Speaker
Roger Jones
(Lancaster University (GB))
Description
The ATLAS experiment has successfully used its Gaudi/Athena software
framework for data taking and analysis during the first LHC run, with
billions of events successfully processed. However, the design of
Gaudi/Athena dates from early 2000 and the software and the physics
code has been written using a single threaded, serial design. This
programming model has increasing difficulty in exploiting the
potential of current CPUs, which offer their best performance only
through taking full advantage of multiple cores and wide vector
registers. Future CPU evolution will intensify this trend, with core
counts increasing and memory per core falling. Maximising performance
per watt will be a key metric, so all of these cores must be used as
efficiently as possible.
In order to address the deficiencies of the current framework, ATLAS
has embarked upon two projects: first, a practical demonstration of
the use of multi-threading in our reconstruction software, using the
GaudiHive framework; second, an exercise to gather requirements for an
updated framework, going back to the first principles of how event
processing occurs.
In this paper we report on both these aspects of our work. For the
hive based demonstrators, we discuss what changes were necessary in
order to allow the serially designed ATLAS code to run, both to the
framework and to the tools and algorithms used. We report on what
general lessons were learned about the code patterns that had been
employed in the software and which patterns were identified as
particularly problematic for multi-threading. These lessons were fed
into our considerations of a new framework and we present preliminary
conclusions on this work. In particular we identify areas where the
framework can be simplified in order to aid the implementation of a
concurrent event processing scheme. Finally, we discuss the practical
difficulties involved in migrating a large established code base to a
multi-threaded framework and how this can be achieved for LHC Run 3.
Primary authors
Benjamin Michael Wynne
(University of Edinburgh (GB))
Charles Leggett
(Lawrence Berkeley National Lab. (US))
Graeme Andrew Stewart
(University of Glasgow (GB))
Roger Jones
(Lancaster University (GB))
