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Computers are no longer getting faster: instead, they are growing more and more CPUs, each of which is no faster than the previous generation. This increase in the number of cores evidently calls for more parallelism in HENP software. If end-users' stand-alone analysis applications are relatively easy to modify, LHC experiments frameworks, being mostly written with a single 'thread' of execution in mind and consequent code bases, are on the other hand more challenging to parallelize. Widespread and inconsiderate changes so close to data taking are out of the equation: we need clear strategies and guidelines to reap the benefits out of the multicore/manycore era while minimizing the code changes. Exploiting parallelism is usually achieved via a) multithreading or b) multiple processes (or a combination of both) but each option has its o...
Comment: 4 pages, 2 figures, contribution to the Conference on Computing in High Energy and Nuclear Physics (CHEP06), 13.-17. Feb 2006, Mumbai, India
The ATLAS high-level trigger (HLT) system provides software-based event selection after the initial LVL1 hardware trigger. It is composed of two stages, the LVL2 trigger and the event filter (EF). The LVL2 trigger performs event selection with optimized algorithms using selected data guided by Region of Interest pointers provided by the LVL1 trigger. Those events selected by LVL2 are built into complete events, which are passed to the EF for a further stage of event selection and classification using off-line algorithms. Events surviving the EF selection are passed for off-line storage. The two stages of HLT are implemented on processor farms. The concept of distributing the selection process between LVL2 and EF is a key element in the architecture, which allows it to be flexible to changes (luminosity, detector knowledge, background c...
Event data from proton-proton collisions at the LHC will be selected by the ATLAS experiment in a three-level trigger system, which, at its first two trigger levels (LVL1+LVL2), reduces the initial bunch crossing rate of 40 MHz to $sim$3 kHz. At this rate, the Event Builder collects the data from the readout system PCs (ROSs) and provides fully assembled events to the Event Filter (EF). The EF is the third trigger level and its aim is to achieve a further rate reduction to $sim$200 Hz on the permanent storage. The Event Builder is based on a farm of O(100) PCs, interconnected via a Gigabit Ethernet to O(150) ROSs. These PCs run Linux and multi-threaded software applications implemented in C++. All the ROSs, and substantial fractions of the Event Builder and Event Filter PCs have been installed and commissioned. We report on performance...
To cope with the 40 MHz event production rate of LHC, the trigger of the ATLAS experiment selects events in three sequential steps of increasing complexity and accuracy whose final results are close to the offline reconstruction. The Level-1, implemented with custom hardware, identifies physics objects within Regions of Interests and operates with a first reduction of the event rate to 75 kHz. The higher trigger levels, Level-2 and Level-3, provide a software based event selection which further reduces the event rate to about 100 Hz. This paper presents the algorithm (/spl mu/Fast) employed at Level-2 to confirm the muon candidates flagged by the Level-1. /spl mu/Fast identifies hits of muon tracks inside the barrel region of the Muon Spectrometer and provides a precise measurement of the muon momentum at the production vertex. The alg...
The ATLAS experiment at the Large Hadron Collider (LHC) will face the challenge of efficiently selecting interesting candidate events in pp collisions at 14 TeV center of mass energy, while rejecting the enormous number of background events, stemming from an interaction rate of up to 10/sup 9/ Hz. The Level1 trigger will reduce this rate to around /spl Oscr/(100kHz). Subsequently, the high level trigger (HLT), which is comprised of the Second Level trigger and the Event Filter, will need to reduce this rate further by a factor of /spl Oscr/(10/sup 3/). The HLT selection is software based and will be implemented on commercial CPUs using a common framework built on the standard ATLAS object oriented software architecture. In this paper an overview of the current implementation of the selection for electrons and photons in the HLT is give...
The ATLAS experiment at the Large Hadron Collider (LHC) will face the challenge of efficiently selecting interesting candidate events in$pp$collisions at 14 TeV center-of-mass energy, whilst rejecting the enormous number of background events. The High-Level Trigger (HLT$=$second level trigger and Event Filter), which is a software based trigger will need to reduce the level-1 output rate of$approx75$kHz to$approx200$Hz written out to mass storage. In this talk an overview of the current physics and system performance of the HLT selection for electrons and photons is given. The performance has been evaluated using Monte Carlo simulations and has been partly demonstrated in the ATLAS testbeam in 2004. The efficiency for the signal channels, the rate expected for the selection, the global data preparation and execution times will be highl...
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