ATLAS TDAQ RoI Builder and the Level 2 Supervisor system

Abstract

The ATLAS High Level Trigger (HLT) uses information from the hardware based Level 1 Trigger system to guide the retrieval of information from the readout system. The Level 1 Trigger elements (jet, electromagnetic, muon candidate, etc.) determine Regions of Interest (RoIs) that seed further trigger decisions. This paper describes the device the RoI Builder (RoIB) that collects these data from the Level 1 Trigger and the Level 2 Supervisors (L2SV) Farm that makes these data available to the HLT. The status of the system design and the results of the tests and integration into ATLAS TDAQ system are presented. I. REGION OF INTEREST CONCEPT AND ROIB/L2SV SYSTEM PROTOTYPING The Level 1 Trigger system (LVL1) identifies a number of RoIs for the HLT spatially limited areas ('roads') in the detector with candidates for phenomena to be triggered. The information from Level 1 includes the selected trigger type and the details of where in η and φ the trigger objects (e/γ, μ, etc.) that caused the event to be accepted originated. Using this information as guidance, specialized algorithms request a sub-set of the event data from the ROSs to perform the event selection. In this way only a few per cent of the event data need to be transferred initially to the HLT system — thus considerably reducing the network bandwidth required. The initial ideas and requirements are summarized in [1, 2]. The RoIB and L2SV system have been prototyped a number of times, have undergone a series of design reviews and tests, and have been integrated with individual parts of the Level 1 Trigger system and the HLT. The Region of Interest Builder (RoIB), located in the underground counting room, takes raw event fragments from various Level 1 Trigger sources, assembles all the fragments of a given event into an RoI record, selects a target Level 2 Supervisor (L2SV) processor in a farm, and then sends the RoI record to this processor. From there the RoI records are distributed to the High Level Trigger (HLT) processors that require them for further event selection and disposition. The Level 2 Supervisor farm, located in the surface TDAQ computing farm barracks, is comprised of commercial rack mounted PCs. They are connected to the RoIB via ATLAS standard S-LINKs and to the High Level Trigger via Ethernet. Each Level 2 Supervisor processor is thus subjected to a fractiono f the Level 1 accept rate avoiding issues of hight input and output bandwidth. The overview of the RoIB/L2SV system is shown in Fig.1. The baseline implementation of the RoIB is a VMEbus system which includes a Single Board Computer (SBC) for configuration, control and monitoring. It is composed of two stages: input and assembly. The input stage consists of Input Cards that receive and buffer the RoI fragments. These cards subsequently send the RoI fragments to Builder Cards in the assembly stage where the RoI fragments are assembled into RoI records. Each Builder Card can service up to four supervisor processors. The number of builder cards within the system is not limited and is dictated by the rate that a supervisor processor can sustain. Figure 1: RoIB/L2SV system overview A prototype of the RoIB was built and tested during the course of 1999. It was based on a pair of 12U VMEbus cards, an Input Card capable of handling six S-LINK inputs, and a pair of Builder Cards able to output to a pair of supervisor processes. This implementation utilized 76 Altera 10K40 FPGAs and 8 10K50 FPGAs. The system and early performance measurements are documented in [3]. Testing has shown that combining RoI fragments from several sources using an FPGAbased device is feasible and that a L2SV consisting of four 300 MHz Pentium II PCs is sufficient to receive the RoIB output rate of 100 kHz. Subsequent tests with prototypes of the muon Central Trigger Processor Interface and the Central Trigger Processor ROD modules of the LVL1 system have made a start on debugging the component interfaces and have further demonstrated that data input to RoIB could be achieved at the required rates [4].