A US university gymnasium filled with people and laptops came close to becoming one of the world's fastest supercomputers this weekend, laying the path for such a collaboration to succeed next time.

FlashMob I, named for the online phenomenon of rallying large numbers of people briefly, took place at the University of San Francisco. Although the approximately 660 computers, mostly laptops plugged into donated switches, failed to generate sufficient speed to crack the top-500 list, organisers say they have no doubt such a temporary supercomputer will soon be assembled.

The graduate students and faculty who organised the event say they hope the data gathered will benefit similar attempts in the future, when people cobble together PCs to solve complex problems with supercomputing. The purpose of the trial run on Saturday was to observe the behaviour of computers with different processors and memory configurations computing in concert.

Supercomputers are typically composed of identically configured PCs, while the connection of so many different systems is new, notes John Witchel, a graduate student in computer science at USF.

"There's virtually no field data available for large-scale heterogeneous environments," Witchel says. "So we've gotten requests from all the national laboratories and researchers... to make sure that we're tracking specific areas of data."

FlashMob stems from a USF graduate computer science class called "Do-it-yourself Supercomputers". As Witchel and his classmates considered how to procure hundreds of PCs necessary to build their own supercomputer, he said suggested that they post a message on online classified-ad site Craigslist.

Word went out on the site, and hundreds of Bay Area computer users responded. At first the FlashMob project sought only laptops, thinking desktops would draw too much power. However, later analysis showed that at top processing speeds desktop PCs don't need significantly more electricity, and so volunteers were allowed to bring in desktops as well, Witchel says.

They plugged into more than 1000 ports on several switches loaned for the project by Foundry Networks and Myricom. All participants ran FlashMob I software on a CD, so their systems' hard drives weren't touched during the project, allaying concerns about privacy and viruses. The program is a modified version of the Knoppix Linux distribution.

"We basically call it an ad-hoc supercomputer," Witchel says. "It's a supercomputer that's built for a day or for a short period of time."

Not all of the hundreds of participating PCs were running when the FlashMob effort hit its peak speed of 180 gigaflops, a billion floating-point operations per second. The Linpack benchmarking software measured progress. To hit the top-500 supercomputing list would require nearly 500 gigaflops, based on current standings. The leaders on that list compute at the speed of teraflops, which are each a thousand gigaflops.

The graduate students wrote the FlashMob program in six weeks, which Witchel says was possible only through the use of open-source software. Both it and statistics about the project's results are available at FlashMobComputing.org.

Organisers envision future FlashMobs that convene to reach a specific research goal, such as running an environmental impact model or conducting complex AIDS research. In theory, a group can cobble together a heterogeneous supercomputer from various PCs, regardless of their make. Most important, in contrast to very expensive supercomputers that cost millions of dollars, such groups can garner a lot of processing power on the cheap.

A different type of ad-hoc supercomputer is a project that invites online users to contribute CPU cycles of idle systems to a sort of virtual supercomputer. One such project is an Intel-supported program that collects processing power for cancer research. Another draws on unused processing cycles to power the SETI project, a search for extraterrestrial life.

Witchel believes FlashMob technology will eventually let home users build their own small supercomputers. By the end of June, he hopes to include MPEG-ripping software with FlashMob, so users could turn a small number of systems on a LAN into a "home render farm" that could rip a DVD in a fraction of the time it currently takes.

"It used to be that you needed a double PhD to do this stuff," says Witchel. "With the FlashMob software we've released, we believe that high schoolers can do this."

The people most likely to use FlashMob, however, are researchers who until now could not afford the millions of dollars necessary to buy a supercomputer.

"I think the real-use case here is in researchers working with modest grants, who don't happen to be associated with a Stanford or a Berkeley or somebody who does have access to supercomputers," Witchell says.