Broadwell includes LLC, just for comparision

By: Simon Farnsworth (simon.delete@this.farnz.org.uk), November 2, 2016 2:48 am
Room: Moderated Discussions
anon (spam.delete.delete@this.this.spam.com) on November 1, 2016 12:56 pm wrote:
> You are all just missing each others points.
> juanrga you are arguing that "overclocking" an existing design won't reduce IPC much, which is true,
> while everyone else is arguing that you'd need to make changes to do that which would reduce IPC.
>
I'd actually been aiming for a slightly more nuanced argument; rephrased, using your "tau" notation for the inherent process delays:

It's not trivial to design a high-frequency, high-IPC CPU. This isn't something you do unintentionally while aiming for a lower frequency target - it's something you have to plan for and have as an explicit goal.

Apple's chip design team is good - possibly one of the best in the industry. If there's a way for them to get higher performance/watt, they'll do it. Thus, they are likely to have critical paths that get a lot done per clock cycle, knowing that tau is high enough to let the critical path complete switching in one clock cycle.

If Apple had been aiming for high clock speeds, these critical paths would be redesigned to cope with the value of tau you'd expect at those higher clock speeds; this, in turn, leaves performance on the table at the clock speeds they do run at, because things that run in one clock cycle in the current designs will take 2 or more clock cycles in the hypothetical high clock speed design.

Thus, if you're asserting that Apple's design is comparable to Intel's, and just needs higher clock speeds, you're asserting one of two things:

1. I'm wrong when I suggest that Apple's team is one of the best in the industry - they're actually egomaniacs who happen to have designed something saleable while trying to massage their own egos.

2. We don't have good factual data for whether Apple really can match Intel's IPC at Intel's clock speeds - there's still too much left unknown when you're talking about over doubling the clock speed of the chip.

I happen to believe that the correct explanation is that Apple's chip design team is very, very good, and that they've focused on getting the highest performance they can in the iPhone/iPad constraint set. I would not be arguing if the claim was that, at 2 GHz, Intel won't get the same performance as Apple, because Intel's demonstrated expertise is in high clock speed cores, and they make up for their performance at lower clock speeds with Turbo Boost to allow them to reach a TDP target, while still providing the interactivity people want.
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            Broadwell includes LLC, just for comparisionanon2016/10/22 08:52 AM
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