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Comment by nottorp | Hacker News Reader

Hmm I need some schooling please:

"The TDP for the latter part is 35 W while the i9-10900F is listed at 65 W, but being an Intel desktop processor that just reflects the TDP for the base clock, with much higher energy demands required for higher clocks (e.g. maximum PL1 has been recorded at 170 W)."

So where do I find the actual maximum power consumption on recent desktop CPUs then? Do I set a limit in the BIOS? Do I read each and every review to see what they measured?

Pointers to relevant links very much appreciated, thankee sai.

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> So where do I find the actual maximum power consumption on recent desktop CPUs then?

Unfortunately you do this by putting it on a motherboard and and generally putting an amp meter on the 12v EPS rail. Either that or just power from wall. The CPU vendors have worked really hard to hide this. Mostly because for most people it's largely irrelevant these days as the CPU will scale to cooling and power delivery. There are some limits though.

You'd think "power consumption for prolonged 100% all-cores workload" would be the most relevant thing when selling server chips specifically, though, since it translates directly into OpEx. And yet even on those data sheets it's nowhere to be found.

So for that it's "Easier" because you check the VRM spec for the socket for server. That's the maximum draw the manufacturer expects any CPU for that socket to ever pull. Then generally most of the higher end parts will vary clock speed vs cores etc to stay on that line.

This works because server motherboard manufacturers only ever build to that spec and not one iota beyond it for cost reasons.

But yeah... average expected draw is going to be interesting and most manufacturers won't give a number to avoid getting sued.

Yeah, but the mobo manufacturer has to plan on you putting in the thirstiest chips on there, when you might be putting lower end chips on because your workload is mostly memory or PCI channel constrained and you are looking to max out the density in the rack.

Of course you also have to consider the cooling capacity of your chassis. 170W in a 1U form factor needs a hell of a lot of airflow to avoid thermal throttling, especially on a dual CPU motherboard. Plus you gotta fit the power supply and everything else in there.

Prolonged 100% all-core workloads, at what clock speed? With what cooling solution?

All of these factors are variable. You could produce a number that says "the maximum clock the part is capable of, with a perfect cooling solution", but that number would never be reached in anything short of a liquid nitrogen testbench, and thus would be pointless outside of an academic setting. Intel was literally demonized for trying something similar to this two years ago, when they benchmarked a 28 core 5Ghz chip without disclosing that it was overclocked, and hooked up to an industrial water cooler [1].

In reality, this number can be found, but it would be an irrelevant number.

[1] https://www.tomshardware.com/news/intel-28-core-cpu-5ghz,372...

If you’re selling server chips you’re likely selling to clients who are gonna do their own tests and trials under their use cases anyways. They aren’t even gonna look at numbers supplied by the manufacturer.

And if you advertise power consumption for those kind of workloads (100% usage) to regular consumers, who are probably gonna buy 1 computer every 3-5 years, you’re disincentivized from optimizing the kind of stuff that have seen real gains, such as designs that aggressively and cheaply turn on/off systems, or clever decisions to run the chip slower than its max output to improve power consumption, without making an impact noticeable by a human user.

But what 100% workload? Is it 100% integer throughput? In which case it'll use a small fraction of the power of a blended integer & float workload. What about AVX-512? Is that considered 100% or beyond 100%? How much RAM is in play? Is it entirely an L1/L2 workload such that memory is never a bottleneck? Or is RAM used and then the CPU isn't really hitting maximum compute throughput? Or is it RAM reads & writes perfectly intermixed with integer & float computations that will always be L1/L2 cache hits, even though literally nothing will ever have that flawless of an execution in terms of power consumption?

With how much of a CPU is dynamically gated & power controlled there is a difference between "maximum theoretical draw" and "maximum a 'real world' 100% load" will hit.

Yeah, it's irrelevant if you buy prebuilt.

If you build it yourself, how do you know how to spec the system? You either waste resources by overspeccing it or you waste money by paying for something that can't run at full tilt on your system.

And if you care about quiet and cool computing, you're screwed even more. A CPU that will run until it hits thermal limits means maxed out fan speed. I don't want maxed out fan speed.

Are there any CPU coolers left that allow you to set a fixed speed manually? :)

You can't find it. Intel has obscured the measurement too much. AMD is guilty of this as well but on a much smaller scale then Intel.

Sheesh. So the only way is to settle on a few cpu models and search for reviews that have measured the actual power consumption?

Even that is specific to that chip/mobo/PSU/Case/CPU cooler setup. So unless you match what was used in the article your mileage will vary a lot.

Sad but true, 35W should translate to less power usage than 65W.

Still waiting for Lenovo Yoga Slim 7 which appears to be at the sweet spot for battery life, power and size.

I don't think reviewers measure it. It's essentially just an average power draw under real-world load at this point.

You look at measured draws for the relevant workloads. Anandtech has a good intro to the various states that an Intel CPU can be in here: https://www.anandtech.com/show/13544/why-intel-processors-dr...

For Intel specifically the TDP is what you need to dissipate to sustain the base frequency. If you want to run beyond base frequency (the "all core turbo"), then you need to handle more. How much more isn't officially documented, nor are the turbo charts either annoyingly (aka, this: https://en.wikichip.org/wiki/intel/core_i9/i9-9900k#Frequenc... )

So for a 9900K looking the official numbers are 95W for 3.6GHz of all core load. But it'll turbo with all cores loaded up to 4.7GHz, for which you'll need some amount of cooling. How much? Well more than 95W that's for sure, but that's all you really officially know. So then you look at reviews and you find that under full load the 9900K pulls down ~170W if it's not hitting thermal limits: https://images.anandtech.com/graphs/graph14605/111362.png

Here is a link to a test of 101 different processors, with corresponding power consumption figures. Alas it is in Russian. https://www.ixbt.com/platform/101-cpu-platform-test-2017.htm...

Oh that's helpful, thanks. Don't need to read the text.

It's also a bit insane how far they go with the marketing lies.

Here is the graph you are looking for: blue = low CPU load yellow = medium red = maximal load

They measure the whole system consumption, so you need to subtract roughly 25-30 watts, to get raw CPU consumption.

I apologize, did not link the graphs: power consumption: https://www.ixbt.com/img/x780/r30/00/02/26/21/31power_334783...

power effciency: https://www.ixbt.com/img/r30/00/02/26/21/33ee_794961.png

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