
Why intel processors draw more power than expected: TDP and Turbo explained - cm2187
https://www.anandtech.com/show/13544/why-intel-processors-draw-more-power-than-expected-tdp-turbo
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magila
Knowing the peak power dissipation at max turbo frequency wouldn't be quite as
informative as you might think. The problem is power consumption varies widely
depending on the workload. Presumably Intel would be expected to report power
for the worst-case workload, but that would be significantly higher than what
most people will ever see. This will only get worse once mainstream CPUs get
512 bit AVX units, further increasing the spread between integer only and AVX
heavy workloads.

I think the basic problem is that power management on modern CPUs has gotten
complex enough that you can't really summarize a CPU's power consumption in
one or two numbers anymore. To really get a clear picture takes a full blown
datasheet with tables showing power consumption under a range of conditions.
Frankly, most enthusiasts aren't equipped evaluate such detailed information.
Instead people rely on a few sparse data points provided by tech review sites
and generally just throw a really big cooler on their CPU and hope for the
best.

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ant6n
The Problem is that both the cooling and the power supply both work with one
simple number, and we have to know what the CPU needs in order to put it in a
system.

~~~
magila
It's easy to make an Intel CPU stay within the limits of a particular power
supply and cooling system by setting PL1 and PL2 appropriately. It's a
Heisenberg's uncertainty sort of situation. You can fix frequency and get
variable power dissipation, or fix power and get variable frequency, but you
can't really fix both at the same time.

Enthusiast users building desktop systems tend to massively over-provision
both power supply and cooling anyways so it's usually not an issue.

~~~
ant6n
Silent PC enthusiasts like to not ridiculously over-provision power and
cooling, and they want to know to what extend they can run a system fanless.

And if you don't want to spend money, you don't want to over-provision either.

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userbinator
_Motherboard manufacturers prefer the 'Unlimited PL2' route, because it puts
their results at the top of benchmark lists._

...and so does every desktop user, no? If the cooling system can "take the
heat", then the CPU should run as fast as it can. It seems like the whole
point of this premature throttling is only to meet some stupid marketing spec,
and the motherboard manufacturers are wise enough to ignore it.

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zrm
> If the cooling system can "take the heat", then the CPU should run as fast
> as it can.

The issue is what happens to benchmarks, because removing the power limit
makes the limit thermal, which makes performance proportional to the efficacy
of the cooling solution and other variables like the ambient temperature in
the room.

There will also be variation between individual processors of the same model,
since it's always been the case that some would run warmer or cooler, but now
that affects out-of-the-box performance. (And with manufacturers sending chips
to reviewers for testing, which ones are they likely to choose?)

This is a pretty big deal if you're a business wanting to buy the kind of
small form factor desktops with cooling solutions that hew close to the
official TDP numbers but you're looking at benchmarks done in larger cases
with gamer-typical cooling solutions. Or people buying always-on home devices
with more stringent cooling or efficiency requirements. Really anyone who
wants to know how the processor would perform in an environment that it isn't
permitted to continuously dissipate 180W.

~~~
userbinator
_The issue is what happens to benchmarks, because removing the power limit
makes the limit thermal, which makes performance proportional to the efficacy
of the cooling solution and other variables like the ambient temperature in
the room._

There surely is an upper limit? Would cooling the CPU with liquid nitrogen,
for example, make it perform appreciably faster even at the same stock clock?

~~~
zrm
There would be an upper limit, but where is it? Maybe something less exotic
than liquid nitrogen (like water cooling), though who knows without trying it.
Maybe even a solid air cooler could do it, or at least get within a few
percent of it.

The concern is that a lot of the cooling solutions that are common in the
market could be _substantially_ worse than that, or even motherboards that
can't supply that much power and are correspondingly configured not to.

It implies that a small form factor machine may be a lot slower even if it has
the same processor in it.

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whateveryou381
TDP="Thermal design power"... (the amount of designed cooling power to get max
cpu performance). ̶W̶h̶e̶r̶e̶ ̶i̶s̶ ̶t̶h̶e̶ ̶m̶y̶s̶t̶e̶r̶y̶?̶

Mystery might be also in the nuance of turbo vs base frequency draw. So even
if you design for TDP as the "max thermal dissipation" you might get only base
frequency performance.

On a side note people often confuse TDP with the max power draw of the
processor. Intel does not specify total power draw probably because processor
scaling is such an important consideration. If you have a system, the easiest
way to figure out what max draw from the power source is to consult the power
brick.

~~~
mbell
> Mystery might be also in the nuance of turbo vs base frequency draw. So even
> if you design for TDP as the "max thermal dissipation" you might get only
> base frequency performance.

That is really what is happening. If you have '4 core 3Ghz Base 4.5Ghz turbo
95W TDP' what that means is that the CPU will pull, by spec, a maximum of 95W
with all cores running at the base frequency under full load. Turbo can exceed
this power spec and maximum power draw under turbo is ill-defined.

> On a side note people often confuse TDP with the max power draw of the
> processor.

It's the same thing, any 'power' a CPU 'draws' is converted to heat, by
definition.

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dpwm
I'm having a hard time reconciling the Anandtech definition of Tau:

> Tau is a timing variable. It dictates how long a processor should stay in
> PL2 mode before hitting a PL1 mode.

with the Intel one:

> Turbo Time Parameter (Tau): An averaging constant used for PL1 exponentional
> weighted moving average (EWMA) power calculation.

I'm assuming that this power calculation is calculated this way, in a similar
way to how Unix load averages are. But the thing is that they are an
exponentially weighted moving average. I understand why: moving averages need
the storage of all the values over that time period and the EWMA only needs
the last value of the moving average.

It's not exactly clear _how_ tau relates to the constant used for the
exponentially weighted moving average. Assuming I want to minimize the
difference between the EWMA and the moving average over a time interval, there
appear to be many valid ways to do so -- and this appears to depend on the
statistical properties of the variable being averaged. But the Anandtech
article shows it as if it's a constant time, which is subtly different.

Is this defined in more detail anywhere?

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std_throwawayay
The tests should be run at the specified TDP to be comparable. Exceeding the
TDP in the tests is a form of overclocking.

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chendragon
IIRC until the recent fix they pushed in a macOS update for the new i9 models,
MacBook Pros don't have any of their power limits set (or have them on the
defaults at 100W). The processor runs at full turbo, until the CPU gets to 102
C and throttles itself to keep it at about 99C.

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chronid
TL;DR: Intel decided to "redefine" what TDP means to them so it aligns what
they're doing and their marketing.

~~~
ajross
TDP means what it always has. It's the maximal thermal power output of the
processor. Input electrical power may fluctuate, but TDP was never an
electrical parameter.

~~~
mtgx
In that case, reviewers and tech sites should either prioritize _peak power_
or at least mention peak power numbers every single time TDP is mentioned,
too, because peak power is _at least_ as important as TDP. I know from past
experience with laptops that tend to get hot, that if the laptop uses
significantly more power than what you'd expect from the TDP, you will have a
bad experience as a customer with that laptop. So at this point, TDP is
definitely quite misleading to customers. Either scrap it completely or at
least mention a more relevant metric to consumers.

~~~
craftyguy
> that if the laptop uses significantly more power than what you'd expect from
> the TDP

You're falling into the "TDP is power input" trap.. TDP and power input are
not the same. You can have a chip that takes less power to operate at a TDP
equivalent to another chip which takes more power to operate.

Edit: There is no way that 100% of output power in a CPU is thermal, else it
would be nothing but a very expensive heater. Also, CPUs do not operate in a
closed environment, like those replying to me here seem to think.

~~~
mindslight
Input power equals output power is a fundamental physical rule, not a "trap".
Yes, average (thermal) output power does not equal instantaneous (electrical)
input power (ie thermal mass), but it averages out.

What the article is pointing out that TDP is thermal _design_ power. Meaning a
nameplate value that was targeted for "standard" operation, even though the
processor can do higher. So if something has a TDP of 100W, yet the heatsink
can maintain proper operating temperature while dissipating 150W, then the
processor might sit there drawing (and dissipating) 150W the _whole time_.

As an aside, I really wish CPU cooler manufacturer/reviews would start simply
stating the _thermal resistance_ \- basically the single number that
characterizes its effectiveness. I know it would undermine the dog and pony
show of building a test system with so-and-so's motherboard etc running
benchmarks, but the entire goal of science is to weed out the extraneous
narrative.

~~~
craftyguy
> Input power equals output power is a fundamental physical rule, not a "trap"

No shit. The trap is thinking that _all_ output power is thermal (it's not).

~~~
pooppaint
All output power is thermal though. This is basic thermodynamics. A negligible
amount of input power will be distributed outside of the CPU die which will be
dissipated as heat a short distance away. Pray tell, what other energy form do
you think this input is becoming anyway?

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p1necone
I imagine some tiny percentage would instead be dissipated as light/radio
waves or sound too - hence my ~99% comment.

There's also this:
([https://en.wikipedia.org/wiki/Landauer%27s_principle](https://en.wikipedia.org/wiki/Landauer%27s_principle))
if you want some light reading.

~~~
pooppaint
Indeed there will be some piezoelectric effect and some EM emissions on the
order of micro watts compared to the 10s of watts burned by the CPU. But the
poster I was replying to seemed quite confused about the fate of power in a
CPU used for computation: It’s dissipated as heat.

