By: David Kanter (dkanter.delete@this.realworldtech.com), August 8, 2014 11:36 pm
Room: Moderated Discussions
> > 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.
No you aren't. Jaguar is a good core design, but the uncore was inappropriate for servers. You are comparing a design with a server-specific uncore vs. one with a client-optimized uncore. It's no surprise that the Jaguar-based design is behind.
> > 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.
ARM is full of legacy crap as well. Not to mention the fact that an ARMv8 requires 3-4 different decoders. I know a few people who have had the pleasure of designing custom ARM cores, and according to them 'ARMv8 decode is just as terrible as x86'.
There really isn't a significant x86 tax. Perhaps 5% for a reasonable core (obviously things are much worse for scalar cores).
> 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.
Those numbers are suspect and also probably not comparing the right things. Much of the cost of a server design is in the cache, coherent interconnects, memory controller, power management, etc. which is necessary for any design, ARM or x86.
David
> > *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.
No you aren't. Jaguar is a good core design, but the uncore was inappropriate for servers. You are comparing a design with a server-specific uncore vs. one with a client-optimized uncore. It's no surprise that the Jaguar-based design is behind.
> > 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.
ARM is full of legacy crap as well. Not to mention the fact that an ARMv8 requires 3-4 different decoders. I know a few people who have had the pleasure of designing custom ARM cores, and according to them 'ARMv8 decode is just as terrible as x86'.
There really isn't a significant x86 tax. Perhaps 5% for a reasonable core (obviously things are much worse for scalar cores).
> 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.
Those numbers are suspect and also probably not comparing the right things. Much of the cost of a server design is in the cache, coherent interconnects, memory controller, power management, etc. which is necessary for any design, ARM or x86.
David
