> If you want to measure how efficient a CPU core is you do so under full load.

Higher wattage gives diminishing returns. Chips will run higher wattage under full load just to eke out a marginal improvement in performance. Therefore efficiency improves if the manufacturer chooses to limit the chip rather than pushing it harder.

Test efficiency using whatever task the chip will be used for. For most ultralight laptops, that will be web browsing etc. so the m1 MacBook/snapdragon results are valid for typical users. Maybe your workload hammers the CPU but that doesn't make it the one true benchmark.

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ben-schaaf 3 days ago | parent [-]

No, and that's exactly the point I'm making. If you try to measure ISA efficiency using a workload where the CPU is idle the vast majority of the time, then your power usage will be dominated by things unrelated to the ISA.

To further hammer the point home, let me reduce do a reductio ad absurdum: The chip is still "in use" when its asleep. Sleeping your laptop is a typical usecase. Therefore how much power is used while sleeping is a good measure of ISA efficiency. This is of course absurd because the CPU cores are entirely turned off when sleeping, they could draw 1kW with potato performance and nothing in this measurement would change.

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fwipsy a day ago | parent [-]

You're not taking into account dynamic clock speeds, just "idle" and "load." Most CPUs have configurable TDP, and if you give them more power they will run more cores at a higher frequency. Suppose I measured the same laptop CPU under "full load" in two different computers at full load, but one is configured for a 30w TDP and the other is configured for a 40w TDP. The first PC will be more efficient, because you get diminishing returns for increased power. But it's the same CPU.

To go back to your original argument, you're claiming that x86 ISA is more efficient than ARM because a certain AMD chip beat certain M1/snapdragon chips at 50w. You can't draw that conclusion because the two chips may be designed to have peak efficiency at different power levels, even if the maximum power draw is the same. Likely the Snapdragon/M1 have better efficiency at 10W with reduced clock speed even if the CPU is not idling.

Hence my response: it doesn't make sense to talk about performance per watt, without also specifying the workload. Not only will different workloads use different amounts of power, they will also rely on different instructions which may give different ISAs or CPUs the edge.

Not to mention -- who even cares about ISA efficiency? What matters is the result for the product I can buy. If M1/snapdragon are able to match AMD on performance but beat it in battery life for my workloads, I don't care if AMD has better "performance per watt" according to your metric.

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ben-schaaf 20 hours ago | parent [-]

> You're not taking into account dynamic clock speeds, just "idle" and "load." Most CPUs have configurable TDP, and if you give them more power they will run more cores at a higher frequency. Suppose I measured the same laptop CPU under "full load" in two different computers at full load, but one is configured for a 30w TDP and the other is configured for a 40w TDP. The first PC will be more efficient, because you get diminishing returns for increased power. But it's the same CPU.

Dynamic clock speeds are exactly why you need to do this testing under full load. No you're probably not getting the same power draw on each chip, but at least you're eliminating the frequency scaling algorithms from the equation. This is why it's so hard to evaluate core efficiency (let alone what effect the ISA has).

What you can do however is compare chips with wildly different performance and power characteristics. A chip that is using significantly more power to do significantly less work is less efficient than a chip using less power to do more.

> To go back to your original argument, you're claiming that x86 ISA is more efficient than ARM because a certain AMD chip beat certain M1/snapdragon chips at 50w.

I never claimed x86 is more efficient than ARM. I did claim the latest AMD cores are more efficient than M1 and snapdragon elite X, while still having worse battery life.

> You can't draw that conclusion because the two chips may be designed to have peak efficiency at different power levels, even if the maximum power draw is the same. Likely the Snapdragon/M1 have better efficiency at 10W with reduced clock speed even if the CPU is not idling.

Firstly this is pretty silly: All chips get more efficient at lower clock speed. Maximum efficiency is at at whatever the lowest clockspeed you can go, which is primarily determined by how the chip is manufactured. Which brings me to my second point: What does any of this have to do with the ISA?

> Not to mention -- who even cares about ISA efficiency? What matters is the result for the product I can buy.

If you don't care about ISA efficiency why are you here‽ That's what this discussion is about! If you don't care about this just leave.

And to answer your question: We care about ISA efficiency because we care about the efficiency of our chips. If ARM was twice as efficient then we should be pushing to kill x86, backwards compatibility be damned. In actuality the reason ARM-based laptops have better efficiency has nothing/little to do with the ISA, so instead of asking AMD/intel to release an ARM-based chips we should be pushing them to optimize battery usage.

	▲	fwipsy 16 hours ago \| parent [-]
		You didn't understand what I said before responding. Please don't waste my time like that.