ARM Challenging Intel in the Server Market: An Overview
by Johan De Gelas on December 16, 2014 10:00 AM ESTRISC vs. CISC Revisited
The RISC vs. CISC discussion is never ending. It started as soon as the first RISC CPUs entered the market in the mid eighties. Just six years ago, Anand reported that AMD's CTO, Fred Weber was claiming:
Fred said that the overhead of maintaining x86 compatibility was negligible, at the time around 10% of the die was the x86 decoder and that percentage would only shrink over time.
Just like Intel today, AMD claimed that the overhead of the complex x86 ISA was dwindling fast as the transistor budget grew exponentially with Moore's law. But the thing to remember is that high ranking managers will always make statements that fit their current strategy and vision. Most of the time there is some truth in it, but the subtleties and nuances of the story are the first victims in press releases and statements.
Now in 2014, it is good to put an end to all this discussion: the ISA is not a game changer, but it matters! AMD is now in a very good position to judge as it will develop x86 and ARM CPUs by the same team, lead by the same CPU architecture veteran. We listened carefully to what Jim Keller, the head of the AMD CPU architect team, had to say in the 4th minute of this YouTube video:
"The big fundamental thing is that ARMv8 ISA has more registers (32), a three operand ISA, and spends less transistors on decoding and dealing with the complexities of x86. That allows us to spend more transistors on performance... ARM gives us some inherent architectural efficiency."
You can debate until you drop, but there is no denying that the x86 ISA requires more pipeline stages and thus transistors to decode than any decent RISC ISA. As x86 instructions are variable length, fetching instructions is less efficient and requires more transistors. The instruction cache is also larger as you need to store pre-decode information. The back-end might deal with RISC-like micro-ops but as the end result must adhere to rules of the x86 ISA, thus transistors are spent on exception handling and condition codes.
It's true that the percentage of transistors spent on decoding has dwindled over the years. But the number of cores has increased significantly. As a result, the x86 tax is not imaginary.
Hardware Accelerators
While we feel that the ARMv8 ISA is definitely a competitive advantage for the ARM server SoCs, the hardware accelerators are a big mystery: we have no idea how large the performance or power advantage is in real software. It might be spectacular or it might be just another "offload works only in the rare case where all these conditions are met". Nevertheless, it is interesting to see how the ARM server SoC has many different integrated accelerators.
Most of them are the usual IPSec, TCP offloading engines, and Cryptographic accelerators. It will be interesting to see if the ARM ecosystem can offer more specialized devices that can really outperform the typical Intel offerings.
One IP block that got my attention was the the Regex accelerators of Cavium. Regular expression accelerators are specialized in pattern recognition and can be very useful for search engines, network security, and data analytics. That seems exactly what we need in the current killer apps. But the devil is in the details: it will need software support, and preferably on a wide scale.
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esterhasz - Thursday, December 18, 2014 - link
But this is exactly why a wider array of machines based on their chips would make sense: the R&D cost is already spent anyways, since iPhone and iPad need chips, selling more units thus reduces R&D cost per unit. Economies of scale.I don't believe a MBA variant with ARM is down the road either, but the rumored iPad Pro could develop into something similar rather quickly.
OreoCookie - Tuesday, December 16, 2014 - link
If you want to talk about ARM on the desktop, that's a whole other discussion, but one that most certainly needs to include price: if the price difference between a Broadwell-based Core M and a fictitious Apple A9X is $200~$230, then this changes the discussion completely. Two other factors are graphics performance (the Core M has »only« 1.3 billion transistors, the A8X ~2 billion, indicating that the mythical A9X may have faster graphics) and the fact that Apple controls the release schedule and can spec the SoC to meet its projected needs. To view this topic solely through the lens of CPU performance is myopic.darkich - Friday, December 19, 2014 - link
Your comparisons missed the picture spectacularly.A8X is a 20nm 2-4W TDP chip with a price that is probably around 70$.
Top of the line Core M5Y70 is a 14nm 4.5 W TDP chip with a price of 270$.
And it has a weaker GPU, btw. (raw performance). And it throttles massively, effectively giving only 50% of the benchmark performance.
If you're going to compare that to an Apple chip, compare it to a 14nm A9X with custom derived PowerVR series 7 GPU,(scales up to 1,4 TFLOPS) vastly expanded memory controllers connected to a much faster RAM (compared to one in the iPad) upclocked to 2GHz, that are available at any time.
darkich - Friday, December 19, 2014 - link
.. *with cores upclocked to about 2GHzFlunk - Tuesday, December 16, 2014 - link
Nintendo already sells ARM systems, the 3DS and the DS before it are both ARM-based. The PSVita is ARM too. I don't see an ARM Macbook Air anytime soon, they need a bigger and higher-clocking chip for that and it doesn't look like that's going to happen anytime soon.Nintendo Maniac 64 - Tuesday, December 16, 2014 - link
Even the Game Boy Advance used an ARM7 for its main CPU.jjj - Tuesday, December 16, 2014 - link
Obviously there are handhelds using ARM but the point was about bigger cores and clearly not handhelds.DLoweinc - Tuesday, December 16, 2014 - link
Don't quote Wikipedia, not suitable for this level of writing.garbagedisposal - Tuesday, December 16, 2014 - link
Says DLoweinc, master of knowledge and scholarly writing.In contrast to your childish and outdated opinion, Wikipedia is a perfectly valid source of information, go read about it and quit crying.
Daniel Egger - Tuesday, December 16, 2014 - link
The problem really is the custom solutions can simply not compete with Intel on any level for general purpose computing (which the majority of applications are), not on performace/price, performance/power and not even on features/price.For instance I can see a huge market for sub-Xeon (or Atom C) performance at a corresponding price -> not going to happen because everyone is targeting > Xeon performance at ridiculous prices because they're expecting the margin to be there however there're simply to many compromises to be made by the buyers so that has to fail.
Also I can see a huge demand for Atom C - Xeon performance at lower power consumption however no one seems to be really targetting this, all we get are Raspberry Pi's and a bit beefier but close from even Atom C. The new virtualisation techniques (Docker et al) opened a whole new can of possibilities for non-x86(_64) devices because virtualisation is suddenly possible and much more lightweight than ever before but no one seems to want to jump this opportunity.
I'd really like to buy some affordable general purpose (BYOM/BYOS) hardware which has a little bit of oomph and takes little power which should be the powerful sides of any of the contenders but somehow all fail to deliver and I don't even see an attempt to change that.
If I want mind-boggling performance at decent performance/price ratio with real virtualisation and 100% standard software compatibility there's no way around the high end Xeons (and maybe AMD iff they manage to get their asses back up) and none of the contenders is ever going to challenge that so they might as well stop trying.