The Large Hadron Collider
The following article is featured in the November issue of Linux Journal (#199). Subscribe to see more articles like this and have them delivered to you every month! http://www.linuxjournal.com/subscribe
Muons and mesons and quarks—oh my! Never fear, Dorothy, the Large Hadron Collider and open-source software will save the day.
What is at the heart of the Large Hadron Collider (LHC) experiments? It should not surprise you that open-source software is one of the things that powers the most complex scientific human endeavor ever attempted. I hope to give you a glimpse into how scientific computing embraces open-source software and the open-source philosophy in one of the LHC experiments.
The Tiered Computing Model
The LHC near Geneva, Switzerland, is nearly 100 meters underground and provides the highest-energy subatomic particle beams ever produced. The goal of the LHC is to give physicists a window into the universe immediately after the big bang. However, when physicists calculated the level of computing power needed to peer through that window, it became clear that it would not be possible to do it with only the computers that could fit under one roof.
Even with the promise of Moore's Law, it was apparent that the experiments would have to include a grid technology and decentralize the computing. Part of the decentralization plans included adoption of a tiered model of computing that creates large data storage and analysis centers around the world.
The Compact Muon Solenoid (CMS) experiment is one of the large collider experiments located at the LHC. The primary computing resource for CMS is located at the LHC laboratory and is called the Tier-0. The function of Tier-0 is to record data as it comes off the detector, archive it and transfer it to the Tier-1 facilities around the globe. Ideally, every participating CMS nation has one Tier-1 facility. In the United States, the Tier-1 is located at Fermi National Laboratory (FNAL) in Batavia, Illinois. Each Tier-1 facility is charged with additional archival storage, as well as physics reconstruction and analysis and transferring data to the Tier-2 centers. The Tier-2 centers serve as an analysis resource funded by CMS for physicists. Individuals and universities are free to construct Tier-3 sites, which are not paid for through CMS.
Currently, there are eight CMS Tier-2 centers in the US. Their locations at universities allow CMS to utilize the computing expertise at those institutions and contribute to the educational opportunities for their students. I work as a system administrator at the CMS Tier-2 facility at the University of Nebraska-Lincoln.
By most standards, the Tier-2 centers are large computing resources. Currently, the capabilities of the Tier-2 at Nebraska include approximately 300 servers with 1,500 CPU cores dedicated to computing along with more than 800 terabytes of disk storage. We have network connectivity to the Tier-1 at FNAL of 10 gigabits per second.
LHC? CMS? ATLAS? I'm Confused.
It is easy to lose track of the entities in the high-energy physics world. The Large Hadron Collider (LHC) is the accelerator that provides the beams of high-energy particles, which are protons. It is located at CERN (Conseil Européen pour la Recherche Nucléaire). CERN is the laboratory, and the LHC is the machine. The Compact Muon Solenoid (CMS) is a large particle detector designed to record what particles are created by the collision of the beams of protons (a muon is an elementary particle similar to the electron). CMS is only one of the experiments at CERN. CMS also is used to refer to the large collaboration of scientists that analyze the data recorded from the CMS detector. Most American physicists participating in CMS are in an organization called USCMS. Other experiments at the LHC include ATLAS, ALICE, LHCb, TOTEM and LHCf. These experiments use their own analysis software but share some grid infrastructures with CMS.
One of the technically more difficult obstacles for CMS computing is managing the data. Data movement is managed using a custom framework called PhEDEx (Physics Experiment Data Export). PhEDEx does not actually move data but serves as a mechanism to initiate transfers between sites. PhEDEx agents running at each site interface with database servers located at CERN to determine what data are needed at that site. X509 proxy certificates are used to authenticate transfers between gridftp doors at the source and destination sites. The Tier-2 at Nebraska has 12 gridftp doors and has sustained transfer rates up to 800 megabytes per second.
It should be noted that the word data can mean a few different things to a physicist. It can refer to the digitized readouts of a detector, Monte Carlo simulation of outputs, or the bits and bytes stored on hard drives and tapes.
The network demands made by the Nebraska Tier-2 site have generated interesting research in computer network engineering. Nebraska was the first university to demonstrate large data movement over a dynamically allocated IP path. When Nebraska's Tier-2 is pulling a large amount of data from the Tier-1 at FNAL, a separate IP path automatically is constructed to prevent traffic from adversely affecting the university's general Internet usage.
Since data transfer and management is such a crucial element for the success of CMS, developing the underlying system has been ongoing for years. The transfer volume of Monte Carlo samples and real physics data already has surpassed 54 petabytes worldwide. Nebraska alone has downloaded 900 terabytes during the past calendar year. All this data movement has been done via commodity servers running open-source software.
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