ESnet Applying Global Networking Expertise to GRETA Spectrometer for Experiments at Michigan Facility
By Jon Bashor
For decades, ESnet engineers have deployed the latest technologies and developed critical tools to build a high-speed network that crisscrosses the nation and spans the Atlantic Ocean. Now, a small team is doing the same for a specialized network that will transport and organize data across distances measured in feet rather than thousands of miles.
Nuclear physicists at Berkeley Lab are building the GRETA experiment, short for Gamma Ray Energy Tracking Array. The gamma ray detector will be installed at the Department of Energy's Facility for Rare Isotope Beams (FRIB) located at Michigan State University in East Lansing.
The GRETA spectrometer will go online with first physics in 2024. When complete it will house an array of 120 detectors that will produce up to 480,000 messages per second -- totaling 4 gigabytes of data per second -- and send them through a computing cluster for analysis. While the data will traverse a network of about 50 meters, the system has been designed so the data could easily be sent to more distant high performance computing systems.
"We will be analyzing everything in real time, on the fly with no intermediate storage," said Mario Cromaz, a Berkeley Lab physicist in charge of the computing component of GRETA. "We had an idea of how we wanted the computing to work, but it was also a networking problem and we didn't have the technical where-with-all so we approached ESnet. That's the kind of expertise we could only find at ESnet."
ESnet network engineer Eli Dart, who is the computing system architect for the project, said ESnet agreed to help so that networking could be integrated into the project early in a way that is scalable and extensible. Dart also sees it as potentially the start of something even bigger -- a system that is a building block for the "Superfacility" concept to seamlessly stitch together experiments, networks and computing resources.
"It's a strategic experiment on ESnet's part -- if we can get in early and help with the design, we can try to help the experiment do things that would otherwise be very difficult," Dart said. "In a deep collaboration like this, we can learn what's important in the context of the experiment, and that can help us improve our services to the scientific community."
First of its kind
GRETA is a gamma ray spectrometer, which will measure the energy of gamma rays created by nuclear collisions inside a compact sphere of high-purity germanium crystals with unprecedented resolution. It consists of a total of 120 highly segmented large-volume, coaxial germanium crystals, combined in groups of four to form a total of 30 Quad Detector Modules.
Cromaz said GRETA is the first of its kind in that it will track the positions of the scattering paths of the gamma rays using an algorithm specifically developed for the project. This capability will help scientists understand the structure of nuclei, which is not only important for understanding the synthesis of heavy elements in stellar environments, but also for applied-science topics in nuclear energy, nuclear forensics, and stockpile stewardship.
Since the excited nuclei emitting the gamma rays are moving very fast -- at a large fraction of the speed of light -- they create a Doppler effect. In order to accurately measure their energy, Cromaz said scientists need to know the angle the ray is coming from. The capability to do this is what makes GRETA unique.
The project team has just finished the design phase and the next formal project review will be in early August. To get this far, a one-quarter version of the experiment, called GRETINA, was built with prototypes to test the concepts and is currently performing experiments at Michigan State University. With a favorable August review, the GRETA team anticipates asking the DOE for approval to commence construction by the end of the fiscal year.
Bringing order to the data
According to Cromaz, the detectors built with field programmable gate arrays will spray out packets of data, which is relatively simple to do. The hard part is creating a buffer to catch the data, and to feed it into the network to the thousands of threads of computation running in the cluster for analysis.
"There are actually two phases to the analysis in the gamma ray tracking array," Cromaz said. "The first phase is locating where the interaction points of the gamma ray with the detector material occurred and the second phase is looking at all interaction points globally in the detector and subdividing/ordering them into likely gamma ray tracks."
The first computing stage derives the number and location of interaction points. This phase only depends on the digitized signals from a given detector crystal (there are 120 crystals which tile the sphere).
"In GRETA, it's advantageous to arrange things this way as converting the raw digitized waveforms to interaction points -- essentially a set of x, y, z coordinates and energies -- reduces the data volume by an order of magnitude," Cromaz said. "This reduces the load on the second phase, the global event builder, and allows us to implement it on a single node, which simplifies the overall design."
Eric Pouyoul, who leads ESnet’s testbed efforts, designed the forward buffer to quickly collect the data, which will then be pulled into analysis jobs by the computing cluster. The forward buffer must receive the high-speed packet streams from 120 detectors with zero packet loss, and then feed the data to the cluster asynchronously.
Pouyoul said the project was challenging on a number of levels, from the physics involved to the nature of the data to the demands of real-time processing. The first step was to write the computing code and algorithms for handling the data. Although he has written high performance code in the past, this project required him to use other skills he’s developed over the years. Once he had the software, he needed to make sure it could handle the outpouring of data.
“The simulation of the crystals was relatively easy,” he said. “But the simulation of the physics--the nuclear behavior at the heart of GRETA--I never did anything like this before.”
Since not all of the crystals would detect every interaction, Pouyoul used a statistical-based model to recreate what would happen inside the detector. He also had to make the code efficient so it could run on the actual hardware GRETA will use. “I was able to build the model of the physics inside GRETA,” he said, “but don’t expect me to really understand it.”
"The first phase is the most computationally intensive part of the process as the maximum data generation rate is 480,000 calculations per second and each calculation requires about five milliseconds per CPU core, hence the requirement for a cluster." Cromaz said.
From there, the data will then pass through a second system also designed by Pouyoul and called the "global event builder." Using software written by Pouyoul, the system looks at the timestamps on all the incoming data and then reassembles them into a single stream of events ordered by the time stamps. Additionally, the algorithm also determines which event each piece of data belongs to and assembles them appropriately. This data will be stored for additional analysis based on timestamps and events.
“This has to happen in real time,” said Pouyoul, who called the project the most exciting work he has done in his 11 years at the lab. “Moving the data from the events through the system to storage cannot take more than 10 seconds.”
While the GRETA project has been gratifying for ESnet, it will also provide more experience toward developing the "Superfacility" concept developed by Berkeley Lab's Computing Sciences organization. The Superfacility framework comprises the seamless integration of experimental and observational instruments with computational and data facilities using high-speed networking. While the concept is straightforward, achieving it requires resolving any number of smaller issues, which vary by facility.
"Because it was designed to be ultimately connected to the wider network, GRETA will be Superfacility-ready," Dart said. "We see GRETA as a strategic experiment on ESnet's part; if we get involved early, we can help with the design and help the experiment do things that otherwise could have been very difficult.
"The fun part of all this is that we would like to see GRETA be a proving ground for this type of environment and then see it be widely adopted," Dart said. "In fact, we're already received inquiries from other sites. If we can help others take advantage of what we've learned, then everybody wins."