Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on May 15, 2013 5:37 pm wrote:
> Ashraf Eassa (aeassa.delete@this.gmail.com) on May 15, 2013 11:59 am wrote:
> > Hi everybody,
> >
> > I've been lurking for years, but the time has come when I would really love to pick the brains of
> > the experts we have here. From my understanding, Atom is a much narrower design than Krait, Cortex
> > A15 and others, and yet, in many benchmarks the older Saltwell core holds its own against even Krait
> > in both FPU/INT, and against A15 in Linux integer benchmarks (but it gets decimated in FPU).
>
> Which Linux benchmarks do you mean? This does not show a single benchmark where dual Atom can keep up
> with dual A15. Even Tegra 3 wins 9 out of 11 benchmarks despite its slow single-channel memory system.

You realize there are an awful lot more tests out there that Phoronix doesn't run, right?

Also, why do we even care about Linux? We care about Android, which is rather distinct from Linux.

> > So, my question is, how do I think about "Silvermont" competitive position against a fairly
> > beefy modern ARM design such as the Cortex A15? From a high level perspective, it looks
> > like on a per-clock basis it should be no contest - A15 is wider and more aggressive.
> > But Intel is claiming that Silvermont is as fast as A15 on a per-clock basis.
>
> "Intel is claiming" - there is your hint... When Atom originally was announced, it was supposed
> to be 5-6 times faster than ARM cores. However when Atom was finally available in phones, it
> actually lagged in performance. This is where Atom is today. Is that competitive?

When did Intel claim that Atom would be 5-6x faster than ARM cores? And which ARM cores? I'd like to see some proof, because that just sounds crazy.

> > A couple of questions then:
> >
> > 1. How can a narrower design pull this off?
>
> It doesn't. Not without trickery anyway - like comparing a highly clocked CPU against a low
> clocked one,

That's not trickery, that's life. Intel has better process technology and is able to hit higher clock speeds. Moreover, there are many A15 implementations that are incredibly power hungry. This shouldn't surprise anyone, since the A15 started out as a server core...but then something happened and ARM tried to shove it into mobiles.

Clock-normalized comparisons are useful as thinking points, but you really need to consider physical design and process technology. Power and frequency are intrinsically tied to physical design and process, as is area. Certainly there are architectural techniques that can have a big impact (I think the A7 omitting a branch predictor is particularly brilliant in that regard), but process has a bigger influence.

>comparing an unreleased CPU against a much older CPU, using different compiler
> versions or optimizing for specific benchmarks (SunSpider). It's called "benchmarketing"...

> About the only area where Silvermont appears to have an advantage over A15 is a lower L2 latency. Everything
> else is like you said, smaller buffers, narrower, simpler and less aggressive. Given the memory system advantage
> I'd expect it to beat A9 by a good margin (although A9R4 might well be competitive). However based on what
> we know you'd have to be extremely optimistic to believe it can get even close to A15 performance.

I claim BS already. If A15 is so good, why do partial register stalls cause a massive drop in performance for Neon? Oh right, maybe it's because someone made a stupid architectural decision they fixed in the A57.

David