large difference between calculated and measured power consumption - Page 1
Forum section: electronics / general

author: m4rtin
author_rank: Regular Contributor
timestamp: May 19, 2021, 10:10:02 pm
content: I have a workstation with two Intel Xeon family CPUs, graphics card with Nvidia GPU, 16 RDIMMs, three NICs, two SSDs and two HHDs. According to Intel, TDP for my CPU is 115 W and according to MSI documentation, the TDP of the graphics card is 145 W. As I understand, thepeak power for a microprocessor is usually 1.5 times the TDP ratingso that in my case the CPUs and graphics card can drawmax(2 * 115 * 1.5) + (145 * 1.5) ~ 560 W? If I sum up the power consumption of the server-grade motherboard itself(~80 W), memory modules(16* 5 W), multi-port NICs(3* 5 W), SSDs(2* 2 W), HDDs(2* 6 W) and fans all under the max load, then I get around 210 W. So in total, the theoretical maximum total power consumption should be around 770 W. If I use thepower supply calculatorand set the CPU utilization to 100%, then the calculated value is similar to my calculations: 730 W. I'm using "80 Plus Titanium" certified 850W PSU so as I understand, then under full load I should expect around 730 * 1.05 = 760 W from the wall? However, when I load all the CPU cores to 90 - 95% using a CPU benchmark program and measure the power using aPM300wattmeter, then I get only 380 - 385 W. I wasn't running anything GPU-intensive at that time and also the storage devices were sitting pretty much in idle, but still, why is the difference between the calculated/expected power consumption and measured power consumption so large? In short, why are my calculations/estimations wrong?

author: golden_labels
author_rank: Super Contributor
timestamp: May 20, 2021, 12:03:28 am
content: Those are values under full load. It is unlikely you are able to fully load any of the components intentionally with a simple task.You have mentioned “90–95%”. Where from is that value taken? Is that CPU load reported by the operating system? If ye, it’s merely the percentage of time the CPU was doing something useful. That doesn’t imply it consumes 90–95% of the power it could consume. That would be very hard to measure.Another issue is that the meter may be not a true RMS, which may introduce an error in the measurement if the measured current is not sinusoidal.

author: SG-1
author_rank: Regular Contributor
timestamp: May 20, 2021, 12:04:33 am
content: Are the stated values actually in watts or are they in va ?

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 20, 2021, 06:09:04 am
content: The figures seem typical to me (and I'm in agreement with golden_labels here).It is difficult to fully load all components of a computer, especially a dual-CPU system. You need carefully written "burn" utilities to do that, and even then, it's hard to do.It is relatively easy to load the CPU and memory only, but then the storage and graphics isn't. It is more likely to accidentally do it once in a blue moon, with a combination of workloads.Because you don't want the system to overheat or crash (due to a brownout), you do need to use a power supply that seems overpowered. In particular, it's just not "power" total; the +5V and +12V supplies must provide enough current. With multiple CPUs and/or graphics cards, the current demands can be quite skewed (but I'm not sure of your particular hardware), and you may need much more powerful power supply than the figures would otherwise indicate.Having the "commonly achieved full load" (whatever that happens to be for you; the workload that utilises the system effectively for you) be about half the rated workload of the power supply is actually a good idea, because most supplies are most efficient at about half the rated load.Quote from: SG-1 on May 20, 2021, 12:04:33 amWatts. Thermal Design Power, in particular, indicates the amount of waste heat expected to be handled by the cooling system in a fully-loaded case.I myself am in the process of building a silent workstation PC (probably Core i7-11700 -based), where the CPU TDP is about 65 watts, with a fully custom enclosure (bench style). I plan on using two Samsung 980 Pro 500GB M.2 SSDs in a software raid configuration for OS and everyday files, plus a couple of spinny-disk drives for media files, and the integrated graphics on the i7, but plenty of memory; the processor being the single biggest power draw. Overall, the theoretical maximum (like you calculated) is around 250 watts or so (depending), but because in practice the system will typically (when not doing heavy computation, but be otherwise used and not sleeping/idle) draw only about 30 watts, I'm looking at PSUs with good efficiency even at the low end. Usually the supplies have their maximum efficiency at about 50% rated load, and the higher-power supplies don't perform too well at this lower end. I'm looking at a Seasonic PX-450, a passive 450W supply, which seems well suited for this particular build.

author: DiTBho
author_rank: Super Contributor
timestamp: May 20, 2021, 08:37:30 am
content: Talking about PSUs, someone says - over-sizing will give you a save overhead in case you need more wattage, but there's no real benefit - oh well, perhaps they forget you will also get some real efficiency gains by running at a lower load, and it's not mostly negligible, at least, according to my measuring.

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 20, 2021, 09:01:57 am
content: Quote from: DiTBho on May 20, 2021, 08:37:30 amA good example is Fractal Design Ion+ 760W, as measured by folks atTechPowerUp:Above 80 watt loads, you'll get 90%+ efficiency; with best efficiency (94%) around 350 watt loads (about half the rated wattage of this PSU).However, note how the efficiency degrades when you get to the low end. If I were to use this one in my low-power workstation, it would waste at least 0.20×30W ≃ 6.0 watts. The 450W passive-cooled Seasonic Prime PX-450 would be 89% efficient at 30W load according to anOCCClub.rureview (in Russian; about 7% of the rated load), in which case only 3.3 watts would be wasted in the supply.Sure, the PX-450 is overpriced for most use cases, but I do want a silent build, and although a watt or three lost in the supply is absolutely nothing, this one just fits well to my particular use case, despite its price. (Where I live, Ion+ 760W and PX-450 are about the same price; both being "80 PLUS Platinum".)My point being, across most typical PC power supplies, they seem to be most efficient at around midway their rated power, with a small drop at the high end; but often a big drop at the very low end. So, you don't want to use a 850W power supply if the computer typically only uses say 30 watts, and peak power is less than 400W (as in my case, but not OP's case; like I said, OP's choice of PSU seems reasonable to me).

author: m4rtin
author_rank: Regular Contributor
timestamp: June 06, 2021, 05:42:32 pm
content: Quote from: golden_labels on May 20, 2021, 12:03:28 amYes, this was the average of all the CPU cores reported bytoputility while runningstress.Has there been any studies, tests or benchmarks which show the actual TDP or power usage while the CPU utilization shown in the OS is close to 100%? If I assume that the TDP is for example 70% or the maximum value specified by the manufacturer while the CPU is running close to 100% and my GPU is sitting in idle and consuming around 10W, then I would actually end up around 430 W for the whole system, which is quite close to the measured 380 - 385 W. I'm simply bit surprised that those PSU calculators seem to overestimate that much.

author: golden_labels
author_rank: Super Contributor
timestamp: June 08, 2021, 04:26:19 am
content: TDP is a design parameter: you do not measure it as a user, you read it from the specs. It’s a constant value associated with a particular device model. It doesn’t depend on load. For that reason the rest of the question makes no sense too.The power actually used for a given workload may be measured by a power meter. Which is a thing you have already done.What calculators are providing is hard to tell without seeing them(1), but most likely the power required to meet power requirements of each component at its maximum power consumption.____(1)Don’t link them for me: I’m not interested in running them.

author: Berni
author_rank: Super Contributor
timestamp: June 08, 2021, 08:29:35 am
content: Such a power consumption calculation was simple back in the Pentium II days when everything constantly ran at the rated clock speed all the time.These days CPUs and GPUs have gotten too power hungry to do this, so the manufacturers implement features in these chips that reduce the clock speed or stop the clock all together to parts of the chip that are not required. This is beyond just the CPU turning down the clock speed on the individual cores (As you can see it doing live with a lot of HW diagnostic software), but it also switches clocks to tiny parts of the CPU core on and off as needed. This means the power consumption of the CPU is highly dependent on what it is doing. For example a core executing a empty "while(1)" will likely use a lot less power than executing a loop that constantly uses wide AVX512 vector float operations. The CPU manufacturers realized this and started taking advantage of it by slightly overclocking cores when the CPU is performing lots of mostly lower power operations. Intel for example calls it "TurboBoost". It monitors the core temperature and the current consumption of the CPUs power rails to determine if there is still enough margin in the TDP to raise the clock speed above stock clock.Most day to day workloads do not load all parts of s CPU and GPU to 100% simultaneously (not even games) so you don't get to see these worst case numbers. There are tools however that are designed to push CPUs and GPUs to as close to the limit as possible for stress testing. I like to use Cinebench for CPU and FurMark for GPU stressing. My most common use for them is stressing an seemingly unstable PC to try and make it reproducible crash, or to test the cooling performance of a new PC build to make sure it performs well under the worst possible cases.