Theoretical question: Uneven number of CPU cores

[CakeMonster]

Is there a reason why there have not been released more CPUs with an uneven number of cores? I know AMD had 3 cores for a while, is there any hardware or software downside to an uneven number?

I'm thinking a theoretical scenario where Intel feels the heat from the competition (yeah, very theoretical) and might want to bump up the mainstream from 4 to 5, and the high end from 6/8 to 8/10. That's a slight increase that is still not rendering the high end as unnecessary or uninteresting.

Could something like this happen, and why or why not?

 

[NTMBK]

Apple uses three CPU cores in its iPad processor.

 

[nenforcer]

Well I don't know if that matters but I believe in the past Intel was using what were MCM (Multi-Chip Modules) on for instance Kentsfield which was essentially two Core 2 Duo dies stuck together.

Then AMD came around and had the first native quad core design with the first Phenom.

I think for AMD it was easier to disable just one of the (non-functioning) cores and call it a 3 core CPU whereas Intel would have had a more difficult time getting 1 /2 of a MCM modules to be functional 3rd CPU. Unlocking extra cores on AMD Phenom ][ chips was also possible giving you either a 2 core, 3 core or sometimes even the full 4 native core chip. They reason they did was to increase yields of otherwise "defective" quad core chips.

Nowadays, however, there is no reason or limitation to why you can't do it this way - in fact I believe the Sandy Bridge-E / Ivy Bridge-E chips all have 6 cores but on the i7 3820 and i7 4820 they have disabled two of them. Some people get better performance on the AMD FX chips by disabling one cpu on each module.

There is not technical reason you couldn't do it but most things come in pairs which is why we have 2, 4, 6 and now 8 core desktop processors. I know in China people are superstitious about the number 8 and would probably prefer that over a 5 or 7 core.

 

[lyssword]

maybe it has something to do with cpu die geometry? like easier to even # of cores into certain square mm?

 

[Thala]

There is not technical reason you couldn't do it but most things come in pairs which is why we have 2, 4, 6 and now 8 core desktop processors

That applies mostly to x86/x64 uarch. On ARM say Cortex A5 MC you can have anything between 1 and 4 cores per cluster.

 

[ShintaiDK]

There is anything hindering it, but its very unlikely to happen.

In case of Intel they just add 2 more and not 1. AMD only did tricores because of yield. And we havent seen it since.

 

[CakeMonster]

There is anything hindering it, but its very unlikely to happen.

In case of Intel they just add 2 more and not 1. AMD only did tricores because of yield. And we havent seen it since.

The 5 core was just wishful thinking on my part.. But it would make a little bit of sense in order not to cannibalize the high end E platform also.

 

[ShintaiDK]

5 isnt changing anything in that regard.

If you wanted more performance, they should simply double the cache to 16MB or add eDRAM. Because its quite clear how much the memory side is bottlenecking the quads.

 

[BigDaveX]

AMD only did tricores because of yield. And we havent seen it since.

We sort of have, with the 3-module Bulldozer and Piledriver CPUs; it's just that they give six threads because of their design. Likewise, any Intel design with an odd number of cores (unless they did a triple-core i5) would also give an even number of threads.

As for why Intel have never done odd numbers of cores, I suspect they keep even-numbered core counts just to simplify their product line. Haswell-EP already goes up to 18 cores, so adding in versions with odd numbers of cores on top of all the different speed bins would just get crazy.

 

[Idontcare]

maybe it has something to do with cpu die geometry? like easier to even # of cores into certain square mm?

As silly as it may seem to some readers, it really is as simple as this.

Software-wise, it matters not whether you are multithreading 2N threads (even) or 2N+1 threads (odd) or 2N-1 threads (also odd).

But why do processors tend to have an even number of cores comes down to geometry, physical geometry of the die.

 

[jpiniero]

As for why Intel have never done odd numbers of cores, I suspect they keep even-numbered core counts just to simplify their product line.

I had suggested moving the desktop i3 to triple core. But then the question becomes - would triple core w/o HT really be better than dual with HT?

 

[Tuna-Fish]

I had suggested moving the desktop i3 to triple core. But then the question becomes - would triple core w/o HT really be better than dual with HT?

But why would they do a triple core without HT as a major model?

Removing HT saves them nothing, and allows practically no salvage of non-functioning chips. Having models without it is entirely about managing their product line to have a smooth progression from less expensive, slower models to more expensive, faster ones. Since models with HT provide better value per mfgt cost, they absolutely want their highest-volume models to be ones with HT enabled.

 

[TeknoBug]

I still have a Dell laptop laying around that has an AMD Turion X3, also built my mother a PC with an Athlon II X3 450 (which unlocked fourth core and renamed itself to Phenom II X4 B55 I think). lol

It is possible but not in Intel's characterstics, next up from 4 cores will end up being 6 cores if Intel ever makes them consumer level. I'm a fan of the i3 line, great performance for power and a triple core with HT would be a fun treat.

 

[jpiniero]

But why would they do a triple core without HT as a major model?

Because the desktop i3 would become cut quad parts instead of full dual core. The purpose is to get rid of the socket dual core die since there is no equivalent mobile version anymore. The desktop Pentium and Celeron would become leaky U parts and/or straight to Atom instead. They could cut the quad die down to 2 and keep HT like they are with the Skylake i3 H models of course.

 

[biostud]

I would suspect that the optimal CPU design involves two rows of cores, and by going uneven, you would always end up with some wasted space.

 

[NTMBK]

I would suspect that the optimal CPU design involves two rows of cores, and by going uneven, you would always end up with some wasted space.

Nah, Intel's desktop chips have had all the CPU cores in one line for quite a while now. Adding an extra CPU on the end (or having one fewer) isn't too great a stretch.

 

[Cogman]

As silly as it may seem to some readers, it really is as simple as this.

Software-wise, it matters not whether you are multithreading 2N threads (even) or 2N+1 threads (odd) or 2N-1 threads (also odd).

But why do processors tend to have an even number of cores comes down to geometry, physical geometry of the die.

This was my understanding. It is harder to lay out a 7 core processor without wasted die space than it is to lay out an 8 core processor.

You can get away with 3 and 6 core processors because it easier to have 2 side by side and 1 on top. However once you start talking about 5, 7, and above it becomes much harder to lay things out in a manner that doesn't waste a lot of space.

 

[crashtech]

New methods of dicing will allow chips to have geometries other than the rectangular shapes that dictate design layouts today. So an odd-core configuration may become more likely in the future, maybe even in a pentagonal shape, who knows?

http://www.npr.org/2015/08/14/43228...ver-15th-convex-pentagon-able-to-tile-a-plane

 

[SlowSpyder]

I don't think there would be any reason for Intel to release a three HT'd core "i6" as anyone looking above an i5 today is looking at an i7 already. I imagine all that would do is get Intel less money in the end. Sure, they may have to discard more faulty silicon, but I think they have the luxury of tweaking their process until they get good yields, as competition is today.

 

[BigDaveX]

Was there ever any truth in the rumour that AMD were deliberately having to sell fully working quad-core Phenom IIs as dual or triple-core chips just to meet demand? If AMD were able to get their yields up to that level, I could certainly buy that Intel don't produce enough defective dies (or I should say, defective to just the right degree) for it to be worth their while making triple-cores.

 

[ShintaiDK]

Was there ever any truth in the rumour that AMD were deliberately having to sell fully working quad-core Phenom IIs as dual or triple-core chips just to meet demand? If AMD were able to get their yields up to that level, I could certainly buy that Intel don't produce enough defective dies (or I should say, defective to just the right degree) for it to be worth their while making triple-cores.

Yep. It was a double edged sword. When tricore demand was low and faulty chips high it was all fun and party. When those two things shifted it just became a burden.

 

[ShintaiDK]

I don't think there would be any reason for Intel to release a three HT'd core "i6" as anyone looking above an i5 today is looking at an i7 already. I imagine all that would do is get Intel less money in the end. Sure, they may have to discard more faulty silicon, but I think they have the luxury of tweaking their process until they get good yields, as competition is today.

When Intel starts mass production yield is usually in the 90% area. While TSMC, Samsung etc starts very low. GloFo for example was out of the A9(they was to produce 30% of them) and allocated to TSMC instead because they couldnt break 50%. (They was around 20-30% yield).

 

[SlowSpyder]

When Intel starts mass production yield is usually in the 90% area. While TSMC, Samsung etc starts very low. GloFo for example was out of the A9(they was to produce 30% of them) and allocated to TSMC instead because they couldnt break 50%. (They was around 20-30% yield).

Then I would imagine they'd only lose money by selling a lower priced three core six HT thread CPU at a lower cost than an i7.

 

[ShintaiDK]

The actual cores are also taking up less and less space today. 4+2 Skylake is what, 35mm2 cores out of 120mm2 or so.

 

[Shivansps]

The mayor problem for them is having to design and keep in production another cpu... for what? The cpu core+cache are in line, its not hard for them to make a single core/tri-core, etc... But just dont worth it.

And they cant do a thing about it whiout having to re-do the whole lineup.... 3-Core Celeron a Pentium? sounds cool but that means I3 must be 3 core + HT, and I5 4 core+HT and I7 6 cores +ht, that whould drasticaly increase costs for them for every market, especially mobile where BOM cost will just shoot up because of higher tdp, for what? there is just no motivation because AMD is a zombie.