Pumping New Life into HPC Clusters, the Case for Liquid Cooling

By Scott Tease

July 10, 2018

High Performance Computing (HPC) faces some daunting challenges in the coming years as traditional, industry-standard systems push the boundaries of data center space, heat and power limitations. As the industry pushes to achieve exascale computing, overcoming performance barriers such as heat and power use will be the first steps in ushering in the next generation of HPC innovations.

A Bit of History

In the last few years, liquid has evolved from novelty to mainstream in HPC. There are multiple reasons why: support for higher bin CPUs for greater computational muscle, extreme density, reduced noise, and, not least of all, lower electric bills. As HPC progresses toward exascale, these factors will continue to impact cluster decision making.

Lenovo’s Scott Tease. Photo by HPCwire.

So, how did we get here? In 2012, Leibniz-Rechenzentrum (LRZ) in Munich, Germany, a supercomputing center supporting a diverse group of researchers from around the world, gave the HPC vendor community a unique challenge: LRZ wanted to dramatically cut the electricity it consumed without sacrificing compute power. The IBM System x team delivered a server that featured Warm Water Direct Water Cooling, piping unchilled water directly to the CPU, memory and other high power consuming components. Thus, was born the era of warm water cooled supercomputers.

Chillers had been a staple of water cooling going back to the old mainframe days. Instead, at LRZ a controlled loop of unchilled water, up to 45°C, was used. In addition to the energy efficiency and data center-level cost savings, several additional benefits emerged. Since the CPUs were kept much cooler by the ultra-efficient direct water cooling, there was less energy loss within the processor, saving as much as five percent more than a comparable air-cooled processor. If desired, the Intel CPUs could run in “turbo mode” constantly, boosting performance up to an additional 10-15 percent. Because the systems had no fans – except small ones on the power supplies – operations were nearly silent. And, the hot water produced from the data center was piped into the building as a heat source. With additional software support savings from SUSE, total savings at LRZ was nearly 40 percent.

And Today…

Several years have passed, and most, if not all, of the major vendors of x86 systems have jumped into water cooling in some manner. These offerings run the gamut from water-cooled rear-door heat exchangers, which act like a car’s radiator and absorb the heat expelled by air-cooled systems to systems literally submerged in a tank full of special dielectrically compliant coolant – something akin to a massive chicken fryer with servers acting as the heating elements. Direct water-cooled systems have evolved too. Advances in thermals and materials now allow intake water up to 50°C. This makes water-cooling a viable option almost anywhere in the world without using chillers. Also, the number of components cooled by water has expanded. In addition to the CPU and memory, the IO and voltage regulation devices are now water-cooled, driving the percentage of heat transferred from the system to water to more than 90 percent.

Unfortunately, not everything in the data center can be water-cooled, so LRZ and Lenovo in partnership with Intel, are in the process of expanding alternative cooling by converting the hot water “waste” into cold water that can be reused to cool the rest of the data center. This process utilizes “adsorption chillers”, which take the hot water from 100 racks with direct-to-node liquid coolin compute nodes and passes it over sheets of a special silica gel that evaporate the water, cooling it. From there, the evaporated water is condensed back into a liquid, which is then either piped back into the compute racks, or into a rear-door heat exchanger for racks of storage and networking gear, which aren’t water-cooled. This process is able to deliver more cold water than the data center can actually consume. This approach to data center design is made possible because the water delivered to the chillers is hot enough to make the process run efficiently. The tight connection and interdependence between the server gear and the data center infrastructure has strong potential.

It’s Not the Humidity, It’s the Heat

The driving force behind processor innovation for the last 50 years has been Moore’s Law, which states that the number of transistors in an integrated circuit will double approximately every two years. Moore’s company, Intel, condensed to double CPU performance every 18 months while costs come down 50 percent. After half a century however, delivering on that prediction has become increasingly more difficult. To stay on the Moore’s law curve, Intel has to add more processing cores to the CPU, which draws more power, and in turn, produces more heat. Look at how the power draw in Intel processors has grown over the last dozen years:

 

Release date Code Processor Core/chip TDP(W) Spec FP Spec FP/W
06/26/06 Woodcrest Intel Xeon 5160 2 80 17.7 0.22
11/12/07 Harpertown Intel Xeon x5460 4 120 25.4 0.21
03/30/09 Nehalem Intel Xeon x5570 4 95 43.8 0.46
03/16/10 Westmere-EP Intel Xeon x5690 6 130 63.7 0.49
05/01/12 Sandy Bridge Intel Xeon E5-2690 8 135 94.8 0.70
01/09/14 Ivy Bridge Intel Xeon E5-2697v2 12 130 104 0.80
09/09/14 Haswell Intel Xeon E5-2699v3 18 145 116 0.80
03/09/15 Broadwell Intel Xeon E5-2699v4 22 145 128 0.88
07/11/17 Skylake Intel Xeon Platinum 8180 28 205 155 0.76

 

To deal with that heat, processors in an air-cooled environment will need larger (taller) heat sinks, which require systems with taller or bigger chassis. ASHRAE has estimated the increased heat load in a standard rack:

Could those 2U4N server “sleds” that have been the staple of HPC clusters over the last several years end up being an endangered species? Probably not, but they face some challenges in order to survive. Simply put, customers will face a difficult trade-off: system density (the number of servers their IT people can cram into a rack) vs. CPU capability (fewer cores). Customers wanting to run higher core CPUs will have to give up space in the rack, meaning more racks in the data center, meaning higher OPEX in real estate, electric and air conditioning costs.

If the customer does not want to (or cannot) go to full direct to node water cooling, but needs density, and computational power, they are in a jam.  This is where technologies like our Thermal Transfer Module (TTM) will come into play. The TTM is an advanced CPU heat sink for air-cooled dense systems, which utilizes liquid to transfer heat away from the processor to a remote area of the system where air cooling is more effective. This would allow them to maintain their profile without compromising on performance.

Going Green

A second factor at work depends on your geography. “Green” data center initiatives have been in place in Europe for a decade. They are what spurred LRZ and others like them to seek alternatives to air cooling. There is even a “Green 500” list of the most energy efficient data centers on the TOP500.org site. As more of these installations are completed, and promoted, other customers may see 50 percent savings on electricity and 15 percent performance improvement as substantial enough justification to take the plunge to water cooling.

Roughly, 55 percent of the world’s electricity is produced from burning fossil fuels, including coal. Data centers consume almost 3 percent of the world’s electricity and can no longer “fly under the radar” when it comes to energy consumption. Other governments may see these results and put regulations in place to reduce data center power consumption.

Saving Green

“Going green” in the data center is not simply an altruistic endeavor. It is, in many places, a matter of necessity. Electricity prices in some parts of the world can exceed $0.20 per kilowatt-hour, in areas it can be double that amount. This accelerates the decision on alternative cooling to today. The largest hurdle is the up-front costs for plumbing infrastructure and the small premium (in most cases less than 10 percent) for water cooled systems over comparable air-cooled.

Finance departments always ask, “How long until a system like this will pay for itself?” and in some cases, it may be one-year. Of course, TCO and ROI are dependent upon the solution itself and the installation costs, but most OEMs have TCO calculators to assist customers in determining payback on a direct water-cooled system.

Summary

Liquid-cooled HPC is now an established alternative to traditional air-cooled systems. In many cases, it is a question of “when” not “if” to make the move.  As core counts, heat sinks and power consumption all continue to grow, circumstances will dictate the shift. Going to liquid cooling technologies can save space, power, money and still deliver the computational muscle needed to run the most demanding HPC workloads.

Scott Tease is executive director of high performance computing and artificial intelligence within Lenovo’s data center group.

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