Why no ARM-based PowerEdge server?

ARM processors are everywhere these days from cellphones to laptops to datacenters. What used to be known as the brain of a very inexpensive single board computer, the Raspberry Pi, targeted at the hobbyist maker market is now fairly ubiquitous:

  • most cellphones, be it iPhones or Androids, use ARM processors,
  • most set-top boxes use ARM processors,
  • most home network and wifi routers use ARM processors,
  • and the list goes on.

This begs the question: why are ARM processors so prevalent nowadays? When I look at where we used to find ARM processors, there is a strong commonality in all those applications: low power. Traditionally, ARM processors have been leveraged in application where good performance with low power is paramount. Cellphones are a great example of such application, as the battery life of a cellphone is directly linked to the amount of power drawn by the CPU, whilst still offering good enough performance to make the user experience enjoyable. This is a fine balance and, for those applications, ARM has dominated the market for a while.

In 2020, another major computer manufacturer, Apple jumped on the ARM bandwagon by releasing its Apple M1 processor. Even though Apple has never been in the business of designing CPUs, it felt that the ever-increasing TDP of x86 CPUs couldn’t offer its customer the battery life and performance they required, so they turned to ARM to offer it with great success and now the general public were exposed to the benefits of the ARM architecture.

In 2018 though, ARM processors moved from consumer type application into the datacenter, with the release of the Graviton CPU by AWS. While Intel and AMD have dominated the datacenter space over the last 20 years, datacenters have always hosted multiple CPU architectures, from x86 to Sun Sparc, IBM Power and others. When it comes to CPU architectures, one attribute that datacenters weren’t really concerned about was power efficiency, but this is rapidly changing as CPU are getting more and more power hungry. The chart below shows the evolution of the TDP (Thermal Design Power), which is a great way to measure how much power a CPU requires, for AMD and Intel processors.

This chart was included in a blog post on the Dell Technologies InfoHub platform: The Future of Server Cooling Part 2 .

As you can see, the trend is getting steeper and steeper and that is part of the reason why AWS decided to design an ARM processor to include in its offering. The goal there is not to offer the best performance, but to offer a lower performance instance at a lower price, thanks to the reduced power consumption of the ARM based instances.

While AWS Graviton CPUs are only available within AWS instances, in 2020, a company called Ampere released a general purpose ARM processor under the Altra brand. The initial Alta processor had 80 cores running at 161W in use. This is a much higher core count than AMD, who was the leader at that time in the x86 space with 64 cores, and a much lower TDP than AMD processors, which were in the 200’s watts at that time. The current generation of Ampere Altra processors offers 128 cores for a TDP of 250W, which is a very strong value proposition, and explains why they power some of Microsoft Azure infrastructure and have been adopted by tier-one server providers, such as HPE and SuperMicro.

Hang on a second, what about Dell?

If ARM processors have such as strong value proposition, then when are we going to see an ARM-based PowerEdge server?

The answer is: not in the foreseeable future. I can hear customers screaming: WHY?

Answering that ‘why’ question was the genesis for this blog post and I felt that the long preamble before getting into the meat of the post was necessary to set the stage for this answer. It is important to understand the value proposition of ARM processors and that value proposition is centered around one metric: W/core. ARM processors have always provided very low W/core. For instance a Rasbperry Pi 4 consumes a maximum of 6 watts when all of its 4 cores are busy, which is 1.5W/core.  While this is an extreme example, Ampere Altra processors have a power envelope of about 2W/core and Ampere Altra Max processors one of about 3W/core, making them extremely power efficient. By comparison, the latest Intel Sapphire Rapids maxes out at 60 cores at 350W, meaning 5.8 W/core, i.e. almost triple the W/core from Ampere Altra processors, which maxes out at 80 cores.

So again, why no ARM-based PowerEdge? I mean, all of the information above only strengthen the value proposition of an ARM-based PowerEdge, right? Then answer is yes, but not really and the reason for this is that our current processor partners, AMD and Intel, aren’t resting on their laurels, especially AMD with its Genoa and Bergamo processors. AMD Genoa maxes out at 96 cores and 192 threads, while offering 4W/core. It is twice as much as Ampere Altra, but it offers significantly more cores and more threads as Ampere Altra only offers 1 thread/core for 80 threads. The exciting one though is Bergamo, as it maxes out at 128 cores and 256 threads, while offering 3W/core, which is very much in line with Ampera Altra Max processors.

On top of offering a similar power envelope, AMD Bergamo offers twice the number of threads, higher memory bandwidth with 12 channels instead of 8 and higher memory performance by using DDR5 instead of DDR4. AMD Bergamo significantly reduces the value proposition of ARM processors and comes with another very significant benefit: it is an x86-based processor, meaning that unlike ARM processors, it doesn’t require applications to be recompiled. While the availability of applications for ARM processors is increasing, not all applications are supported on ARM today.

All of those reasons are why there won’t be an ARM-based PowerEdge server in the foreseeable future. It doesn’t mean that ARM processors are bad, it just means that we value the relationship we have with Intel and AMD and they offer processors that are competitive against ARM processors and with none of the downside of ARM processors. It also doesn’t mean that we, as Dell, will stop evaluating the value proposition of ARM processors, but at this time, we don’t think that that value proposition is strong enough to warrant being included in the PowerEdge lineup.

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