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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  • Kurge - Wednesday, March 13, 2013 - link

    Yeah, should have had two teams - each with goal to optimize on each platform. The Xeon team would not (lol) load up 24 VM's to serve the same web app. It's silly. Go bare metal in that use case.

    There will be different needs for different cases. The "lets load up a bunch of VMs" is useful to cloud providers and in other cases, but not for "I want to feed this app to as many users as possible".
  • dig23 - Tuesday, March 12, 2013 - link

    Interesting article and great first effort but felt bit outdated on both ATOM as well as ARM front, I am not blaming you, just saying.
  • JarredWalton - Tuesday, March 12, 2013 - link

    Outdated in what sense? No one else has really made a serious attempt to review thee Calxedas stuff, and while there are better Atom option out there, as Johan notes we were unable to get any in-house in time for testing. Or do you mean Calxedas' use of Cortex-A9 is outdated? If so, that's more of a case of laying the groundwork I think. Assuming they have their A15 option be backwards compatible with the current system (e.g. just get a new set of cards with the updated SoCs), that would be very cool.
  • JohanAnandtech - Wednesday, March 13, 2013 - link

    I can only agree with Jarred. There are no A15 server chips AFAIK, and unless I have missed a launch, I think the Atom N2800 is not outdated at all (Dec 2011).
  • aryonoco - Wednesday, March 13, 2013 - link

    This was a fabulous and most informative write up. You answered so many of my questions with this article. Excellent job covering an area that no one else is, and also kudos for running such great benchmarks.

    This really is tech journalism at its best. Thank you Johan, and thank you Anand for employing such high-quality writers.

    We all know how memory constrained the ARM A9 is. Even something like Krait would solve a lot of A9's traditional weak areas. And yet, it looks like the Calxeda makes sense in enough niches to be sustain their R&D and development efforts. Low-to-medium traffic web hosting, media streaming and storage. Each one of those areas is a sizeable market and the Calxeda solution offers enough to be seriously considered in these makets.

    And when one thinks about how many years of x86 optimisation has gone into the toolchain in things like the gcc, one realises the potential that lies ahead for ARM in this market. ARM's future roadmap is well known, next is Cortex A15 and then Cortex A57. Meanwhile there will be more software optimisation, and the management/deployment side will also improve. With all these in mind, I think it's more than conceivable that ARM will grab up to 20% marketshare in the server market by 2015.
  • JohanAnandtech - Wednesday, March 13, 2013 - link

    Thanks! Good summary... and indeed 20% marketshare is not impossible. The real questions is whether Intel give the Atom it is long overdue architecture update, or will Haswell put some pressure from above? Exciting times.
  • beginner99 - Wednesday, March 13, 2013 - link

    Isn't it much easier to administer 24 virtual servers than 24 physical ones (cost of personnel)? When all servers have the same workload it look sgood for ARM but the virtualized intel environment easily wins if some servers get a lot more requests than others, meaning too much for one ARM SOC to handle. The tested scenario is basically the best one could ever hope for the ARM server and pretty unrealistic (same load for all servers). That's fine but then also post worst-case scenarios...Intel server is a lot more flexible.
  • hardwaremister - Wednesday, March 13, 2013 - link

    I completely agree with the other readers that this writing is just absolutely superb. Fantastic novel job Johan.
    However, I also agree with the above commenter: a big part coup on virtualizing a "fat" core system is to be able to properly utilize the resources of the machine across VMs. By equally loading "tiny tiles", the obvious advantage of the inherent load balancing of a virtualized infrastructure completely disappears.
    Under current the current "fat" VM infrastructure you can accomodate individual VMs with heterogeneous loading levels, with extra provisioning in the resource pool.
    That is just not simply the case for these tests based on an army of individual machines against a many VMs virtualized under a few "fat" cpus.
    I don't mean to be overcritical, but this is a proper apples vs oranges comparison.
  • bobbozzo - Wednesday, March 13, 2013 - link

    A lot of shared hosting ISP's use lightweight virtualization with Linux or BSD "Containers". I would like to see you re-benchmark with those on both servers instead of using VMs.
    You should see higher performance vs full virtualization. I'm not sure how it would affect the ARM performance, but it shouldn't hurt much, and there is more potential for better load sharing if some sites are busier than others.
  • Jambe - Wednesday, March 13, 2013 - link

    Surprising, indeed! Thoroughgoing as usual, and excellently written.

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