Extending the IceCube DAQ system by integration of the generic high-speed sorter module TESS

In the extreme environment of Antarctica at the South Pole, the IceCube experiment, the world's first kilometer-scale neutrino telescope, collects cosmic ray events. IceCube consists of over 5000 digital optical sensor modules (DOMs) deployed on 86 instrumentation lines each extending 2.5 km deep in the antarctic ice. The array of optical modules monitors the Cherenkov light emitted by passing radiation, which, when digitized and timestamped to nanosecond precision, is used as input to sophisticated reconstruction algorithms that determine the direction, energy, and type of the incident cosmic ray event. In order to achieve this goal, the IceCube data acquisition system merges the digital data streams from each photodetector into a single time-ordered list which is presented to online triggers that determine, in realtime, whether or not a given pattern of hits is noise or signal. At the present time, the data provided to the triggers is limited by the performance of sorting and merging algorithms: the 500 Hz raw data rate from each sensor (2.5 MHz array aggregate rate) is beyond the capability of the central sort and merge. The current solution adopted by the IceCube detector is to impose a hardware-based pre-trigger coincidence on hits emanating from the DOMs which reduces the rate by a factor of 20. While this pre-trigger coincidence has negligible impact on the detector sensitivity for the principal goal of high-energy neutrinos from galactic or extragalactic sources, other low-energy physics searches are affected. This presentation details work done to develop and implement a system, TESS, which is capable of merging the full raw data stream being produced by the IceCube DOMs. TESS is designed as a pipelined architecture with three major modules: server, selector and the client glued together by circular buffers. The three modules runs in only three threads and since the architecture is self synchronizing and uses no data copying maximum performance can be achieved for global sorting of payloads. The TESS sorting architecture was originally designed to provide a globally sorted data stream for triggers targeting low-energy events from annihilation of hypothesized dark-matter particles, however its utility is generalizable to any IceCube trigger which requires inspection of the full data stream. The IceCube online supernova detection system is a notable example. Moreover, the algorithm is generic to any system involving multiple, independently sorted data streams which must be merged into a single sorted data stream.