Writing ROOT Data in Parallel with TBufferMerger

Experiments at the Large Hadron Collider (LHC) produce tens of petabytes of new data in ROOT format per year that need to be processed and analysed. In the next decade, following the planned upgrades of the LHC and its detectors, this data production rate is expected to increase at least ten-fold. Therefore, optimizing the ROOT I/O subsystem is of critical importance to the success of the LHC physics programme. This contribution presents ROOT’s approach for writing data from multiple threads to a single output file in an efficient way. Technical aspects of the implementation—the TBufferMerger class—and programming model examples are described. Measurements of runtime performance and the overall improvement relative to the case of serial data writing are also discussed