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Comment by jojobas | Hacker News Reader

Does the word "license" actually mean "even though the hardware is capable, you didn't pay enough to not downclock" or something else here?

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No, nothing like that.

It's just a term probably used by the designers to define the particular "regimes" a chip can operate in: if it is doing lightweight stuff it can run at a lower voltage, "closer to the edge" if you will, because the worst case dI/dt event is relatively small so maximum voltage drop is limited. This saves power and keeps the chip cooler.

If the chip wants to suddenly start running some heavy-duty 512-bit wide floating point multiplies, it needs to inform the power delivery subsystem of it's intent and then wait for permission: this permission arrives in the form of a license, like "ok, you are now licensed to run L1 operations (heavy 256-bit or light 512-bit)".

Sometimes this license change can be satisfied by just bumping up the voltage without changing the frequency. Other times there might not be enough headroom to increase the voltage enough and still stay within the operating envelope so a combination of voltage and lowered frequency is used, which is how you get license-based downclocking.

This earlier article [1] explains in some detail the nature of these license transitions, including voltage levels and halted periods.

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[1] https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html

The three links in the third paragraph of the article describe the meaning of license in this context:

https://stackoverflow.com/a/56861355

https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html

https://lemire.me/blog/2018/09/07/avx-512-when-and-how-to-us...

Excerpted from the Stack Overflow answer:

There are three frequency levels, so-called licenses, from fastest to slowest: L0, L1 and L2. L0 is the "nominal" speed you'll see written on the box: when the chip says "3.5 GHz turbo", they are referring to the single-core L0 turbo. L1 is a lower speed sometimes called AVX turbo or AVX2 turbo, originally associated with AVX and AVX2 instructions. L2 is a lower speed than L1, sometimes called "AVX-512 turbo".

Clear as mud. If it means "we can't make these instructions work at nominal clock" rather than "we won't let you", why can server chips do it?

The server chips tend to be sold with a lower base clock than the consumer equivalents. If you underclock your consumer CPU to whatever the server equivalent currently is, you aren't going to see much throttling under any workloads. But you're also going to be the guy that has a 2.2GHz processor when all your friends claim to have a 5GHz processor.

In my experience, the limiting factor tends to be power delivery. I had a 6950x that I overclocked and I could get it to consistently hard crash just by starting an AVX task. My filesystems did not appreciate that! (I eventually spent a ton of time debugging why that happened, and it was just that the power supply couldn't keep the 12V rail at 12V. Turns out that Intel knows what sort of equipment is out on the market, and designed their chips accordingly. I did upgrade the power supply (1200W Corsair -> 850W Seasonic Prime) and got stable AVX without downclocking. But the whole experience killed overclocking for me. It just isn't worth the certainty that your computer will crash for no good reason at random times.)

Server chips have the same license-based limitations. In fact, the limitations first appeared on those chips and in some generations only on those chips.

The term "license" comes from Intel terminology, I didn't invent it.

It has nothing to do with legal/software licensing, but rather the chip having permission (a "license") from the power management unit and voltage regulator to run certain types of instructions that might place a large amount of stress on the power delivery components.

Note that HEDT Skylake-X all let you choose the clock speed for each license in the bios. It is definitely not a "we won't let you".

Server chips are sometimes already down clocked for stability or are binned for ability to handle more heat/voltage/clock, and so may have more headroom in some cases.

Doing computation produces heat, doing more computation at once produces more heat. If you build a cpu doing 512bits of math at a time, and get it to max clock, you'll be able to do math 32bits at a time, at a higher clock, because its a bit cooler. It is just physics.

They can't.

A Google search doesn’t do much to define the term, but from context I believe it means a frequency allowance that’s baked into the CPU rather than one that’s enforced dynamically depending on power/thermals.

I doubt Intel has validated the hardware outside its “license”. This appears to be a much earlier design constraint than anything they could reasonably expect to bin.

The license is an open-loop, predetermined safe operating area for AVX. The other limits, like RAPL and temperature, are closed-loop control systems. I think we can imagine that the static SOA exists for all these parts, but in the newer part it's so large that it doesn't tread much on the territory of the closed-loop systems. Eventually the SOA will be large enough to be irrelevant in practice.

This is an excellent description. May I use, with credit, part of this text, perhaps edited or paraphrased, as a clarifier in the blog post?

Sure, feel free. I certainly don't need to be credited for the idea that semiconductors have limits.

Well yes, but you gave a good bit more detail than "that semiconductors have limits" ;).

That's what I assumed it means too. And the article said nothing to clarify, but now I learn from comments here it means something quite different...

If it does that's terrible

Sun or SGI used to do approximately that. They sold downclocked/locked core chips that could later be unlocked for a fee (from memory, time-limited fee).

Intel did that on low-end desktop CPUs for once, but soon discontinued it after criticism.

* Intel Upgrade Service

https://en.wikipedia.org/wiki/Intel_Upgrade_Service

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