Upgrading and Analyzing Apple's Nehalem Mac Proby Anand Lal Shimpi on July 13, 2009 5:00 PM EST
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The Crossroads of Simplicity and Sophistication
Choices. Choices. Apple doesn’t like to present the end user with many choices. Too many choices can confuse, if left unchecked they can become overwhelming. The overburdening of choices is something that most PC OEMs fall victim to. I recently spoke with ASUS and brought this up in a conversation about the Eee PC. Three and four digit model numbers are how you tell one Eee PC apart from another. Perhaps you have the Eee PC 901, or the Eee PC 1000HA or the S101. To an enthusiast who has time to research these things, the model numbers aren’t that hard to figure out - it’s easier than Calculus after all. To someone just looking to buy “one of those Eee things”, it’s overwhelming.
Try buying an Apple notebook and you’re faced with two models: the MacBook and the MacBook Pro. If you’re a consumer, buy a MacBook, if you’re a professional buy the Pro version. Then just select your screen size and you’re done. That’s how Apple wants it to work and for the most part, it does. Very well.
Apple’s simple approach works quite well for consumers, but once you start getting into the high end content creation world it’s not quite so easy. How do you simplify the decision between two very fast cores and four slower cores or eight even slower ones? It wouldn’t really fit within Apple’s well kept home to ask its customers whether they run predominantly single threaded, lightly threaded or heavily threaded applications. Much to my surprise, the two new Mac Pros do effectively that. They present the end user with an option to choose four faster cores or eight slower ones. And there’s much more to the numbers that what Apple publishes on its own website.
These are the CPUs Apple offers on the new Mac Pro:
|Apple Mac Pro (2009)||Quad Core Model||Eight Core Model|
|Default CPU||1 x Xeon W3520 (2.66GHz)||2 x Xeon E5520 (2.26GHz)|
The clock speed difference appears to only be 17% at first glance, but there’s much more to the story.
Four or Eight Cores and the Magic of Nehalem
There are effectively three classes of applications that we have to consider when wondering whether or not the new Mac Pro is indeed a good buy. On one end of the spectrum we have single-threaded applications and tasks.
These days CPU performance improvements happen along three vectors: ILP, clock speed and TLP. The first vector of performance improvement is ILP (Instruction Level Parallelism). These improvements are changes to the micro-architecture. They could be as simple as adding a larger/faster cache, or as complex as a faster/more capable SSE unit. These days there are minor improvements in ILP between microprocessor generations. The second vector, clock speed, is also fairly stagnant. The Nehalem based Xeons run at about the same clock speed as the Woodcrest, Clovertown and Harpertown based Xeons that the older Mac Pros used. The final vector, TLP (Thread Level Parallelism), is where we’ve seen some of the biggest gains this round. As the name implies, execute more threads in parallel and you can get more performance. You increase the number of threads you can execute by running multiple threads on a core (SMT or Hyper Threading) or by adding more cores to a chip. Quad-core is still the sweet spot configuration for Xeons, but the Nehalem architecture brings Hyper Threading back to the limelight and now each of those four cores can work on two threads of instructions at the same time.
Well let’s look at how ILP, clock speed and TLP compare from Harpertown to Nehalem (for more details on what makes Nehalem tick, err tock, be sure to read our architectural analysis):
|Apple Mac Pro (2009) vs Apple Mac Pro (2006 - 2008)||Upgrade||Downgrade|
|Instruction Level Parallelism (ILP)|| Faster memory access |
Minor microarchitectural updates
|Smaller L2 caches|
|Clock Speed||Minor clock speed advantage in some cases||Minor clock speed disadvantage in others|
|Thread Level Parallelism (TLP)||Large L3 cache shared by all cores |
2x threads per core (Hyper Threading)
Looking at the table of improvements you should already know where to expect the Nehalem Mac Pro to excel. With each chip being able to execute twice as many threads as those used in the old Mac Pro, if you’re running a well threaded application then you’ll certainly see performance improvements on the new Mac Pro. What sorts of applications are “well threaded”? Generally things like 3D rendering and professional video encoding. The easiest way to find out is to fire up activity monitor and see how many of your cores are taxed while you’re using your system. If all of the bars are full of blue on a quad-core machine then you’d probably appreciate a Nehalem Mac Pro.
The clock speed improvements are minimal. In a non thermally constrained environment you can add 133MHz to whatever clock speed Apple puts on the box. So the 2.26GHz Mac Pro will most likely run at 2.40GHz and the 2.66GHz Mac Pro will spend most of its time at 2.80GHz, if you’re doing something CPU intensive that is. This is of course do to Intel’s Turbo mode.