ARM based servers hold the promise of extremely low power and excellent performance per Watt ratios. It's theoretically possible to place an incredible number of servers into a single rack; there are already implementations with as many as 1000 ARM servers in one rack (48 server nodes in a 2U chassis). What's more, all of those nodes consume less than 5KW combined (or around 5W per quad-core ARM node). But whenever a new technology is hyped, it's important to remain objective. The media loves to rave about new trends and people like reading about "some new thing"; however, at the end of the day the system administrator has to keep his IT services working and convince his boss to invest in new technologies.

At first sight, the relatively low performance per core of ARM CPUs seems like a bad match for servers. The dominant CPU in the server market is without doubt Intel's Xeon. The success of the Xeon family is largely rooted in its excellent single-threaded (or per core) performance at moderate power levels (70-95W). Combine this exceptional single-threaded performance with a decent core count and you get good performance in almost any kind of application. Economies of scale and the resulting price levels are also very important, but the server market has been more than willing to pay a little extra if the response times are lower and the energy bills moderate.

A data point proving that single-threaded performance is still important is the evolution of the T-series of Oracle (or Sun if you prefer). The Sun T3 had 16 cores with 128 threads; the T4 however had only 8 cores with 8 threads each, and CEO Larry Ellison touted more than once that single-threaded performance was massively improved, up to five times faster. Do we really need another server with a flock of slow but energy efficient cores? Has history not taught us that a few "bulls" is better than "a flock of chickens"?

History has also shown that the amount of memory per server is very important. Many HPC and virtualization applications are limited by the amount of RAM. The current Cortex-A9 generation of ARM CPUs has a 32-bit address bus and does not support more than 4GB.

And yet, the interest in ARM-based servers is growing, and there is more to it than just hype. Yes, ARM-based CPUs still lack the number crunching power and the massive amount of DIMM slots that Xeon's memory controller can handle, but ARM CPUs score extremely well when it comes to cost and power consumption.

ARM based CPU have also made giant steps forward when it comes to performance. To give you a few data points: a dual ARM Cortex-A9 at 1.2GHz (Samsung Exynos 1.2GHz) introduced in 2011 compresses more than 10 times faster than the typical ARM 11 based cores in 2008. The SunSpider performance increased by a factor 20 according to Anand's measurements on the iPhones (though part of that is almost certainly thanks to browser and software optimizations). The latest ARM Cortex-A15 is again quite a bit more powerful, offering about 50% higher performance. The A57 will add 64-bit support and is estimated to deliver 20 to 30% higher performance. In short, the single-threaded performance is increasing quickly, and the same is true for the amount of RAM that can be addresssed. The ARM Cortex-A9 is limited to 4GB but the Cortex-A15 should be able to address 16GB while the A57 will be able to address a lot more.

It is likely just a matter of time before ARM products can start to chip away at segments of the server market. How much time? The best way to find out is to look at the most mature ARM server shipping today: the Calxeda based Boston Viridis. Just what can this server handle today, where does it have the potential to succeed, and what are its shortcomings? Let's find out.

It's a Cluster, Not a Server


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  • thenewguy617 - Wednesday, March 13, 2013 - link

    I would like to see the results with the website running on bare metal. I would like to, but I don't believe you when you say the virtualization overhead is minimal.
    Also, did you include the power used by the switch? as we scale the xeon cluster we will add a lot of cost and power in the network, however Calxeda fabric should scale for free.
  • thebeastie - Thursday, March 14, 2013 - link

    I think a lot of you are missing the main point or future potential of this server technology. And that is that intel like to make an absolute minimum of $50 per CPU they make, in server CPUs it's more like $300.

    These Arm CPUs are being sold at around $10 a CPU.
    Sure Caldexa have gone the hard yards making such a server and want a lot of money for it. BUT once these ARM servers are priced in relative context of their actual CPu costs its going to be the biggest bomb drop on Intels sever profits in history.
  • Silma - Thursday, March 14, 2013 - link

    Assuming you are right and ARM is becoming so important that it can't be ignored, what's to prevent Intel to produce and sell ARM itself? In fact what's to prevent Intel to produce the best ARM socs as it has arguably the best fabs?
    There are rumors that Apple is asking Intel to produce procs for them, this would certainly be very interesting if it proves to be true.
  • thebeastie - Friday, March 15, 2013 - link

    The fact that Intel would practically look at other businesses then produce SoC/CPUs for $10 each, x86 or ARM based doesn't matter in the face of such high portability of code. Reply
  • Metaluna - Friday, March 15, 2013 - link

    The problem is that ARM cores are pretty much a commodity, so ARM SoC pricing is inevitably going to end up as a race to the bottom. This could make it difficult for Intel to sustain the kind of margins it needs to keep it's superior process R&D efforts going. Or at least, it would need to use its high-margin parts to subsidize R&D for the commodity stuff which could get tricky given the overall slowing of the market for the higher end processors. I think this is what's happening with the supposed Apple deal. There have been reports that they have excess capacity at 22nm right now so it makes sense to use it. And, since Apple only sells its processors as part of its phones and tablets, it doesn't directly compete with x86 on the open market.

    Of course, all the other fabs are operating under the same cost constraints, so there would be an overall slower pace of process improvements (which is happening anyway as we get closer to the absolute limits at <10nm).
  • wsw1982 - Wednesday, April 3, 2013 - link

    And so does those companies, run into bottom. What can they do to even their R&D, by put the server chip into mobile phone? Reply
  • Krysto - Monday, March 18, 2013 - link

    Yup. This is actually Intel's biggest threat by far. It's not the technical competition (even though Intel's Atom servers don't seem nearly as competitive as these upcoming ARM servers), but the biggest problem by far for them will be that they will have to compete with the dozen or so ARM server companies on price, while having more or less the same performance.

    THAT is what will kill Intel in the long term. Intel is not a company built to last on Atom-like profits (which will get even lower once the ARM servers flood the market). And they can forget about their juicy Core profits in a couple of years.
  • wsw1982 - Wednesday, April 3, 2013 - link

    So your argument is because the ARM solution is more expensive than Intel solution now, therefore it must be cheaper than Intel solution in the feature? The mobile ARM is cheap, so does the Intel mobile chips. Reply
  • Silma - Thursday, March 14, 2013 - link

    1300$ difference / server, that's a lot electricity you have to spare to justify the cost, especially as it is better that Xeon servers only in a few chosen benchmarks.

    Can't see how this is interesting in production environment.
    It's more for testing / experimenting I guess;
  • Wilco1 - Thursday, March 14, 2013 - link

    The savings are more than just electricity cost, you also save on cooling costs and can pack your server room more densely. If you do a TCO calculation over several years it might well turn out to be cheaper overall.

    This is the first ARM server solution, so it's partly to get the software working and test the market. However I was surprised how competitive it is already, especially when you realize they use a relatively slow 40nm Cortex-A9. The 2nd generation using 28nm A15 will be out in about 6 months, if they manage to double performance per core at similar cost and power then it will look even better.

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