Overclocking Intel’s Xeon
The first thing to remember about dropping a 2P processor in a desktop platform is that not every motherboard recognizes Xeon CPUs. Asus’ Rampage III Formula does (the company says it tries to give all of its relevant RoG platforms this capability). I’ve also heard good things about certain EVGA platforms, though I don’t have any of the company’s motherboards in-house to cross-check. Should you decide to follow this path, double-check that your board will, in fact, take a Xeon processor.
Limitations
Now, right upfront, we know that there is a practical limit where Intel’s reference clock, known as the BCLK, gets hung up. That ceiling is generally in the 220 MHz range. Multiply that number by the E5620’s highest supported ratio, 19x, and you get a fairly feasible 4.18 GHz operating frequency. That’d be nearly a 1.8 GHz overclock—not bad…not bad at all.
Remember, though, that upping the BCLK from 133 to 220 MHz throws a lot of other frequencies out of whack. Intel arms the Xeon with a handful of divisors to help narrow the range of clock rates you can use, but they’re fairly limited. For example, you’ll only want to use the 800 or 1066 MT/s memory ratios. Similarly, QPI needs to be set to 4.8 or 5.86 GT/s. Although the Asus Rampage III Formula motherboard I used for this experiment supports more aggressive settings, using them prevents the platform from booting. Not that we would have wanted to anyway—setting each field to the lowest possible value gives us the most headroom for an aggressive overclock.
As you start pushing frequencies other than the core clock beyond their specifications, it often becomes necessary to goose voltage levels, too. This will almost always be the case for Intel’s Xeon E5620, Core i7-930, or Core i7-970—the three CPUs on our bench today.
Working Around Them
A 221 MHz BCLK setting was already pushing my Xeon E5620 sample fairly hard for a 4.2 GHz clock rate. Even with the lowest DDR3-800 and 4.8 GT/s ratios set, I was forcing a 7976 MT/s QPI rate. This was actually doable with a 1.4 V CPU voltage, 1.425 V QPI/DRAM voltage, and 1.35 V IOH voltage. Those sound fairly high, but our Xeon processor handled them well, never exceeding 75 degrees Celsius with eight threads active in Prime95.
Dialing in 4.3 GHz required a 226 MHz BCLK setting—well beyond where this board wanted to go. That was an 8156 MT/s QPI data rate, with memory clocked at DDR3-1359 (not a problem for my 2000 MT/s Patriot Sector 7 kit), and a 3625 MHz uncore frequency. At this point, I had to pull out a couple of tricks. An increased PCIe clock (110 MHz) was needed to even boot up. Moreover, the QPI Link Data Rate had to be set to Slow Mode or, again, the machine simply wouldn’t boot. Be careful with PCIe voltage adjustments, though. After bumping up the PCIe frequency and IOH/ICH PCIe voltages, I fried the onboard Intel gigabit Ethernet controller. It simply wouldn't show up in Windows afterward.
Cranking the BCLK up to 231 MHz, yielding 4.4 GHz, might have even been viable. Unfortunately, no combination of voltages, differential amplitudes, or clock skews could lock in stability with an 8337 MT/s QPI data rate. Had this CPU been unlocked, though, I’m confident it would have handled 4.4 GHz without a problem.
My goal wasn’t to find the most extraneous settings possible before popping a processor, though. So I dialed things back a bit for this comparison.
