Fine-grain Parallel Processing On A Commodity Platform: A Solution For The Atlas Second-level Trigger

From 2005 on, CERN expects to have a new accelerator available for experiments: the Large Hadron Collider (LHC), with a circumference of 27 kilometres. The ATLAS detector produces 40 TeraBytes/s of data. Only a fraction of all data is interesting. A computer system, called the trigger, selects the interesting data through real-time data analysis. The trigger consists of three subsequent filtering levels: LVL1, LVL2, and LVL3. LVL1 will be implemented using special-purpose hardware. LVL2 and LVL3 will be implemented using a Network Of Workstations (NOW). A major problem is to make efficient use of the computing power available in each workstation. The major contribution of this designer's project is an infrastructure named MESH. MESH enables CERN to cost- effectively implement the LVL2 trigger. Furthermore, due to the use of commodity technology, MESH enables the LVL2 trigger to be cost-effectively upgraded and supported during its 20 year lifecycle. MESH facilitates efficient parallel processing on PCs interconnected by Ethernet. Over the years, efficient Input/Output (I/O) for workstation clusters focused on parallel computing has had much attention. In that context, fault-tolerance issues, such as fault confinement and failure behaviour, have either been ignored or given very little consideration. Since these fault-tolerance issues are of major importance to the ATLAS trigger, the software systems resulting from such research could not be used. On the other hand, individual techniques, resulting from such research, have proven to be essential. This work combines the latest techniques to equip workstations with efficient I/O. It extends this research to the use of commodity hardware instead of specialised hardware. The I/O system is tightly integrated with an efficient special-purpose scheduler. Some of the latest scheduling techniques, developed in the context of parallel computing, have been adapted and improved. CERN considers an Ethernet-based solution for LVL2 to be very promising, and has therefore developed an Ethernet- based prototype of the ATLAS LVL2 trigger. This prototype uses MESH as communication and scheduling infrastructure. (Abstract shortened by UMI.)