[einpoklum]

Wow, I can't believe how BS this TDP is! I feel like a total idiot! I've always assumed it's sorta-kinda a tight upper bound on power consumption, perhaps with some allowance for "imperfections" in the dissipation properties of the CPU, and that I shouldn't sweat the details.

Couldn't this count as false/misleading advertizing though?

[gruez]

It's thermal design power, ie. it's the power that it's designed for, not absolute max.

[db48x]

No, they don’t design the chip with these numbers in mind. The marketing department picks the number they want based on how they want customers to think about the chip, and which competitors they want you to compare it against. They just plug in whatever numbers are needed into the formula so that the number comes out how they want it.

[wahern]

That seems a little too cynical. It matters how a customer might use a chip, such as the type of cooling that would be expected in a typical system using that model, and that's informed by the advertised specifications. Base clocks and the amount of SRAM also figure into TDP. No doubt there are completely arbitrary aspects to TDP driven purely by profit-focused market segmentation, but it's not just that.

That said, it's definitely very frustrating as someone who does the occasional server build. Not only does TDP not reflect minimum or maximum power draw for a CPU package itself, but it's also completely divorced from power draw for the chipset(s), NICs, BMCs (ugh), etc, not to mention how the vendor BIOS/firmware throttles everything, and so TDP can be wildly different from power draw at the outlet. The past 5 years have kind of sucked for homelab builders. The Xeon E3 years were probably peak CPU and full-system power efficiency when accounting for long idle times. Can you get there with modern AMD and Intel chips? Maybe. Depends on who you ask and when. Even with identical CPUs, differences in motherboard vendor, BIOS settings, and even kernel can result in drastically different (as in 2-3x) reported idle power draw.

[db48x]

No, clock speed and cache have nothing to do with TDP. AMD uses a simple formula to calculate TDP. It is the temperature of the IHS minus the air temperature measured at the cpu cooler’s intake fan, divided by a conversion faction in °C/W.

But they don’t use real temperatures from real systems. They just make up a different set of temperatures for each CPU that they sell, so that the TDP comes out to the number that they want. The formula doesn’t even mean anything, in real physical terms.

I agree that predicting power usage is far more difficult than it should be. The real power usage of the CPU is dependent on the temperature too, since the colder you can make the CPU the more power it will voluntarily use (it just raises the clock multiplier until it measures the temperature of the CPU rising without leveling off). And as you said there are a bunch of other factors as well.

[magicalhippo]

> The formula doesn’t even mean anything, in real physical terms.

From your description the formula is how you would calculate the power for which a certain heatsink at a given ambient temperature would result in the specified IHS temperature.

The °C/W number is not a conversion factor but the thermal resistance[1] of the heatsink & paste, that is a physical property.

So unless I misunderstood you it's very much something real in physical terms.

[1]: https://fscdn.rohm.com/en/products/databook/applinote/common...

[db48x]

It might be a useful formula _if_ the numbers were real. Note that when AMD tells you that a 9900X cpu is has a 120W TDP, that's because they picked three numbers to plug into that formula that result in 120 popping out. They picked the result of 120 first, and then found numbers to put into the formula so that it gives you that result.

But the reason I say that it’s physically meaningless is that real heat dissipation is strongly temperature dependent. The thermal conductivity of a heatsink goes up as the temperature goes up because heat is more effectively transferred into the air at higher temperatures.

[gruez]

>The marketing department picks the number they want based on how they want customers to think about the chip, and which competitors they want you to compare it against. They just plug in whatever numbers are needed into the formula so that the number comes out how they want it.

Are you just describing product segmentation? ie. how the ryzen 5700x and 5800x are basically the same chip, down to the number of enabled cores, except for clocks and power limit ("TDP")?

[db48x]

Yep. The 5800X is a higher bin specifically because it can clock higher than the ones in the 5700X bin. That certainly makes them draw more power, so they give them a higher TDP number too. But the TDP doesn’t have anything to do with how much power the cpu will draw or how much heat it will generate in practice. Those numbers vary quite a lot; the CPU continuously adjusts it’s own frequency multiplier based on it’s own measured temperature, meaning it’ll draw more power if you cool it better.

[gruez]

>But the TDP doesn’t have anything to do with how much power the cpu will draw or how much heat it will generate in practice. Those numbers vary quite a lot; the CPU continuously adjusts it’s own frequency multiplier based on it’s own measured temperature, meaning it’ll draw more power if you cool it better.

I don't get it, are you referring to the phenomenon that different workloads have different power consumption (eg. a bunch of AVX512 floating point operations vs a bunch of NOPs), therefore TDP is totally made up? I agree that there's a lot of factors that impact power usage, and CPUs aren't like a space heater where if you let it run at full blast it'll always consume the TDP specified, but that doesn't mean TDP numbers are made up. They still vaguely approximate power usage under some synthetic test conditions, or at the very least is vaguely correlated to some limit of the CPU (eg. PPT limit on AMD platforms).

[db48x]

No, the TDP number doesn’t even vaguely approximate anything. You can’t use the number to predict anything, or to plan, or to estimate your electric bill, or anything like that.

[lazide]

Apparently that’s not actually true?

[taneq]

Huh, I always thought it was “total dissipated power”. Like you’d use to spec a power supply.

[vel0city]

Its pretty insane to see someone say something like: “TDP is about thermal watts, not electrical watts. These are not the same.” Watts are watts.

But yeah, TDP means nothing. If you stick plenty of cooling and run the right motherboard board revision your "TDP" can be whatever you want it to be until the thing melts.

[o11c]

"TDP is about average watts, not peak watts" would be an honest way of saying it.

[vel0city]

But in the end that's still not actually true in many modern desktop chips. You can take a 65W part, and with a "stock" motherboard firmware, good cooling, and the right workload end up averaging way more than 65W. Or if you have it in a hot room it just might end up using less than 65W.

TDP is more of a rough idea of how much power the manufacturer wanted to classify the part as. It ultimately only loosely relates to the actual heat or electrical usage in practice.

[kllrnohj]

> Couldn't this count as false/misleading advertizing though?

For what, exactly? TDP stands for "thermal design power" - nothing in that means peak power or most power. It stopped being meaningful when CPUs learned to vary clock speeds and turbo boost - what is the thermal design target at that point, exactly? Sustained power virus load?

[einpoklum]

> For what, exactly? TDP stands for "thermal design power"

The chip is not designed for this rate of power dissipation; and it is not the rate of power dissipation that you can expect to get from the chip.

[kllrnohj]

> The chip is not designed for this rate of power dissipation

Says who? AMD advertises the chip as having a base clock of 4.3 GHz over all cores. The 9950X pulls somewhere around 220W at 5ghz all cores and with how power scales, 170W at the advertised 4.3 GHz seems more than plausible. Seems perfectly within reason that the advertised frequency and the advertised TDP are aligned.

I wish Anandtech was still around as iirc they did have charts for all this, which nobody else seems to do :/

> and it is not the rate of power dissipation that you can expect to get from the chip.

Again, says who? Who's expectations? This is a consumer chip, and the expectations of a consumer chip is not that it spends 100% of its time running prime95 or a similar "power virus" workload. I expect that if I buy this chip while I would have intervals of >170W, I'd also have long periods of much less than 170W. If I have a cooler designed to sustain 170W of cooling, that's going to work out on average just fine as there's thermal mass in the system.

[einpoklum]

> Says who?

Says AMD and says Intel, apparently. At the link, there is an official explanation (sort of) how the TDP figure is derived.