4K

    - Robert Munafo - mrob.com/pub/comp/mac-models.html
    Reply
  • Auralwiz - Friday, July 13, 2012 - link

    Did you ever try the 2.26 8 core CPU upgrade?
    I tired two w35675s and failed to boot. Is there two big of a speed increase limit?

    Michael
    Reply
  • Highjnx - Sunday, November 7, 2010 - link

    I recently tried to upgrade the E5520 procs in my 2009 Octo Nehalem with a set of X5560 (2.8GHz). I was able to get the machine to power up with no error lights. However I didn't get a chime and it refused to boot.

    In reading Anand's article on upgrading with a set of X5570 chips I didn't see any mention of the X5560 as an option. Is there something written into the bios that prevents this from posting.

    The other thing I question is the added height created by the Integrated Heat Spreader doesn't give a solid connection between the daughter board and the heat sink fan plug. Has anyone run across a how-to on removing the IHS?

    Any insight would be appreciated as I have access to several set's of X5560.

    todd-
    Reply
  • Auralwiz - Friday, July 13, 2012 - link

    I purchased two w3565 LG 1366 processors for my 8 core dual 2.26 GHz 2009 Mac Pro 4,1.
    These CPUs are 3.2 Ghz.

    The install was easy but the machine would not boot up. I re-installed the two 2.26 original CPUs and the system returned to normal. Any idea why it didn't work on an 8 core system?
    I had the same experience as user Highjinx.

    Michael
    Reply
  • Soren4 - Sunday, June 12, 2016 - link

    As a 2009 Mac Pro owner, I've followed the upgrade discussions for these machines carefully for some time. There appears to be a lot of discussion and instruction on how to swap out processors but usually involving lidded CPU's instead, which pose potential risks of damaging the motherboard in the process.

    I found an interesting video tutorial on Youtube today that clearly explains, while demonstrating, how to effectively remove the soldered IHS off the CPU, with the cleaning off of the solder, without damaging it.

    It seems to me that this would be a lot easier to do than having to modify the heat sync pads and add washers etc., all the while allowing the processors to stay cooler due to the removal of the IHS.

    Here is the link for review.

    https://www.youtube.com/watch?v=aPhDfUkll-o

    Thanks
    Reply

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