One of the check-off features of Intel’s big Haswell-E CPU is support for quad-channel DDR4 memory, but my testing shows it may not matter much.
Think of memory channels as shotgun barrels. You know from video games that two barrels are better than one. Now think of quad-channel RAM is the four-barrel shotgun of computers: The more memory channels, the more memory bandwidth available to the CPU.
For each channel in a modern PC, you need an individual stick of RAM. This also depends, of course, on the CPU. Consumer chips such as the Core i7-4790K and the new Core i7-6700K support up to two channels, while consumer chips such as the Core i7-5960X can support up to four channels
Normally this doesn’t matter. You wouldn’t buy an expensive X99 motherboard and pricey Core i7-5960X, then intentionally gimp its quad-channel memory performance by installing only two pieces of RAM instead of four.
Meet Asrock’s X99E-ITX/ac
The problem? You can’t actually fit Intel’s Haswell-E chip and the four memory slots it needs into a smaller Mini-ITX motherboard. They just don’t physically fit using full-sized memory modules. Without access to Intel’s Haswell-E CPU, that means miniature PCs are limited to quad-core CPUs at best.
Asrock’s crazy solution was simply to leave off two of the slots on its X99E-ITX/ac motherboard. Yes, that cuts your bandwidth in half, but it lets you build such crazy machines as this Falcon Northwest or this exotic CyberPower Trinity Xtreme and run more than four CPU cores. The big question is: How much of a hit do you take?
How we tested
I decided to test just how much real performance you give up by leaving half your system bandwidth behind, As the Asrock X99E-ITX/ac is permanently restricted to dual-channel memory, the only way I could test this was to use a full-size X99 motherboard.
For that I turned to a MicroExpress B20 system we reviewed. It has a full-size Asus X99 Pro motherboard and a six-core Core i7-5820K CPU, along with a GeForce GTX 970 card and 16GB of DDR4/2666 RAM in quad-channel mode, using four 4GB modules. I ran several benchmarks with it in quad-channel mode, then swapped out the four sticks of RAM for two sticks of 8GB DDR4/2666 in dual-channel mode.
I could have just pulled two of the systems' original four memory sticks but I decided some would be concerned the 16GB vs. 8GB of total RAM would affect the results. It wouldn’t, but I’ll humor you. So for the record: We’re testing 16GB of DDR4/2666 in dual-channel mode vs. 16GB of DDR4/2666 in quad-channel mode.
Sisoft Sandra Memory Bandwidth
My first test was SiSoft Sandra’s memory bandwidth test. This jack-of-all trades benchmark suite measures and pokes just about everything in your PC. It's long been a standard to measure available memory bandwidth in a PC. The results were as expected (and also a good way to double-check that I hadn’t put the modules in the wrong slots). Going from dual-channel DDR4/2666 to quad-channel DDR4/2666 nearly doubles the available memory bandwidth. Woohoo! Go home, right?
Nope. This chart is probably the only good news for quad-channel memory, but I’ll let you bask in the bandwidth for now. Read on for the real performance impact.