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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jjj - Tuesday, December 16, 2014 - link
If you look at phones and tabs ,we might be getting some rather big custom cores in 2015 and 2016. Apple and Nvidia already have that, ofc much smaller than Intel's core when adjusting for process (actually that's an assumption when it comes to Denver since don't think we've seen any die shots).Intel at the same time in consumer is pushing for more non-CPU/GPU compute units and low power and they might face a tough question about core size and even process (if they target low clocks, low power , or the opposite).Got to wonder if at some point they'll have to go for a big core just for server.Would make things even more interesting.
Might not matter but Apple kinda has the perf for an ARM Macbook Air if they go quad. Not something worth doing for such low volume but doable when they go quad on all ipads or sooner if they launch a bigger ipad. Could be a trigger for others pushing more ARM based Chromebooks and beyond. That would set the stage for even bigger ARM cores.
Also got the feeling Nintendo will go ARM in 2016 and not many reasons for Sony and M$ not to go that way if they ever make a new gen- just another market for bigger ARM cores, any significant revenue helps with dev costs so it matters.
CajunArson - Tuesday, December 16, 2014 - link
1. The Core-m is widely derided as not being fast enough for the MacBook Air.2. The Core-m is easily twice as fast as the A8X in benchmarks that count... even Anandtech's own benchmarks show that. Furthermore, when you step away from web browsers and get to use the advanced features of the Core-m like AVX, that advantage jumps to about 8x faster in compute-heavy benchmarks like Linpack.
3. Even the mythical A9 coming in 2015 is expected to have roughly a 20% performance boost over the A8x.
4. Any real computer using an ARM chip would have to have a translation layer just like the old Rosetta to run the huge library of x86 software out there. Rosetta sort of worked because the Core 2 chips from Intel were *massively* faster than the PowerPC parts they replaced. Now you expect to run the translation overhead on an A9 chip that is slower -- by a large margin -- than the Core-m parts you've already derided as not being good enough?
Yeah, I'm not holding my breath.
fjdulles - Tuesday, December 16, 2014 - link
You may be right, but remember that ARM chips using the same power budget as Intel core i* will no doubt be clocked higher and perform that much better. Not sure if that will be competitive but it would be interesting to see.wallysb01 - Tuesday, December 16, 2014 - link
Only if you want a glorified tablet as a laptop. The software most people use in real work on laptops/desktops is not going to be ported over to ARM at an speed, even if ARMs could do that work reasonably well.Kevin G - Wednesday, December 17, 2014 - link
I'm under the impression that a good chunk has already been ported. MS Office for example is native ARM on Windows RT. Various Linux distributions have ARM ports completed with ARM based office and desktop software. The main thing missing are some big commercial applications like Photoshop etc.The server side of thing is similar with Linux and open software ports. MS is weirdly absent but I suspect that an ARM based version of Windows 2012/2014 is waiting of major hardware to be released. Much of the Windows base is already ported over to ARM due to Windows RT.
Kevin G - Wednesday, December 17, 2014 - link
Indeed. Performance of ARM platforms once power constraints have been removed is a very open question. So far all the core designs in products have been used in mobile where SoC power consumption is less than 5 W. What a 100 W product would look is an open and very interesting question.Ratman6161 - Wednesday, December 17, 2014 - link
If they "use the same power budget as an Intel core i*" then what would be the point?jjj - Tuesday, December 16, 2014 - link
Ok you are focusing on the wrong thing but lets do that anyway.I have never claimed that Apple's own SoC would beat Intel's current SoCs, just that the perf would be enough if they go quad and obviously higher clocks.
When you talk Core M you should remember that the price at launch was $281 so it's not good enough for anything.
Anyway how about you compare a possible Apple SoC with a MacBook Air from 2011, lets face it the Air is a crap machine anyway , not much perf and TN panel for w/e ridiculous price it costs now and it's users are certainly not doing any heavy lifting with it.
At the same time Apple's own 15- 20$ SoC would allow them a much cheaper machine and a presence in a price segment they never competed in, adding at least 5B of revenue per year (including cannibalization) and a share gain in PC of 2-3%.
But then again the point was that there are a bunch of trends that could favor bigger ARM cores.
Morawka - Wednesday, December 17, 2014 - link
it might cost them $20 for the A8X in fab cost, but the R&D for that chip is in the 10's of millions. Factor that in, to however many they ship, and it adds at least another $20 per chipjospoortvliet - Wednesday, December 17, 2014 - link
Even more obvious then that this would save them money by spreading out the fixed costs over more devices...