A Few Words on Power Consumption

When we tested the first wave of Skylake-X processors, one of the take away points was that Intel was starting to push the blurred line between thermal design power (TDP) and power consumption. Technically the TDP is a value, in Watts, to which a CPU cooler should be designed to cope with heat energy of that amount: a processor with a 140W TDP should be paired with a CPU cooler that can dissipate a minimum of 140W in order to avoid temperature spikes and ‘thermal runaway’. Failure to do so will cause the processor to hit thermal limits and reduce performance to compensate. Normally the TDP is, on average, also a good metric for power consumption values. A processor with a TDP of 140W should, in general, consume 140W of power (plus some efficiency losses).

In the past, particularly with mainstream processors, and even with the latest batch of mainstream processors, Intel typically rides the power consumption well under the rated TDP value. The Core i5-7600K for example has a TDP of 95W, and we measured a power consumption of ~61W, of which ~53W was from the CPU cores. So when we say that in the past Intel has been conservative with the TDP value, this is typically the sort of metric we will quote.

With the initial Skylake-X launch, things were a little different. Due to the high all-core frequencies, the new mesh topology, the advent of AVX-512, and the sheer number of cores in play, the power consumption was matching the TDP and even exceeding it in some cases. The Core i9-7900X is rated at 140W TDP, however we measured 149W, a 6.4% difference. The previous generation 10-core, the Core i7-6950X was also rated at 140W, but only draws 111W at load. Intel’s power strategy has changed with Skylake-X, particularly as we ramp up the number of cores.

Even though we didn’t perform the testing ourselves, our colleagues over at Tom’s Hardware, Paul Alcorn and Igor Wallossek, did extensive power testing on the Skylake-X processors. Along with showing that the power delivery system of the new motherboards works best with substantial heatsinks and active cooling (such as a VRM fan), they showed that with the right overclock, a user can draw over 330W without too much fuss.

So for the two processors in the review today, the same high values ring true. Almost to the point of it being alarmingly so. Both the Core i9-7980XE and the Core i9-7960X have a TDP rating of 165W, and we start with the peak headline numbers first. Our power testing implements a Prime95 stress test, with the data taken from the internal power management registers that the hardware uses to manage power delivery and frequency response. This method is not as accurate as a physical measurement, but is more universal, it removes the need to tool up every single product, and the hardware itself uses these values to make decisions about the performance response.

Power: Total Package (Full Load)

At full load, the total package power consumption for the Core i9-7960X is almost on the money, drawing 163W.

However the Core i9-7980XE goes above and beyond (and not necessarily in a good way). At full load, running an all-core frequency of 3.4 GHz, we recorded a total package power consumption of 190.36W. This is a 25W increase over the TDP value, or a 15.4% gain. Assuming our singular CPU is ‘representative’, I’d hazard a guess and say that the TDP value of this processor should be nearer 190W, or 205W to be on the safe side. Unfortunately, when Intel started designing the Basin Falls platform, it only was designed to be rated at 165W. This is a case of Intel pushing the margins, perhaps a little too far for some. It will be interesting to get the Xeon-W processors in for equivalent testing.

Our power testing program can also pull out a breakdown of the power consumption, depending if the registers are preconfigured in the software. In this case we were also able to pull out values for the DRAM controller(s) power consumption, although looking at the values this is likely to include the uncore/mesh as well. For both CPUs at load, we see that this DRAM and mesh combination is drawing ~42W. If we remove this from the load power numbers, that leaves 121W for the 16-core chip (7.5W per core) and 140W for the 18-core chip (7.8W per core).

Power: Cores Only (Full Load)

Most of the rise of the power consumption, for both the cores and DRAM, happens when the processor is loaded to four threads - the Core i9-7980XE is drawing 100W+ when four threads are loaded. This is what we expect to see: when the processor is lightly loaded and in turbo mode, a core can consume upwards of 20W, while at load it will migrate down to a smaller value. We saw the same with with Ryzen, drawing 17W per core when lightly threaded down to 6W per core when loaded. Obviously the peak efficiency point for these cores is down nearer the 6-8W range than up at the 15-20W range.

Unfortunately, due to timing, we did not perform any overclocking to see the effect it has on power. There was one number in the review materials we received that will likely be checked with our other Purch colleagues: one motherboard vendor quoted the power consumption of the Core i9-7980XE, when overclocked to 4.4 GHz, will reach over 500W. I think someone wants IBM’s record. It also means that the choice of CPU cooler is an important factor in all of this: very few off-the-shelf solutions will happily deal with 300W properly, let alone 500W. These processors are unlikely to bring about a boom in custom liquid cooling loops, however for the professionals that want all the cores and also peak single thread performance, start looking at pre-built overclocked systems that emphasize a massive amount of cooling capability.

A Quick Run on Efficiency

Some of our readers have requested a look into some efficiency numbers. We’re still in the process of producing a good way to represent this data, and take power numbers directly during the benchmark to get a full accurate reading. In the meantime, we’re going to take a benchmark we know hammers every thread of every CPU and put that against our load power readings.

First up is Corona. We take the benchmark result and divide by the load power, to get the efficiency value. This value is then reduced by a constant factor to provide a single digit number.

In a rendering task like Corona, where all the threads are hammered all the time, both the Skylake-X parts out-perform Threadripper for power efficiency, although not by twice as much. Interestingly the results show that as we reduce the clocks on TR, the 1700 comes out on top for pure efficiency in this test.

HandBrake’s HEVC efficiency with large frames actually peaks with the Core i5 here, with the 1700 not far behind. All the Skylake-X processors out-perform Threadripper on efficiency.

Benchmarking Performance: CPU Legacy Tests Performance Per Dollar Analysis


View All Comments

  • Spunjji - Monday, September 25, 2017 - link

    Super relevant, because they indicate how badly thermally limited the CPU is - which is hella good info to have if you're, say, considering delidding a $1999 processor because the manufacturer used toothpaste under the IHS. Reply
  • tricomp - Monday, September 25, 2017 - link

    Poor AMD...
    No chance they are going to supply (even more) cpu's demand after posting this article..
    I am trying to purchase at list 7 systems for my customers in my country but there's nowhere I can find them beasts here..
  • iwod - Monday, September 25, 2017 - link

    I wish someone could do an article on that too. GF doesn't seems to be the limitation here. GF, should in theory more then enough capacity in their Fab 8 for AMD. Unless GF have some other big customers, otherwise AMD should really be bumping out as much unit as possible. Reply
  • Atom11 - Monday, September 25, 2017 - link

    Can we please see one test (!), if you could possible manage, that shows the advantage of AVX-512 in compare to AVX2 when doing:

    1.) matrix multiply
    2.) FFT
    3.) convolution
  • ZeDestructor - Monday, September 25, 2017 - link

    Give us a comparison to AVX1 and SSE4 too! Reply
  • Gothmoth - Monday, September 25, 2017 - link

    threadripper delivers 80+% of the perfromance for less than 50% of the price.... you don´t have to be a genius to see what the better deal is (price germany: TR 1950x = 950 euro, 7890xe =2300 euro) Reply
  • Spunjji - Monday, September 25, 2017 - link

    Don't let that stop them equivocating about how companies who need that power yet somehow have no need for ECC don't care about cost because something something software TCO blah blah. Reply
  • spdragoo - Monday, September 25, 2017 - link

    I'm trying really, really hard to think of a company that, at some point or another, doesn't say, "Equipment X may outperform Equipment Y, but the extra cost to buy Equipment X is too much, we'll just make-do with Y instead." Especially since 100% of companies have a limit on their budgets.

    What's that, you say? Multi-billion dollar corporations don't have to worry about the money they spend? Someone apparently didn't pay attention in their Econ 200 class, or their Introduction to Accounting coursework.

    By definition, every business has a *finite* amount of money they can spend, based on a) how much money they collect from their customers, b) how much they can recoup on the sale of assets (tangible or intangible), & c) how much they can get from "other sources" (mostly bank loans or by selling stock shares, or sometimes government grants, but you might find the occasional situation where a generous benefactor just bequeaths money to a company...but I doubt you'll even see that happen to 1% of the companies out there -- & no, venture capitalists pouring money into a company is *not* a situation where they "give the money away", they're getting something for their money, usually stock shares or guarantees of repayment of the loans). Of that money, some of it is earmarked for employee compensation (not just the executives, but the office drones & lower-level employees that do 99% of the actual work), some of it goes towards taxes, some of it pays for rental payments, some for loan payments, some for utilities (telephone, Internet, electricity, gas, water, etc.), some of it may get set aside for "emergencies", some gets earmarked for dividends to the shareholders, etc. That means that a (relatively) small portion is set aside for "equipment replacement". Now, if the company is lucky, the lion's share of that budget is for IT-related equipment...but that covers more than just the office drones' machines, that covers everything: server racks, storage services, cloud vendor payments, etc.

    And that is where the price comes into play. For probably 90% of office users out there, not only is Threadripper an overpowered product, so are these products. Heck, we're in the middle of an upgrade from Windows 7 to Windows 10, & they're taking the opportunity to replace our old Sandy Bridge i5 machines with Skylake i7 machines. Sure, they're running faster now...but the main reason they're running faster is because we went from 32-bit Windows to 64-bit Windows, so our PCs now have 8GB of RAM instead of 4GB. That helps with our workload...which primarily revolves around MS Office & using browsers to access & modify a number of massive databases. Having an 8C/16T CPU, let alone a 16C/32T CPU, wouldn't provide any boost for us, since the primary slowdown is on the server side.

    These are going to be expensive systems for specialized purposes...& those individual companies are going to look at their budgets very closely, as well as the performance benchmarks, before deciding to purchase these systems. Sure, they may hold the performance crown...but not by that big of a margin, & especially when compared to the margin that gives them the "most expensive price" crown.
  • BrokenCrayons - Monday, September 25, 2017 - link

    Human labor is more expensive than hardware. The 20% additional performance for $1000 more can be earned back quickly by the increased productivity of your workforce (assuming your management staff is effective enough to keep the employees gainfully employed of course and that's certainly not always the case). Reply
  • vladx - Tuesday, September 26, 2017 - link

    Indeed the difference in price is pretty much negligible in a professional setting. Reply

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