Abstract

The CMS experiment at the LHC uses a two-stage trigger system, with events flowing from the first level trigger at a rate of 100 kHz. These events are read out by the Data Acquisition system (DAQ), assembled in memory in a farm of computers, and finally fed into the high-level trigger (HLT) software running on the farm. The HLT software selects interesting events for offline storage and analysis at a rate of a few hundred Hz. The HLT algorithms consist of sequences of offline-style reconstruction and filtering modules, executed on a farm of 0(10000) CPU cores built from commodity hardware. Experience from the 2010–2011 collider run is detailed, as well as the current architecture of the CMS HLT, and its integration with the CMS reconstruction framework and CMS DAQ. The short- and medium-term evolution of the HLT software infrastructure is discussed, with future improvements aimed at supporting extensions of the HLT computing power, and addressing remaining performance and maintenance issues.

Highlights

  • THE CMS [1] trigger and data acquisition system [2] (Fig. 1) is designed to cope with unprecedented luminosities and interaction rates

  • These events are read out by the Data Acquisition system (DAQ), assembled in memory in a farm of computers, and fed into the high-level trigger (HLT) software running on the farm

  • The HLT algorithms consist of sequences of offline-style reconstruction and filtering modules, executed on a farm of 0(10000) CPU cores built from commodity hardware

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Summary

INTRODUCTION

THE CMS [1] trigger and data acquisition system [2] (Fig. 1) is designed to cope with unprecedented luminosities and interaction rates. At the LHC design luminosity of 1034cm-2s-1, and bunch-crossing rates of 40 MHz, an average of about 20 to 40 interactions take place at each bunch crossing. Two trigger levels are employed in CMS: the Level-1 Trigger (L1T), implemented using custom electronics reduces the initial event rate by a factor of 100 [3] using custom electronics. Events accepted by the Level-1 are read-out and assembled by the DAQ Event Builder (EVB) [4]. High Level Trigger (HLT) analyzes complete CMS events at the Level-1 accept rate of 100 kHz. The HLT provides further rate reduction by analyzing full-granularity detector data, using software reconstruction and filtering algorithms on a large computing cluster consisting of commercial processors, the Event Filter Farm. In this paper we describe recent experience with the CMS HLT during collision runs, as well as ongoing and planned development of the system

TRIGGER AND DAQ GENERAL ARCHITECTURE
HIGH LEVEL TRIGGER ARCHITECTURE
RECENT EXPERIENCE WITH THE CMS HLT
EVOLUTION OF THE HLT INFRASTRUCTURE
Inter-Process Communication
Further Isolation of Physics Algorithms
Deployment of New Hardware
Findings
SUMMARY
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