  The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will
soon carry out an unprecedented wide, fast, and deep survey of the sky in
multiple optical bands. The data from LSST will open up a new discovery space
in astronomy and cosmology, simultaneously providing clues toward addressing
burning issues of the day, such as the origin of dark energy and and the nature
of dark matter, while at the same time yielding data that will, in turn, pose
fresh new questions. To prepare for the imminent arrival of this remarkable
data set, it is crucial that the associated scientific communities be able to
develop the software needed to analyze it. Computational power now available
allows us to generate synthetic data sets that can be used as a realistic
training ground for such an effort. This effort raises its own challenges --
the need to generate very large simulations of the night sky, scaling up
simulation campaigns to large numbers of compute nodes across multiple
computing centers with different architectures, and optimizing the complex
workload around memory requirements and widely varying wall clock times. We
describe here a large-scale workflow that melds together Python code to steer
the workflow, Parsl to manage the large-scale distributed execution of workflow
components, and containers to carry out the image simulation campaign across
multiple sites. Taking advantage of these tools, we developed an extreme-scale
computational framework and used it to simulate five years of observations for
300 square degrees of sky area. We describe our experiences and lessons learned
in developing this workflow capability, and highlight how the scalability and
portability of our approach enabled us to efficiently execute it on up to 4000
compute nodes on two supercomputers.
