I’ve written about Asrock’s interesting decision to sacrifice memory bandwidth for core count a few times now. The reaction is usually to recoil from those who just don’t want to make that compromise in memory bandwidth. Give up half your memory bandwidth just to make a smaller system with six or eight cores? Never!
I had the same reaction myself originally. After running my tests though, I’m not sure it matters. I’m sure that somewhere out there beneath the pale moonlight, there’s a task or benchmark that truly pays the dividends you’d expect by doubling the available system bandwidth, but I’m not seeing it here.
Why? I suspect one reason might be the massive 15MB cache in the 6-core Core i7-5820K processor I used. The quad-core Core i7-4790K has an 8MB cache. That’s almost double the cache with only two more cores added to the equation. Could going to an 8-core Core i7-5960X show the weaknesses of cutting system memory bandwidth in half? After my tests today, I’m not so sure it will.
I will say, if I built or bought a full-size X99 Haswell-E machine, I’d still want quad-channel memory, because there’s just no reason to give it up. But if I had to choose a small box where I got six cores instead of four, and my workloads benefited from the extra CPU cores? I’d have absolutely no problem making that decision to throw memory bandwidth overboard.