The Evolution of the ATLAS Region of Interest Builder

The ATLAS trigger system is deployed to reduce the event rate from the Large Hadron Collider bunch crossing frequency of 40 MHz to 1 kHz for permanent storage using a tiered system. In the PC trigger farm decisions are seeded by Regions of Interest found by the custom hardware trigger system. The Regions of Interest are collected and distributed to the farm at 100 kHz by the ATLAS Region of Interest Builder. The Evolution of the Region of Interest Builder from a crate of custom VME-based electronics to a commodity PC hosting a single custom PCIe card has been undertaken to increase the system performance, flexibility, and ease maintenance. The functionality and performance of the Region of Interest Builder previously only possible using FPGAs and a custom backplane VME Crate, has now been implemented in a multi-threaded C++ software library interfaced to a single PCIe card with one Xilinx Vertex 6 FPGA. The PC-based system was installed in the ATLAS Data Acquisition system between the 2015 and 2016 data taking periods. This presentation describes the requirements and functionality of the ATLAS Region of Interest Builder, and the PC-based implementation as well as measurement results from controlled test setups and in situ operation of the ATLAS experiment.