anon (anon.delete@this.anon.com) on August 6, 2014 7:54 pm wrote:
> juanrga (nospam.delete@this.juanrga.com) on August 6, 2014 11:55 am wrote:
> >
> > I liked the part when they appeal to "independent research" and then mention the same crap paper
> > that every Intel fanboy mentions. I still recall the first time that I did read the paper by
> > Blem, Menon, and Sankaralingam. After my initial perplexity on how almost any decision they took
> > (from hardware to compiler version) seemed orientated to favor x86 over ARM, I did search further
> > info about the senior author and found that one of the coworkers of his research team is an Intel
> > lab guy, that several students in his group are awarded Intel grants... LOL
>
> Yes, the answer is not a "strong" one at all.
>
> That paper was discussed here when it came out, and numerous issues were pointed out with
> it. Not to mention that it absolutely does *not* show that ISA does not make a difference.
> The most it really attempts to show is that when looking at devices ranging from A8 to i7,
> microarchitecture perf/power/cost target is the first order effect. Which it is. What it does
> not show is whether the ISA made, say, 10% difference when holding all else equal.

They compared older hardware. Migrating from SB-i7 to HW-i7 introduces little benefits in performance (except when using new AVX2 extensions to x86) but in ARM each gen is not a mere 5-10% faster than former gen but much more. Their choice favored x86.

They used only Intel designs. Using only AMD or a mixture of AMD and Intel would change both performance and efficiency. Their choice favored x86.

They computed the power consumption incorrectly. Their methodology choice favored x86.

They used an old compiler that didn't optimize the code for ARM all that would do. Their choice favored x86.

And so on.

> Then we have this
>
> "Intel's then-mobile chief Mike Bell clearly stated that the concept of an 'x86 tax' simply isn't true."
>
> Which is obviously something a marketing/executive person would say, but it's also completely false. The
> *concept* of an x86 tax is absolutely true. And not just the concept, but even in implementation, we can
> take a really simple example which is the instruction decoding complexity, and point to that.[*]
>
> There have also been engineers in the past acknowledge some inefficiencies and estimate
> "x86 tax" for then-PC class designs. Whether those are still valid with x64 and ever more
> complex CPUs is up for debate, but certainly the *concept* of an x86 tax is there.
>
> It's also not really disputed that at the very small scale, x86
> designs can't compete with simple ARM based microarchitectures.

Take a modern A57 core. According to AMD the A57 Opteron is faster than jaguar based Opteron but consumes less power. The ARM core performance is ~40% faster, and consumes roughly one half.

Jaguar is considered a good x86 design and even competitive against Intel last designs. Thus we are seeing the x86 tax in action.

> I have also heard from many people (it's possible this is just an uninformed 'echo chamber effect',
> but I think there is some merit to the idea) that x86 cores take significantly more design skill
> than an equivalent ARM core. Whether this is due to compatibility, or decoders, or necessity of
> more capable memory pipline and caches, I don't know, but it seems to also be an x86 tax.

E.g. a x86 decoder is more difficult to implement than an ARM64 decoder, because the former has to match instructions of variable length. Also the x86 ISA is full of legacy instructions, which have to be implemented in hardware and then verified/tested which increases development costs and time of development.

According to Feldman an entirely custom server chip using the ARM architecture takes about 18 months and about $30 million. By contrast, it takes three or four-year time frame and $300--400 million in development costs required to build an x86-based server chip based on a new micro-architecture.