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A fresh look at processor value

...oh my God—it's full of graphs!

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Evaluating the latest processors based on a price-performance analysis is turning into a yearly tradition here at TR. Our first stab at the concept dates back to June 2007, and we tried again with a new batch of CPUs last May. Setting the stage for these articles is a bit like waiting for the stars to align, because a number of criteria must be met: we need performance numbers from a broad enough cross-section of current processors, we need to skirt new processor launches, and we need to wait for prices to be reasonably stable.

The stars have now fallen into place again (or so it would seem), and we've therefore taken another gander at CPU price-performance relationships using fresh numbers from our Socket AM3 Phenom II review. Once again, we threw our test results and official pricing information into a big spreadsheet, laid out the data into a veritable smorgasbord of graphs, and compiled everything neatly here for your reading pleasure.

Before inviting you to bask in the glow of our many charts and scatter plots, we should clarify this article's purpose. This isn't an exhaustive value assessment of all current desktop processors, nor should it be your one-stop guide to picking a new CPU. Rather, this is an attempt to determine how our collection of test processors—almost exclusively enthusiast items priced above $100—compare when we study both pricing and performance simultaneously.

Is a Core i7-920 worth the extra cost over a Phenom II X4 940 for video encoding buffs? Are dual-core CPUs like the Core 2 Duo E8400 still compelling choices compared to low-end triple- and quad-core offerings? Those are some questions this article should help answer.

The test subjects
With that in mind, let's take a look at which CPUs we'll be comparing today. Here's the list on the Intel side of the playground:

Model Clock speed Cores/threads L2 cache/L3 cache Fab process TDP Price
Core i7-965 3.2GHz 4/8 1MB/8MB 45nm 130W $999
Core i7-940 2.93GHz 4/8 1MB/8MB 45nm 130W $562
Core i7-920 2.66GHz 4/8 1MB/8MB 45nm 130W $284
Core 2 Quad Q9550 2.83GHz 4/4 12MB 45nm 95W $266
Core 2 Quad Q9400 2.66GHz 4/4 6MB 45nm 95W $213
Core 2 Quad Q9300 2.5GHz 4/4 6MB 45nm 95W $266
Core 2 Quad Q6600 2.4GHz 4/4 8MB 65nm 95W $183
Core 2 Quad Q8200 2.33GHz 4/4 4MB 45nm 95W $163
Core 2 Duo E8600 3.33GHz 2/2 6MB 45nm 65W $266
Core 2 Duo E8400 3GHz 2/2 6MB 45nm 65W $163

...and on the AMD side:

Model Clock speed Cores/threads L2 cache/L3 cache Fab process TDP Price
Phenom II X4 940 3GHz 4/4 2MB/6MB 45nm 125W $225
Phenom II X4 920 2.8GHz 4/4 2MB/6MB 45nm 125W $195
Phenom II X4 810 2.6GHz 4/4 2MB/4MB 45nm 95W $175
Phenom X4 9950 2.6GHz 4/4 2MB/2MB 65nm 140W $173
Phenom II X3 720 2.8GHz 3/3 1.5MB/6MB 45nm 95W $145
Phenom X3 8750 2.4GHz 3/3 1.5MB/2MB 65nm 95W $122
Athlon X2 6400+ 3.2GHz 2/2 2MB 90nm 125W ~$90

Since retail and e-tail prices oscillate a little too much for our liking, we took our prices straight out of Intel's official list and AMD's processor pricing page. These are figures for bulk orders, but they should only be within a few dollars of retail prices. The only exception is the Athlon X2 6400+, which doesn't appear on the AMD page—we got that CPU's $90 price tag from Newegg.

Why include a discontinued CPU to begin with? Though it's growing long in the tooth, the Athlon X2 6400+ should be roughly representative of the type of performance you can expect from some of today's faster sub-$100 dual-core processors. It should thus serve as a useful baseline against which to compare newer and dearer offerings.

Low-end processors aside, you may see some other missing links in the lists above. There, too, time constraints forced us to make some compromises and exclude CPUs like the Core 2 Duo E7400 or Core 2 Quad Q8300. Thanks to our relatively broad cross-section of data, however, figuring out where those chips would be situated shouldn't be too hard. (For instance, the Core 2 Quad Q8300 should perform somewhere between the Q8200 and Q9300 with a price tag of around $185.)

Finally, sharp-eyed readers might notice we didn't factor platform or power costs into in our processor prices. That's partially true. We'd rather keep things simple for now, but we'll have a look at platform and power costs a little later.

Laying out our data
So, how does one represent value in graph form? We've re-enlisted our two trusty friends from previous value articles: performance-per-dollar bar charts and performance-versus-price scatter plots. The former should be self-explanatory—think "score points per dollar" or "frames per second per dollar." In cases where performance is measured as a time in seconds (and the shortest time is best), we'll use "rate" as our metric. We'll usually define rate in kilohertz or megahertz, which we work out with a formula like "1/seconds × 1000" or "1/seconds × 1000000."

You can use the perf-per-dollar charts to get a more precise look at which CPUs offer more bang for your buck, but be careful: getting the most thingamajigs per dollar isn't the whole story. You've gotta look at the whole picture. Processor prices don't rise linearly with performance, so faster offerings will almost always seem like poorer deals than low-end dual-core chips. That doesn't mean extra performance (and the resulting time saved) isn't worth it.

Our scatter plots look like so, mapping performance to the Y axis and price to the X axis:

As you'd expect, the best possible processor would sit at the top left of the plot, offering the highest performance at no cost. Conversely, the poorest choice would be at the bottom right.

In a nutshell, our scatter plots provide a visual representation of the value curve, which should help locate the most interesting combinations of pricing and performance. The performance-per-dollar bar charts come as complements to these plots, laying out the same data (more or less) in a purely numerical fashion.

In our view, the best deals often lie where either performance stops rising substantially while prices keep rising or where prices suddenly shoot up without performance following suit. You'll see what we mean once we get into our comparisons.