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.

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  • wackazong - Wednesday, September 9, 2009 - link

    Hello,

    this may be the right place to ask: What's the difference between the Xeon and the (much cheaper) i7 processors? Couldn't you put an i7 into a Mac Pro?
    Reply
  • sdevenshire - Saturday, August 29, 2009 - link

    Hi,

    I purchased a 2xQuad core mac in Jan 2008 and I would like to upgrade the cpu to the new Nehalem. Apple suggested it could be done but they don't do it. I contacted a number of Mac repair places and they said it can't be done. I realize that upgrading the cpu probably means upgrading the motherboard, but that's fine with me.

    Any suggestions on where I could get this done or where I might get instructions for doing it myself?

    TIA,
    Shane
    Reply
  • 529th - Thursday, July 30, 2009 - link

    If you’ve read our Nehalem articles you’ll know that each chip has three 64-bit wide memory controllers, thus you’ll want to install DIMMs in triplets. You can install four DIMMs, but accessing memory in the fourth module will be slower - something you’ll never notice if you’re wondering. ???

    This is hindering me from buying a 4 channel UD3R X58 board. My main goal of an i7 build is for editing AVCHD files through Premiere Pro CS4. Being that tri channel will get me 6g and PP CS4 likes more memory, will adding memory to the 4th module screw things up?
    Reply
  • newrigel - Tuesday, July 28, 2009 - link

    Man, take your hackysack and go buy some laundry soap to clean the crap out of your drawers! You guy's talk about a couple thousand dollars like it's buying a damn house or some huge purchase LOL!
    Macs are really cost efficient and yes... PC's are cheaper but who gives a damn! If you want to be cheap... be cheap! Hackintoshes (LOL) are just that... a POS and your getting what you pay for! You cheap asses probably hit your ol' ladies up for gas money to get to work hehe...
    MACS RULE!
    Reply
  • ditchmagnet - Monday, July 27, 2009 - link

    Just for fun I went to apples site and customized the mac pro, I just maxed out the hardware choices and then went to newegg and priced out an equivalent PC (Server board, with the same CPUs and everything except more RAM)
    Total for the apple including shipping and tax is over $20,000
    the newegg build is under $9,000
    I bet my 920 build at 4.5ghz is faster than the mac pro though, and all for under $1000
    Reply
  • moltentofu - Tuesday, July 21, 2009 - link

    wander over to the egg and buy a lian li. No flashy lights really, no idiot clear side panels. For some reason the hard drive access light and the power light are different colors on mine though.

    You can get a combo case with a seasonic 550W power supply with it. I put a phenom II x4 3.2Ghz 16 Gigs of RAM and a 1 Gig 4870 in there with aftermarket cooling all around (air not liquid) and it cost me 900 bucks - all from the egg.

    If you think you're going to find performance arbitrage basically anywhere in the market you couldn't be wrong-er. Just pick your price/performance point and stick to it.

    Thanks for the awesome reviews as usual Anand!
    Reply
  • moltentofu - Tuesday, July 21, 2009 - link

    With one big 'ol caveat to the arbitrage thing and that is: building your own system really does seem to be cheaper right now, and also I can't find component setups in the big name companies right now that make me quite happy.

    I miss when Dell Outlet used to be affordable. I'm afraid these Macs are just waaay out of range of my meagre salary.
    Reply
  • fmaste - Monday, July 20, 2009 - link

    Everybody talks about how expensive the Mac Pro is, but, has somebody compared with other brands? Look at this:

    I customize two Dell Precision Workstations with the same components as the base configurations Apple offers for the Mac Pro.
    The results!!

    Mac Pro Quad-Core: $2,499.00
    Dell Precision T5500: $3,427

    Mac Pro 8-Core: $3,299.00
    Dell Precision T7500: $3,427

    BOTH APPLE OFFERS ARE BETTER!!!!!
    Both with the same processors, same amounts of memory at the same speed.
    The only difference is instead of a NVIDIA GeForce GT 120 with 512MB you get a 256MB NVIDIA® Quadro® NVS 295. And that Dell options have hard drives with less capacity, 500GB vs 640GB. I also added the second Gigabit Ethernet card to both Dells. Dell has no bluetooth option and you may need to add a sound card to them.
    Remember, you get a more expensive PC with windows Vista and an ugly chasis.
    Reply
  • fmaste - Tuesday, July 21, 2009 - link

    And here is HP

    Mac Pro Quad-Core: $2,499.00
    Same specs configurable HP Z800 Workstation: $3,942.00

    Mac Pro 8-Core: $3,299.00
    Same specs configurable HP Z800 Workstation: $3,702.00
    Reply
  • excalibur3 - Friday, July 17, 2009 - link

    So when you hypothetically created your i7 hackintosh, what were it's specs? I'm thinking about doing this (as a thought experiment only of course) and I'm wondering what such a system would price out. How would I know what motherboard to use to be compatible? Reply

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