Feature - Tier-3 computing centers expand options for physicists
Researchers at Texas Tech University work more than 5,000 miles from CERN, but they will have just as much chance of making new physics discoveries using the data collected at the Large Hadron Collider as scientists in Switzerland.
Texas Tech runs a Tier-3 computing center that is part of the CMS collaboration, allowing physicists there to host and analyze data from the experiment locally. Tier-3 centers make up one of four tiers in the LHC Computing Grid.
“Tier-3 sounds like the bottom of the chain,” said Alan Sill, a senior scientist who runs the center, “but in a way it’s the top of the chain. It’s the first level at which physicists have access to data under their own control.”
A single Tier-0 site, at CERN, processes raw information and sends it around the world to Tier-1 centers. CMS’s seven Tier-1 centers reprocess the data and break it into subsamples to send to about 35 CMS Tier-2 centers. Tier-2 centers host data for the experiment and devote much of their time to running centralized simulations, but also provide resources for individual physicists’ analysis jobs.
U.S. Tier-1 and Tier-2 centers are funded by the Department of Energy and the National Science Foundation. Tier-3 centers must find their own funding for the hardware, operational support and network connections they need to obtain data remotely from the experiment, said Ian Fisk, deputy manager for CMS software and computing.
Freed from the reprocessing and simulation responsibilities of Tier-1 and Tier-2 centers, they can devote their resources to their own studies. Physicists face less competition for time and resources at a Tier-3 than at a Tier-2. They have more freedom to run the analyses they want and more flexibility in testing and debugging the code before it’s used.
“There are fewer constraints on bringing up a Tier 3,” Fisk said. “This is really the growth area for the experiment right now.”
CMS currently runs eight Tier-3 centers in the U.S. Some, like the one at Texas Tech, can run hundreds of analyses simultaneously. Others are as simple as a handful of central processing units capable of running about 60 analyses at a time.
“Tier-2s and Tier-3s are where physics analysis is going to happen,” said Burt Holzman, who gives computer support to U.S. Tier-3 centers from Fermilab's Tier-1 center. “Having a Tier 3 gives physicists the leverage to be creative and explore.”
—Kathryn Grim, Fermilab