Above: A map of the temperature field halfway through this important epoch in the evolution of the universe. Reionization converts a cold and neutral gas into a warm and highly ionized medium.

Originally, the researchers' job was scheduled to run continuously for five days on Columbia. The users had several "start up" problems and the job ran much slower than expected. During the course of the run, the job was stopped multiple times, and was restarted with changes in the code checkpoint algorithm to resolve performance problems.

"Johnny provided us with diligent support in the setup and running of our simulations," Trac said, adding, "We much appreciate the pleasant, informative, and very helpful communications with him."

Chang’s extensive code support included set-up and running of Cen and Trac’s state-of-the-art simulation, which required large computer processor and memory resources—about 2 terabytes—on the Columbia supercomputer. He performed transfer and backup of important files and data, and diagnosed code performance problems. Chang also tracked down issues, ran experiments, and determined the cause of various problems. His suggestions for improvement sped up the code by 10%. Another suggested optimization—running the code on every other core—sped the code by another 15%.

Chang estimated that the collective changes would save two days of runtime (over an 8-day run) in the future. Cen and Trac commended Chang’s dedication, making himself available late at night and on weekends during a ten-day period to solve the problem, enabling the simulation to complete.

Contact: Johnny Chang, Johnny.Chang@nasa.gov, NASA Advanced Supercomputing Division, Computer Sciences Corp.