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Exploring the impact of memory speed on Core i7 performance

A look at channels, bus speeds, and latencies
— 11:24 PM on December 1, 2008

One of the defining features of Intel's Core i7 processor is its integrated memory controller. There's certainly nothing revolutionary about moving this logic onto the processor die—AMD's been doing it for years, ever since the first Opteron launched way back in 2003. However, with support for three channels of DDR3 memory at speeds up to 1600MHz and beyond, the Core i7's integrated memory controller is clearly a cut above what's available in even AMD's latest Phenom processors.

With three channels of lowly DDR3-1066, you're looking at a whopping 25.6GB/s of memory bandwidth. Crank the memory clock up to 1600MHz, and bandwidth jumps to an even more impressive 38.4GB/s. This fat pipe, combined with the low access latencies inherent to on-die memory controllers, gives the Core i7 a formidable memory subsystem. It also poses an interesting question: how bound by memory speed is Intel's new processor architecture?

That might seem like a purely academic question on the surface, the sort of thing an especially geeky hardware reviewer would address to satisfy their own obsessive technical curiosity. But it's an issue that leads to many more pertinent questions that should be on the minds of prospective Core i7 system builders. For example, is there any tangible performance benefit—beyond higher scores in synthetic memory benchmarks—to pairing a Core i7 with fancy DIMMs with lower access latencies, or are you better off saving a few bucks with more pedestrian DIMMs that run at looser timings? What about memory frequency? Does the Core i7's performance scale up if you drop a little extra coin on memory capable of running at 1333 or 1600MHz? And while we're at it, does performance really suffer if you drop down to just two memory channels? Join us as we throw multiple memory configurations at a pair of Core i7 processors to find out.

A multitude of memory configurations
We've split testing between a Core i7-920 and 965 Extreme due to the former's lack of official support for memory faster than 1066MHz. Core i7-920 and 940 processors have a maximum memory speed of 1066MHz that motherboard makers haven't yet found a way to circumvent, but the 965 Extreme is free to use multipliers that run its memory bus at 1333, 1600, 1866, and even 2133MHz. Finding DIMMs capable of running at those higher speeds may prove difficult, though. Intel's maximum recommended memory voltage for Core i7 processors is just 1.65V. While that's a smidgen above the 1.5V called for by the DDR3 spec, it's well below the voltage required by most high-speed DDR3 modules already on the market. Speedy DDR3 chips that can get by with only 1.65V appear to be in short supply, but Kingston was able to hook us up with a triple-channel trio of KHX14400D3K3/3GX modules rated for operation at up to 1800MHz at 1.65V.

With a fistful of DDR3, we've run a number of different configurations on each CPU. We'll start with straight memory scaling on the 965 Extreme, since that's the most straightforward. Here we've run the system's memory at 1066, 1333, and 1600MHz. The Kingston DIMMs are rated for 7-7-7-20 timings at 1066 and 1333MHz, and 8-8-8-24 timings at 1600MHz, and we've stuck with those values. These modules are also capable of running at 1800MHz with 9-9-9-27 timings, but our Core i7 motherboard only has the necessary multipliers for an 1866MHz memory clock, which proved a little too fast for our modules. Even topping out at 1600MHz, these configurations should give us a good look at how the flagship Core i7 responds to faster RAM.

Next, we turn our attention to the 920 to answer several additional questions about Core i7 memory performance. The first and perhaps most important of these is whether you can get away with budget DIMMs that have looser memory timings. To find out, we've tested the 920 with 1066MHz memory at 7-7-7-20 and 9-9-9-27 timings. We've also addressed the ultimate cheapskate question of whether you lose much performance pairing the Core i7 with only two DIMMs, leaving one of its memory channels on the table. For this dual-channel config, we've stuck with 1066MHz memory at 9-9-9-27 timings.

Despite the 920's lack of official support for faster memory, we can push its memory clock higher by overclocking the processor's base clock speed. We've done just that, dialing our Core i7-920's base clock up from 133 to 167 and 200MHz, which allows us to run the memory at 1333 and 1600MHz, respectively. Since we're focusing on memory performance, we lowered the 920's core multiplier to 16X at 167MHz and 13X at 200MHz. That gives us the same 2.66GHz core clock with a 167MHz base clock and 2.6GHz at 200MHz, which is close enough. As we did with the 965 Extreme, we've stuck with our DIMMs' default latencies of 7-7-7-20 at 1333MHz and 8-8-8-24 at 1600MHz. These results should let us know what happens to the i7's performance when you push both its base and memory clocks.

Since we're trying to isolate the impact of memory performance, we disabled the Core i7's Turbo mode for testing. As a result, the 920's core clock won't exceed 2.66GHz, and our 965 Extreme won't tick up from 3.2GHz. However, the changes we make to memory and base clock speeds will affect other elements of the Core i7 processor, notably the speed of its memory controller, L3 cache, and QuickPath Interconnect. These so-called uncore elements of the processor can impact performance, so we've listed the various speeds in a nifty table below.

Core Memory Memory controller L3 cache QPI
965 - 1066 3.2GHz 1066MHz 2.66GHz 2.66GHz 3.2GHz
965 - 1333 3.2GHz 1333MHz 2.66GHz 2.66GHz 3.2GHz
965 - 1600 3.2GHz 1600MHz 3.2GHz 3.2GHz 3.2GHz
920 - 1066 2.66GHz 1066MHz 2.13GHz 2.13GHz 2.4GHz
920 (16x167) - 1333 2.66GHz 1333MHz 2.66GHz 2.66GHz 3.0GHz
920 (13x200) - 1600 2.6GHz 1600MHz 3.2GHz 3.2GHz 3.6GHz

Our memory controller clocks come from CPU-Z, which lists the speed of the Core i7's memory controller as the "NB frequency." Based on discussions we've had with Intel, we believe the L3 cache's clock speed is equal to the memory controller speed in Core i7-920 and 965 Extreme processors. The higher memory controller speeds required to run faster memory must therefore also increase the speed of the L3 cache. Higher L3 speeds are likely to improve performance with data sets that fit within the Core i7's 8MB of available cache. They can potentially reduce memory access latencies, as well.