Introducing HPiC, the Hartree Centre’s Raspberry Pi Cluster

The Hartree Centre has a new pocket-sized addition to our data centre! One of our Research Software Engineers, Tim Powell tells us all about it…

HPiC has been created as a host for software demonstrations and for outreach events. It simulates a supercomputer by networking together 20 Raspberry Pi 3 Model B’s, allowing them to communicate and execute parallel programs.

The Raspberry Pi is a low-cost, low-power, single-board computer designed to make computer science more accessible to amateur developers, schools, and developing countries. Released in 2013, Raspberry Pis can be used for a wide range of applications – from robotics, to music streaming, to smart mirrors! The incredibly versatile Raspberry Pi 3 computer has a Quad Core 1.2Ghz ARM processor at its heart, 1GB of RAM, WiFi, Bluetooth capabilities and a whole host of device connectivity via a GPIO connector.

​​HPiC replicates high performance computing (HPC) techniques and can perform over 1,000 million instructions per second. HPiC has 19 ‘worker’ nodes (1 node = 1 raspberry pi), each with a quad-core ARM processor, resulting in 76 cores to utilise for parallel computing. The remaining node is called the ‘Head Node’ and allows us to interact and submit jobs to the ‘worker’ nodes.

HPiC’s case is built to mimic the Hartree Centre’s machine room. Currently, there are two demos available on HPiC: a Smoothed Particle Hydrodynamics (SPH) Simulation and a Mandelbrot Set Race (PiBrot). Both of these show key supercomputing techniques and we’re sourcing more demos at the moment.

The SPH simulation in action.

The SPH simulation shows how water interacts in a variety of environmental conditions by changing gravity, viscosity and density, etc. The simulation runs in parallel on several nodes at the same time by utilising domain decomposition. This means that each processor is assigned a different part of the simulation space. A dynamic load balancing algorithm adjusts the domains to ensure each processor has approximately the same volume of fluid (or number of SPH particles), maximising the performance of the simulation.

Mandelbrot set calculation race underway. Left: single node, right: 18 node.

PiBrot, however, is a race… a race to calculate a Mandelbrot set. However, one side has an advantage as it uses 18 nodes to calculate the set, whereas the other side only uses a single node. This demonstrates how mathematical calculations can be spread across several nodes to speed up the process with proper parallelisation of code.

Building a prototype foam core case.

Designing and building HPiC was an interesting and fun opportunity. When designing the case, I had to think how to best portray the Hartree Centre and supercomputing, whilst making it as accessible and friendly to the public as possible. Once the idea to mimic the machine room had been established I mocked up a temporary case out of foam core, this was mainly to check that all the hardware could fit in! I then took my physical mock-up and converted it into a digital 3D model.

Finally, the plans were sent off to an external company who built the case. After a late night at the office, too many cable ties, and copious amounts of electrical tape the Raspberry Pi cluster was assembled!

Finished! HPiC complete with case.

By this point there was quite a lot of discussion about what to actually call our cluster so I held a competition to decide on the name with a raspberry flavoured cake as the prize! We settled on HPiC as it can stand for both High Performance Computing and Hartree Pi Cluster!

Last month my fellow Research Software Engineer, Aiman Shaikh attended the annual EuroScience Open Forum in Toulouse, France, helping to inspire attendees with our amazing science and technology at the United Kingdom stand. In pride of place was the new mini supercomputer… it was HPiC’s first outing. Aiman did an amazing job showcasing HPiC, and encouraging delegates to interact with the SPH simulation and PiBrot race. Amongst visitors from across Europe keen to see a demonstration of HPiC, were Sharon Cosgrove, Executive Director STFC Strategy, Planning and Communications and Rebecca Endean, UKRI Strategy Director.

Aiman demonstrating HPiC to Rebecca Endean, UKRI Strategy Director.

What’s next? We’re currently developing some more demos to run on our little HPiC as well as looking to get the case engraved with the name and our logo, we can’t wait to take it out to more events and get more people excited about the world of HPC!

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