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big.LITTLE Processing: Defining the Future of SoC Architecture - https://www.samsung.com/semiconductor/minisite/exynos/newsro...

With this CPU design some cores are optimised for performance (at the expense of using more power) while some cores are optimised for efficiency (using the least power at the expense of computing performance). This makes sense for laptops and smartphones, as it can save power and thus run longer when being powered by batteries. But (in my opinion) not for Desktop PC's where most people care more about computing performance than saving a few watts.

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> not for Desktop PC's where most people care more about computing performance than saving a few watts.

Not for all workloads. Daemons and services don’t usually need much oomph, and being able to run them at double the efficiency and avoid them competing for resources with software which actually wants the higher power cores would be amazing: less context switching, less cache thrashing, less random variability, …

Hell, it would also help make low-power software more reliable e.g. foobar and the audio output don’t need much but they need it. If something else is loading the machine to the gills they can get starved and audio will start breaking down.

I do understand the pros and cons. My point was that it doesn't matter when you prefer peak performance vs efficiency on a desktop PC. I'd rather have 8 Firestorm cores on my PC, with one or two of these fast cores dedicated for the "lighter" workload even if it consumes a few more watts. When your desktop computer is slow, doing a CPU resource intensive task, nobody is going to appreciate that it is consuming less power - everyone will be cribbing about the slow speed.

Assuming die area is the constraint, the choice is between 4 firestorm + 4 icestorm and 5 firestorm, because a firestorm core is 4x bigger than an icestorm core.

Or between 8 firestorm and 7 fire + 4 ice (or 6 fire + 8 ice).

I thought the idea was that not all processes are the same class of citizen. Some of 'em go to high performance cores, but some of them always get assigned to what's basically a second computer running in parallel and handling interface stuff or system management, typically a lot less powerful (though according to this article, far from helpless).

In this way the CPU-intensive stuff never has to break its concentration by sullying its tiny processors with utility calculations, but keeping the observable system responsive (a much less demanding task) goes to the second, weaker computer, that's running as if nothing else is happening. So neither thing ends up being slow, because neither thing particularly has to be interleaved with the other.

Source: I'm no CPU or SoC designer, but I own a Mac M1 laptop for the purposes of compiling open source plugins to the new machines, so I've done hundreds of XCode compiles on the machine, and lots of housekeeping work in Finder. None of it seems slow, by any metric or perspective.

I understand that. But that wasn't the point. Historically laptop and desktop PC CPUs have been different because of the different requirement - laptop's prioritise efficiency whereas desktop pc users prioritise performance. Apple instead has used the same CPU on their tablet, laptop and Desktop PC.

(And note that even Intel / AMD processors do the kind of power scaling that you described without any specialised low power cores - all cores are designed to run at varying speed. Thus, when a small task is executing, the core executing it runs it at the minimum acceptable speed and saves power. But the same core can run at full speed on demand. That's a more acceptable tradeoff for me for Desktop PCs, especially considering that ARM CPUs already are very efficient.)

> This makes sense for laptops and smartphones, as it can save power and thus run longer when being powered by batteries. But (in my opinion) not for Desktop PC's where most people care more about computing performance than saving a few watts.

You're probably right about laptops and smartphones. I don't have a view on whether you're right about desktops, but I think it's probably not true for the big datacentre providers, who absolutely optimise for power and heat.

If the only niche that doesn't care about power is the desktop, and desktops are absolutely a niche when compared to the total market of smartphones, laptops and datacentres, I suspect power efficiency is coming to the desktop whether you want it or not, whether it's a positive or not, if only because of economies of scale as far as chip development and production goes.

Yes, I realise that datacenters do care about power consumption too, and that is why they find the idea of a ARM server cpus so attractive. My point ofcourse was directed at home users of Desktop PC - I don't like the idea of sacrificing performance to save a few watts, when the ARM cpus are already very efficient. (I mean, when you are rendering a 3d scene, or encoding a video, or compiling a code, no one I know is going to appreciate the longer times it takes to execute these tasks because not all cores are equal).

I'm not sure that you could make a case for this not making sense in a desktop computer, as everything is ultimately a trade-off.

It's fairly clear that the Icestorm cores represent a performance gain in terms of performance per watt, but also die area. The four Icestorm cores and their support infrastructure takes up about the same physical space as one Firestorm core with its support infrastructure.

I doubt that an M1 with five Firestorm cores would perform as well as the eight cores we did get.

My case for having 8 fast Firestorm cores instead of 4 fast + 4 slow cores is simple - the majority of Desktop PC users don't care about sacrificing performance to save a few watts. When a PC is running slow because of a cpu intensive task, nobody is going to appreciate that it is saving a few more watts.

And from the figures in the article, it looks like 1 Firestorm core is very roughly equal to 2 Icestorm cores -

    Relative to their Firestorm times, Icestorms performed more slowly by:

      190% running assembly language
      330% running simd (Accelerate) library functions
      280% running simple Swift
      550% running ‘idiomatic’ Swift

So 5 or 6 Firestorm cores could have matched the current 4 + 4 config (disregarding the valid point you made about the die area).

The rumours are that this year's MacBook pro's will feature an M1X processor with an 8 + 2 configuration which seems like it might well be a good tradeoff to me. The 2 small cores reduce context switching on the big cores making overall performance a lot snappier.

I'm going to guess that if you could have 8 Firestorm, you will always be better off having 6 Firestorm/8 Icestorm.

Always. In practical use. Even for the most isolated tasks there'll be enough 'maintenance work' for the system to do, in practice, that the pool of Icestorms will justify their real estate.

If I'm mistaken, it'll be because there proves to be a maximum number of Icestorm cores for the 'maintenance work' that's called for: say if you had a 64/64 system, the idea is that maybe you get to a point where you can have 'too many cores' on the Icestorm side, wastefully so.

But that assumes they play NO role in setting up the Firestorms to perform optimally. All this stuff, all this load balancing and assigning, isn't magically done by some GrandMasterCore, it's being worked out by the same CPUs that are benefitting from the load balancing.

If you can use Icestorm cores to better set up and load balance the Firestorm cores, if you can burn unused CPU on the Icestorm side to optimize the Firestorm side… then there is NEVER a situation where you'd want the Firestorms to outnumber Icestorms. If that's happening then the Icestorms are doing some grunt work to clear a path for the Firestorms, and if you can burn more cycles to further optimize that, well…

More Firestorm cores means more die area, which means the SOC gets significantly more expensive and/or supply won't keep up with demand.

Obviously it would be great to have 8 Firestorm cores in a desktop Mac—and this will probably be what we'll get in a future A1X—but as long as we're doing silicon fan fiction, why not ask for 32 Firestorm cores? That would be cool.

The M1 is cheaper than previous Intel i7 Mac Minis. So I don't think it would have mattered if the CPU was slightly more expensive. People would have been willing to pay that price. It was just less riskier, and cheaper, for Apple to use the same chip on both laptops and Desktop PCs even though the CPU trade-offs and consumer expectations of both are different.

It’s worth remembering that the M1, a chip which has exceeded all expectations of performance, is still just a low end CPU, not merely a “laptop” part. It’s purpose is to be efficient in respect to cost. And this includes the parts cost of higher wattage power delivery and the parts cost of more aggressive heat dissipation.

Future M1 derivatives and successors will offer higher performance at a higher price.

> the majority of Desktop PC users don't care about sacrificing performance to save a few watts

If you ask on those terms, agreed. But if quantified in terms of value delivered, it’s a different story. For example, if your computer can be always on with tasks like updates or if you can dedicate tasks to the slow cores there may be a value there.

> not for Desktop PC's

Intel don't agree. They are using a heterogeneous architecture for desktop class chips in upcoming Alder Lake processors [0]

[0] https://www.anandtech.com/show/16881/a-deep-dive-into-intels...

A small home server running 24/7 on low load mostly off of the small cores that can seamlessly ramp up to big cores for power when needed sounds exactly like the sort of thing you'd want.

Depending on the specific workload, if the “slow” cores are more efficient per watt it may be preferable to have only a small number of “fast” cores for single-threaded applications and many slow cores. Or possibly vice versa if that’s not true.

The M2 processor could be 4-big 64-LITTLE, or 16-big 8-LITTLE or some other combination.

Not unthinkable. It depends on how the load balancing is, especially if the little cores are far from helpless on their own.

Most of the time your PC isn't working hard, and it makes sense to use lower power cores to perform basic tasks.

I understand that perfectly. But how willing are you to sacrifice performance for that, is my point. There's a reason why CPU makers focus on building faster chips - that's what people care about most on Desktop PCs.

You don't need to sacrifice performance since CPUs step down automatically. For a hybrid design they only use the smaller cores for less demanding tasks. In fact this design gives a perceived increase in performamce as the author has written about.

Ofcourse you are sacrificing performance when you put slower cores alongside fast cores, instead of using all fast cores only. And for what - ARM cpu cores already are very efficient. So saving a few more watts with the slower cores, at the expense of performance (faster cores) isn't a worthwhile tradeoff when you are doing some cpu intensive task on your desktop. Saving a few extra watts matters on phones / tablets / laptops because you can use it longer on battery.

Saving watts means lowering fan RPM, meaning less noise. And that's a big priority for many.

Also, aren’t desktop CPUs constrained by thermal load at some point or can we use ever bigger coolers? Personally, I find it almost obscene that my desktop PC consumes roughly as much as a good old incandescent lightbulb (60+W) while idling. My laptop uses as much under full load.

Your PC uses 60w idling? Is that with screen? It's not too much in that case. CPUS and GPUS have gotten a lot better at idle power consumption, and PSUs are also quite efficient these days.

As discussed here [1], a fairly beefy Ryzen/RTX setup is often found idling around that number.

[1] https://hardforum.com/threads/killawatt-owners-whats-the-idl...

Downvolt and downclock your RAM when possible. Turn off VRM phases when not needed (also on the GPU; RAM should be handled properly already on it).

Allow package-C6. Turn on PCIe power saving stuff.

There is a bunch of stuff that mildly hurts performance and greatly improves efficiency in intermittent workloads, by letting hardware sleep/power-off when not needed.

Not only that, i'd prefer to have a small amount of ram and cpu to be running 24/7 for always-on features that I'd love to have my PC doing.

I don't like having to run a PI for some stuff just because i don't want my huge tower running all the time, it would be really neat if it could run at anything between 5 - 600W, not sure though if the PSUs would be able to offer that range.

I guess so - I really don't care about the fan noise (I'd rather PC makers think more about sound proofing for that rather than reducing fan RPM thus reducing performance).

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