The Large Hadron Collider

Once the data is stored at a Tier-2, physicists need to be able to analyze it to make their discoveries. The platform for this task is Linux. For the sake of standardization, most of the development occurs on Red Hat Enterprise-based distributions. Both CERN and FNAL have their own Linux distributions but add improvements and customizations into the Scientific Linux distribution. The Tier-2 at Nebraska runs CentOS as the primary platform at our site.

With data files constructed to be about 2GB in size and data sets currently hovering in the low terabyte range, full data set analysis on a typical desktop is problematic. A typical physics analysis will start with coding and debugging taking place on a single workstation or small Tier-3 cluster. Once the coding and debugging phase is completed, the analysis is run over the entire data set, most likely at a Tier-2 site. Submitting an analysis to a grid computing site is not easy, and the process has been automated with software developed by CMS called CRAB (CMS Remote Analysis Builder).

To create a user's jobs, CRAB queries the CMS database at CERN that contains the locations where the data is stored globally. CRAB constructs the grid submission scripts. Users then can submit the entire analysis to an appropriate grid resource. CRAB allows users to query the progress of their jobs and request the output to be downloaded to their personal workstations.

CRAB can direct output to the Tier-2 storage itself. Each CMS user is allowed 1 terabyte of space on each Tier-2 site for the non-archival storage of each user's analysis output. Policing the storage used by scientists is a task left to the Tier-2 sites. HDFS's quota functionality gives the Nebraska Tier-2 administrators an easily updated tool to limit the use of analysis space automatically.

Figure 3 shows a simulated event seen through CMS, and Figure 4 shows an actual record event.

Figure 3. How a physicist sees CMS—this is the event display of a single simulated event.

Figure 4. An actual recorded event from CMS—this event shows radiation and charged particles spilling into the detector from the beam colliding with material in the beam pipe.

The LHC will enable physicists to investigate the inner workings of the universe. The accelerator and experiments have been decades in design and construction. The lab is setting new benchmarks for energetic particle beams. Everyone I talk to about our work seems to get glossy-eyed and complain that it is just too complex to comprehend. What I want to do with this quick overview of the computing involved in the LHC is tell the Linux community that the science being done at the LHC owes a great deal to the contributors and developers in the Open Source community. Even if you don't know your quark from your meson, your contributions to open-source software are helping physicists at the LHC and around the world.