iMac Pro Apple custom Xeons in iMac Pro

[mpe]

Is there any definitive answer about what is actually different about these tailored Xeon chips in iMac Pros and why they are underclocked compared to stock Xeons?

I remember during initial press coverage during iMac Pro launch in December, I heard someone quoting Apple that these custom-made Xeon chips are optimised for AVX-512 performance so that they can perform AVX-512 optimised code without applying negative AVX offset. Unlike the standard chips that are optimised for general instruction performance, but have to throttle down when too dense AVX-512 code is executed.

However, I can't source this information anymore. I was wondering if someone has more information about this.

 

[SecuritySteve]

I keep reading that these Xeons are under clocked ... but thats a bit of a misconception.

If they made a separate model for Apple, then it is a separate model and shouldn't be considered under clocked. The 8 core version seems to have the most complaints, but from what I can tell that is because it was special produced to be the cheap option. The 10 core seems to have a sacrificed base frequency, but again that could be for the same sourcing reasons.

From intel ark vs Apple:
8 Core: 3.7/4.5 GHz --- 3.2/4.2
10 Core: 3.3/4.5 GHz --- 3.0/4.5
14 Core: 2.5/4.3 GHz --- 2.5/4.3 GHz (IDENTICAL)
18 Core: 2.3/4.3 GHz --- 2.3/4.3 GHz (IDENTICAL)

You get what you pay for. I'm not suggesting every workflow should favor a higher core count, as there are some applications that definitely favor single core performance. But in reality, the single core performance of all of these chips are going to be about the same at 4.2-4.5 GHz.

TL;DR - The base chips are custom made to be cheaper and more affordable.

 

[AppliedVisual]

And if Apple did offer the 3.3GHz base clocked 10-core, then the 14-core option would be pointless as one would lose far too much with the 800MHz difference in base clock to gain 4 cores. As it is, I opted for the 14-core over the 10-core, but waffled for a few days before placing the order. While I think I probably made the right choice given most of my apps this system will be used for, I would have been just fine with the 10-core and would have had the computer a few weeks ago. And I think even with the slower 3GHz base of the 10-core option, the 10 and 14 core models are going to be very similar in overall performance. I think one will edge out the other at opposing ends of the core use spectrum and nominally using 8 to 16 threads, the systems will be nearly indistinguishable.

 

[CWallace]

Is there any definitive answer about what is actually different about these tailored Xeon chips in iMac Pros and why they are underclocked compared to stock Xeons?

Marco Armett spoke with an Apple PR person and they noted that the W-2140B and W-2150B have two AVX-512 units and implied the W-2145 and W-2155 only had one, but according to Intel those CPUs both have two units, as well.

Apple also confirmed that the CPUs were not down-clocked due to thermal limitations of the cooling system. With reports that the iMac Pro's fans never really spin up no matter how hard you work it, it could very well be that the decision was made to down-clock the CPU and GPU to allow the cooling system to operate at "quiet" levels regardless of load.

 

[FredT2]

Is there any definitive answer about what is actually different about these tailored Xeon chips in iMac Pros and why they are underclocked compared to stock Xeons?

As to why, I like the theory that it was to get better price separation, particularly between the 8 and 10-core processors. Intel's list price for the 10-core is only $325 more than the 8-core. The difference between 10 and 14 is $500, and between 14 and 18 it is $600. I think Apple needed a cheaper part at the bottom end to be able to charge $800 for each step up. Either that or the 8-core is a super bargain. Whatever the reason, the 8 and 10-core seem to perform very well.

 

[mpe]

Would be good to see how the Apple CPUs compare with stock parts in AVX intensive workloads. Intel advertised TDP, base and turbo clock doesn't apply to AVX. There is a different set of clock/turbo mutlipliers for AVX which is currently unknown.