The ATLAS High Level Trigger Steering Framework and the Trigger 
Configuration System.

The ATLAS High Level Trigger Steering Framework and the Trigger Configuration System. The ATLAS detector system installed in the Large Hadron Collider (LHC) at CERN is designed to study proton-proton and nucleus-nucleus collisions with a maximum center of mass energy of 14 TeV at a bunch collision rate of 40MHz. In March 2010 the four LHC experiments saw the first proton-proton collisions at 7 TeV. Still within the year a collision rate of nearly 10 MHz is expected. At ATLAS, events of potential interest for ATLAS physics are selected by a three-level trigger system, with a final recording rate of about 200 Hz. The first level (L1) is implemented in custom hardware; the two levels of the high level trigger (HLT) are software triggers, running on large farms of standard computers and network devices. Within the ATLAS physics program more than 500 trigger signatures are defined. The HLT tests each signature on each L1-accepted event; the test outcome is recorded for later analysis. The HLT-Steering is responsible for this. It foremost ensures the independent test of each signature, guarantying unbiased trigger decisions. Yet, to minimize data readout and execution time, cached detector data and once-calculated trigger objects are reused to form the decision. Some signature tests are performed only on a scaled-down fraction of candidate events, in order to redu ce the output rate and further limit the execution time. For some signatures it is important to physics analysts to know the would-be decision of that test when it was scaled out. For this the HLT-Steering is equipped with a test-after-accept feature. The HLT-Steering receives the setup of the signatures from the trigger configuration system. This system dynamically provides the online setup for the L1 and HLT, e.g. L1 trigger thresholds and multiplicities, and HLT algorithm configuration and trigger objects selection criteria. It also archives the trigger configuration for analysis, which is crucial for understanding trigger efficiencies. The configuration system uses a relational database (TriggerDB) to store all configuration information. A graphical user interface, the Trigger Tool, is provided to interact with the TriggerDB. Services are provided to access configuration data for online, offline, and Monte Carlo processing. A web interface to the TriggerDB is available to the ATLAS user to look at the trigger configuration remotely. This poster will introduce the ATLAS trigger with focus on the trigger configuration, steering, and resource monitoring system. It will point out the implications of the system design for physics analysis. It will also present performance figures from the first year of successful operation with proton-proton and heavy ion coll i sions.