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A closer look at the Core i7-940


How does the real McCoy perform?
— 11:45 PM on December 11, 2008

When we first reviewed the Core i7 processor, we had two chips on hand: the high-end Core i7-965 Extreme and the more affordable Core i7-920. Sandwiched in between them in Intel's product lineup is the Core i7-940. Since we didn't have one of those to test, we employed a trick we sometimes use and turned down the clock speed on our Core i7-965 Extreme from its native 3.2GHz to the 940's 2.93GHz frequency. Given the breadth of CPU model ranges these days, we find ourselves using this trick fairly often. In fact, in this case, Intel even recommended that reviewers use this method to test Core i7-940 performance and provided the media with instructions for setting the proper clock speeds.

But such an approach always comes with caveats. Although we're usually confident that the performance results of a "simulated" product model, using the same silicon and the same clock speed, will be true to the original, we're less than confident that the power consumption results will match the actual chip. In fact, power consumption can vary from one individual chip to the next, along with the voltage, which is set at the factory. Beyond that, there is the slight possibility that our simulated product might somehow not match the original in terms of its configuration—and thus performance.

We discovered such a problem with our "simulated" Core i7-940 just ahead of the publication of our initial Core i7 review. Turns out that both our test configuration and Intel's reviewer's guide had overlooked an important characteristic of the Core i7: it has multiple clock domains that govern its processing cores and what Intel calls its "uncore" elements, such as the memory controller, QuickPath interconnect, and L3 cache. We had set the proper CPU core and QPI link speeds for our simulated Core i7-940, but the memory controller and "uncore" clock were incorrect—they were set to run at 3.2GHz, not the 940's default speed of 2.13GHz for both. Not only that, but unlike the "unlocked" Extreme edition, the 940's "uncore" clocks are limited to 2.13GHz and cannot go any higher (at least, not without overclocking.)

After the nice EMT people used the paddles to revive me from my minor coronary event, I decided to go ahead with the publication of our Core i7 review with the simulated Core i7-940 performance numbers intact, but I promised in the text of the review to follow up with numbers from a properly configured Core i7-940.

You see, what we didn't know at that time was the precise impact of the fast L3 cache speed on the Core i7-940's performance. But we knew from experience with similar architectures, such as the AMD Phenom, that raising the L3 cache speed of the Core i7 from 2.13GHz to 3.2GHz was potentially what we call in the industry A Very Big Deal; it would affect memory latency, cache bandwidth, and even power consumption. Put plainly, our original review of the Core i7 overstated the 940's performance by some amount, possibly five to ten percent.

In order to rectify this situation, we got our thermal paste-stained hands on the genuine article: a retail boxed Core i7-940 processor, in order to see how the real thing performs. Here's a nice picture of the soothing blue box:


We popped this puppy (the processor, not the box) into the exact same test rig we'd used for our initial Core i7 testing and ran it through the entirety of our CPU test suite.

In the end, the differences between our original, simulated Core i7-940 and the retail copy of the same weren't huge, but they were measurable and sometimes fairly significant. Rather than hit you with a full-on deluge of performance data, I've selected some of our key tests—including synthetic memory benchmarks, productivity applications, and games—and provided results from them on the following pages. The scores from the two key contenders in this matchup are colored red in the graphs for easy identification. Our simulated Core i7-940 with the too-fast uncore clocks is marked as (sim), while the real McCoy simply goes by its proper name. Read on to see how much (and sometimes how little) difference the actual product's slower L3 cache and memory controller can make.