Abstract
During the long LHC shutdown, ATLAS experiment is preparing several fundamental changes to its offline event processing framework and analysis model. These include moving to multi-threaded reconstruction and simulation and reducing data duplication during derivation analysis by producing a combined mini-xAOD stream. These changes will allow ATLAS to take advantage of the higher luminosity at Run 3 without overstraining processing and storage capabilities. They also require significant improvements to the underlying event store and the I/O framework to support them. These improvements include: 1) an overhaul of the Run 2 I/O framework to be thread-safe and minimize serial bottlenecks, 2) introduction of new immutable references for object navigation, which don’t rely on storage container entry number so data can be merged in-memory, 3) using filter decisions to annotate combined output stream to allow for fast event selection on input and 4) selecting optimized compression algorithms and settings to allow efficient reading of event selections.
Highlights
The ATLAS[1] experiment is one of the 4 experiments collecting data from the LHC at CERN
In this paper we describe the developments in the ATLAS Event Store and the I/O framework that were done to meet the new requirements
The I/O framework had to be adapted to accept overlapping write requests coming from the Stores, providing thread safety and allowing as much concurrency as possible in order to ensure good performance
Summary
The ATLAS[1] experiment is one of the 4 experiments collecting data from the LHC at CERN. Each consecutive Run generates an increasing amount of ever more complex physics data that needs to be processed. In order to keep up with the increasing processing needs, the experiments modify their software frameworks to utilize the available CPU resources in the most efficient way. The I/O components have to evolve as well - to handle the resulting higher data rates and to function effectively in the multi-threaded environment. The upcoming LHC Run 3 and especially Run 4 will require significant software performance improvements. In this paper we describe the developments in the ATLAS Event Store and the I/O framework that were done to meet the new requirements
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