We sit down with Intel's Paul Zagacki to talk Devil's Canyon, Haswell-E and overclocking. Then, we overclock five retail Core i7-5960X CPUs to their limits.
Intel’s Broadwell architecture was introduced in early June as the fifth-gen Core processor family (if you missed our coverage, check out Broadwell: Intel Core i7-5775C And i5-5675C Review). Skylake, the sixth-gen design, is expected soon. But if you’re a power user running really well-threaded applications, then both of those “mainstream” platforms remain secondary to Haswell-E—the beefiest incarnation of Intel’s fourth-gen architecture
To be sure, Haswell-E remains the enthusiast's top choice almost a year after its introduction. There’s the fact that you find it in six- and eight-core configurations, of course. Huge 15 and 20MB last-level caches are also great for performance. A quad-channel DDR4 memory controller serves up unprecedented bandwidth. And, depending on the model, you get 28 or 40 lanes of PCIe 3.0 connectivity. Oh, and don’t forget that all three models—the Core i7-5820K, -5930K and -5960X—sport unlocked multipliers for overclocking.
As a matter of personal preference, I tend not to spend a bunch of time overclocking when a CPU is first introduced. There’s the fact that the samples typically come directly from Intel or AMD, motherboard BIOSes aren’t particularly mature and, well, it’s just a long loop of trial and error ahead of time-sensitive launch coverage. But during a recent conversation with the folks at Intel, the subject of Haswell-E and overclocking was brought up. I realized that Tom’s Hardware hasn’t done much with Intel’s highest-end platform since last August, and I had an idea for an experiment.
Actually, the discussion started with Devil’s Canyon, which takes us even further back to Core i7-4790K Review: Devil's Canyon Tantalizes Enthusiasts. The Core i7-4790K was the processor Intel introduced after causing consternation amongst power users when they saw Haswell's lackluster overclocking. Among the improvements cited in our review, a more efficient thermal interface material and additional capacitors to stabilize power delivery were the most notable.
Underneath that superficial summary, which in no way conveys the effort that goes into bringing a new product to market, I knew that Intel had its own engineers working on characterizing the -4790K’s frequency scaling in such a way as to facilitate more aggressive stock clock rates. It also needed to substantiate the claims it was making about a significant population of processors hitting 5GHz. We didn’t get that far in our review, though we did use a conservative maximum voltage and still had quite a bit of thermal headroom in our sample. I wanted the inside story, straight from the horse’s mouth.
And so Intel’s hooked us up with Paul Zagacki, principal engineer with the company’s client computing group, who walked us through some of what the team went through in bringing Devil’s Canyon up and subsequently optimizing the platform for higher stable clock rates.
During the course of our talk, Paul did hit us with a tip that we were quick to put to good use: around 80 °C is where the processor starts encountering a roll-off—it’ll actually overclock better under 80 degrees than if you let it continue climbing.
Devil’s Canyon Lessons
The idea came up to revisit Devil’s Canyon using retail samples. It seemed like most of last June’s Core i7-4790K coverage saw reviewers landing around 4.7 or 4.8GHz with four cores under load. Might refinements made over the last year change that? And what about the impact of boxed parts rather than our pre-production CPU?
Haswell-E is going to endure beyond the launch of Skylake, though. It made more sense to do an overclocking test using Core i7-5960X instead. Plus, the -5820K, -5930K and -5960X all use solder as an interface material instead of a polymer. It dawned on me that we never really asked Intel ahead of the Haswell-E launch whether those parts benefited from the lab work gleaned from Devil’s Canyon. I followed up after our initial chat with a series of email questions, which Paul answered in more depth than we often get from Intel:
Tom’s Hardware: Did Intel use any of the experience that went into Devil’s Canyon to improve overclocking on Haswell-E?
Paul Zagacki: Yes. In fact, we used many of the learnings from the Devil’s Canyon analysis and optimization to improve the overclocking experience of HSW-E. With double the core count, -5960X’s total consumption under extreme overclocking can be very high. To ensure those four extra cores scaled well, we allowed additional power delivery by uncapping Icc Max. We also added a knob to allow enthusiasts to increase the thermal throttling point (we didn’t want a few degrees to get in the way of that last bin). This is something we explored for Devil’s Canyon, but found the throttling knobs weren’t limiting, so they were never released into the wild.
Tom’s Hardware: Was the impact any different on the larger HSW-E die compared to quad-core Haswell?
Paul Zagacki: In general, a larger die area benefits single-threaded thermals with more area under the IHS to spread the power. But all cores overclocked becomes more dependent on overall effectiveness of the thermal solution to bring the extra power out of the package. All of those cores running can quickly become power delivery-limited. Power = CV^2F (more or less) and is a cruel mistress, especially when it takes any extra voltage to get the frequency. Adding cores linearly scales the switching capacitance for any given workload.
Tom’s Hardware: What other knobs don’t get enough attention when you see enthusiasts trying to maximize an overclock or dial in stability?
Paul Zagacki: As you know, the extreme overclockers leave no stone unturned. But casual overclockers don’t seem to play with the real-time knobs accessible via Windows. Many of Intel’s overclocking settings can be changed live within the OS. Overclockers can improve their reliability by dynamically changing frequency, voltage, power limits, etc. Intel’s Extreme Tuning Utility, for example, offers a streamlined interface for making such adjustments. It’s a handy tool for overclocking and has some options to dynamically apply different profiles based on the applications running on your system. For example, XTU can dynamically overclock the processor when a specific game starts up. On top of that, we have put a lot of thought into how to recover a system when you’ve gone off the deep end with a set of overclocking knobs. There are a number of features within the CPU and chipset that sense when we are hung and can automatically reset the platform to a place where a simple recovery to a safe operating point is possible.
Tom’s Hardware: With all of this talk about Devil’s Canyon and Haswell-E, what are the top overclocks you’ve seen in Intel’s lab?
Paul Zagacki: On Core i7-4790K, we were seeing a strong population of parts reaching 4.9 and 5.0GHz for 4C/8T on water cooling. We have the ability to look at a lot more silicon than your average overclocker, and then can correlate those results back to data we see in manufacturing and test to project against huge populations of data. Our focus for Devil’s Canyon was on what a typical enthusiast would do in this space, generating stable results and focusing less on things like getting the best-case frequency snapshot, single-threaded or LN2.
Core i7-5960X overclocking has also been amazing. Even with eight cores, the top clock rate is only slightly lower than the four-core -4790K, with some units clocking in as high as 4.7-4.8GHz. This is pretty amazing considering all the additional power/temperature and resulting voltage drops you have to manage with 4x the switching capacitance from the additional cores. Also, we’ve also seen DDR4 overclocking above 4,000 MT/s. This is truly amazing—DDR4 was just released and the officially supported maximum frequency is 2133. With all those cores running, you have to feed them, so the DDR4 overclocking helps you realize a lot of performance you gain from tuning your cores.
How We Tested
Not long after our talk, a box showed up with five Core i7-5960X CPUs in sealed retail packaging. I reached out to the folks at NZXT for a reasonably-priced closed-loop liquid cooler and, a few days later, was greeted by a Kraken X41 at the office.
Test System Components
Over the course of a weekend, we got all five CPUs tested, recording their top stable eight-core clock rate, single-core clock rate, maximum full-load temperature, peak single-core temp and the voltage required to get each sample stable for an hour of RealBench. Along the way, we took a number of interesting notes.
Let’s start with the big number—top clock rate with all eight cores fully utilized. In four out of five cases, we hit a ceiling at 4.5GHz. The fifth made it to 4.6GHz. That’s not to say those are absolute maximums, though. As Paul mentioned, a full load taxes the thermal solution’s ability to move heat away from the IHS. NZXT’s Kraken X41 is a great closed-loop cooler for around $110, but it’s not the largest or most aggressive of its kind. And even then, given the complexity of Haswell-E (a 2.6 billion-transistor die) compared to Haswell (at 1.6 billion, much of which is integrated graphics), 4.5GHz is a strong increase.
Working within the bounds of our cooler, we started tuning the other direction—find the top clock rate, flirting with voltages up to around 1.4V, and then back the voltage down to the edge of stability for a lower temperature.
Knowing that 80 degrees was where overclocking began to roll off, it became immediately apparent that 1.4V was too much for our cooling setup. So, most serious attempts started around 1.35V. From there, we’d back off in coarse .1V increments before adding voltage back in .025V steps at the first sign of instability. And although there’s no clear correlation between our top overclock and the voltage it took to get there, there is some consistency in the temperatures we saw.
Generally, the CPUs that were stable at 1.2V encountered the lowest temperatures. We’re reporting readings from the hottest core—meaning seven others ran cooler. Sample 5 looks like a sweet little piece of silicon. And although Sample 4 is the one that ran stably at 4.6GHz using a 1.3V Vcore setting, its hottest core is, well, five degrees hotter than what we see from any other processor.
Single-core overclocking was a little trickier, even though the Kraken had a lot less thermal energy to contend with. As you can see in the image above, Windows likes to schedule single-threaded workloads on different cores. So, your first five loops at 5GHz might be successful, only to crash on the sixth as a weaker core buckles under the load. Not surprisingly, then, it’s a real pain to loop something like our iTunes or LAME workload over and over looking for instability.
But after many hours of testing, we’re pretty confident that these chips could all take 4.8 and 4.9GHz across their eight cores, so long as only one was active at a time.
Depending on the core under load, temperatures were all over the place. We kept our eye out for the warmest-running one and made it representative of that CPU.
A Sign Of Things To Come
I’ve made no secret of my disappointment with Ivy Bridge and Haswell. After the gloriousness that was Nehalem and then Sandy Bridge, the following two architectures made me question Intel’s commitment to the desktop PC.
The expediency with which Intel made Devil’s Canyon happen after receiving the community’s feedback on Haswell signified a change in direction, though. And while we haven’t gotten our hands on a sample capable of 5GHz yet, Intel says it’s seeing a notable population of parts capable of that symbolically significant clock rate.
Instead, we were able to test a quintet of something even juicier—the eight-core i7-5960X. Retail examples all, we had little trouble taking all five to 4.5GHz (a 1.2GHz overclock over the chip’s 3.3GHz stock Turbo Boost setting with eight cores active). Single-threaded frequencies bounced between 4.8 and 4.9GHz. Despite Haswell-E’s business-class pedigree (these are the same dies used to make Xeon CPUs), Intel exposes a handful of additional tuning options to make the Core i7s even more attractive to enthusiasts. Needless to say, we're happy to see it.