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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  • JohanAnandtech - Wednesday, March 13, 2013 - link

    Hmmm ... There is almost no info on how that hypervisor works. It is hard to imagine that kind of system would scale very well. How does it keep Cache coherent? Do you have info on that?
  • timbuktu - Wednesday, March 13, 2013 - link

    I can't speak directly to ScaleMP, but it looks similar to NUMALink.

    Reading through this article about Calxedas, great job BTW, I couldn't help but think about the old SGI hardware that seemed pretty similar with MIPs (and later Itanium) processors connected through a switch with NUMALink. I haven't played with NUMALink directly in almost a decade, but back then cheaper Altix slabs were ring topology while higher end hardware was switched. In the end though, you could put together a bunch of 1U racks together and have a single system image. Like you mentioned though, cache coherency was exceptionally important. Since we have a uv here, I can point you to the documentation for that box.

    Everything old is new again, I suppose. Well, except NUMAlink never went away. =D
  • Tunrip - Wednesday, March 13, 2013 - link

    I'd be interested in knowing how the Xeon compared if you did the same test without the virtual machines.
  • JohanAnandtech - Wednesday, March 13, 2013 - link

    The website won't scale to 32 logical cores I am afraid... but we can try to see how far we can get
  • Colin1497 - Wednesday, March 13, 2013 - link

    A better question might be "is 24 VM's a logical number to use?" Would more or fewer VM's work better? The appearance is that you have 24VM's because you have 24 ARM nodes?
  • duploxxx - Wednesday, March 13, 2013 - link

    very interesting, loved reading it. But although early in the ball game I do think there are other way better solutions in the pipe-line from the big OEM:

    HP Moonshot
  • JohanAnandtech - Wednesday, March 13, 2013 - link

    Isn't remarkable how PR people manage to fill so many pages with "extreme" and "the future" without telling anything. Frustation became even higher when I clicked "get the facts" page. That is more like "You are not getting any facts at all".
  • DuckieHo - Wednesday, March 13, 2013 - link

    Since these are set up as webservers, what's the power consumption at say 20-40% load? Usually there is some load instead of completely idle.
  • JohanAnandtech - Wednesday, March 13, 2013 - link

    Good suggestion... you'll like to see a step by step power measurement like SpecPower right? Let me try that.
  • DanNeely - Wednesday, March 13, 2013 - link

    I'd be interested in seeing where, and what happens when you start pushing single chips to and slightly beyond their limits. Calxeda's hardware's proved competitive on a very friendly workload (which I didn't really expect would happen until their A15 product); but in the real world a set of small websites are unlikely to all have equal load levels. Virtual servers on larger CPUs should give more headroom for load spikes; so knowing what the limits on Calxeda's hardware are strikes me as fairly important.

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