From Supercomputers to PDAs: Cactus on an iPAQBerlin 27 August 2002 Cactus was originally designed as a framework for physicists with largescale applications which, to model complex natural phenomena with adequate fidelity, need the kind of computational resources only supplied by the world's largest supercomputers. One important design requirement has been to ensure that the physicists developing and running their simulations had a common working environment. Their codes should be able to be developed and debugged on local laptops and workstations, and then be immediately run on any final production machine, whether it be a Linux cluster, a Cray T3E or any other supercomputer.
Cactus has been used for simulations running across thousands of processors distributed across several machines but Cactus can also be run on a iPAQ Pocket PC from HP, one of the latest generations of PDAs.
Physicists Horst Beyer and Denis Pollney, from the Numerical Relativity Group at the Max Planck Institute for Gravitational Physics in Germany, are more used to working with machines at the other end of the Compaq product line. One of the groups main resources is the 3000 processor Compaq Alphaserver "Terascale Computing System" called Lemieux at the Pittsburg Computing Center in the USA, currently the 3rd fastest computer in the world.
Swapping this 6 Teraflop, 3 TeraByte monster machine for a pocket sized 206 Megaflop, 32 MegaByte iPAQ 3630 Pocket PC, the duo compiled and ran a Cactus application, using the Intimate Linux Distribution, graphing the output data also on the handheld with the xgraph graphics package.
To compare this to Lemieux, 100000 iPAQ Pocket PCs would be needed to provide the same amount of computing power as the supercomputer. At todays prices buying these would cost around $50 Million.
Using a Linux distribution on the iPAQ Pocket PC meant that porting the the Cactus code and the xgraph graphics package was relatively straightforward. Although they used a simple WaveToy application for this example, most of Cactus thorns written in C, C++, or F77 could have been used. As yet there is no F90 compiler.
While it certainly isn't planned yet to start running black hole simulations on handhelds, physicists do want to start using such devices to complement their high-performance computing work.
With the monitoring and steering Web interfaces which can be used with any remotely running Cactus simulation, physicists can use their handhelds, connected wireless to the internet, to interact with their simulations wherever they happen to be. From a Web browser on the handheld, they can check on all their simulations, visualise output data, change parameters, and stop and restart them as necessary.
Running Cactus on an iPAQ Pocket PC also demonstrates the emerging Grid-orientated view for HPC. Portable applications can make effective use of all of the computing resources available to their users. Intelligent resource brokers and schedulers can send tasks to the most appropriate machines for execution, taking into account factors such as cost, efficiency, and time to completion. Grid-enabled applications can migrate from resource to resource, spawning subtasks to smaller machines. In this picture a users iPAQ Pocket PC is just another, albeit small, available resource.
Cactus is a main application for the European GridLab project, which contains a whole workpackage on mobile computing using on a range of devices including laptops and iPAQ Pocket PCs. The project web site is
http://www.cactuscode.org.
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