Jean Sexton

Affiliation and Research Interests

I am a member of the Center for Computational Sciences and Engineering (CCSE) group in the Computing Sciences Directorate at the Lawrence Berkeley National Laboratory. I am interested in improving the scalable, performance portable efficiency of simulations involving time integration methods. I am currently working on AMReX and supporting Nyx.

Much of my work is open source and can be followed on my GitHub page.

Previous Work

As a postdoctoral researcher in CCSE I worked on improving efficiency for simulations involving time integration methods, including the AMReX-based applications  Nyx  and Castro. I primarily worked on the Nyx N-body hydro code for computational cosmology. Cosmological simulations allow us to investigate physical parameters which are not directly observable. 

This work included porting Nyx to GPU's, and transitioning to a fully C++ code for performance portability and maintainability. More efficient numerical methods and improved parallelism allow for more highly refined and complex models. Part of this work was algorithmic improvements, including adapting known splitting methods to a cosmological setting. Specifically, I improved the numerical coupling between the hydrodynamic solver and the integration of the heating and cooling source terms as well as the overall algorithm efficiency. I implemented a spectral deferred correction coupling strategy (which accounts for the comoving frame) in order to improve Nyx's heating-cooling integration efficiency and robustness. 

I received my doctorate in Computational and Applied Mathematics from Southern Methodist University in late 2017. My dissertation research fits broadly in the applied mathematics fields of scientific computing and numerical analysis. Specifically, I focused on the development of numerical methods for the time integration of problems with multiple characteristic time scales. These methods are motivated by multiphysics, multiscale real-world application problems which are constructed by coupling physical processes with potential disparate length and time scales together. I developed a family of efficient, fully coupled fourth-order multirate method with comparable stability properties to leading existing third-order multirate methods. These methods were based on existing Recursive Flux-Splitting Multirate methods using Generalized Additive Runge-Kutta theory to analyze order conditions.

Publications

• Jean Sexton, Zarija Lukic, Ann Almgren, Chris Daley, Brian Friesen, Andrew Myers, and Weiqun Zhang. “Nyx: A Massively Parallel AMR Code for Computational Cosmology” Journal of Open Source Software, 6(63), 3068, https://doi.org/10.21105/joss.03068
• Max P. Katz, Ann Almgren, Maria Barrios Sazo, Kiran Eiden, Kevin Gott, Alice Harpole, Jean M. Sexton, Don E. Willcox, Weiqun Zhang, and Michael Zingale. 2020. “Preparing nuclear astrophysics for exascale” In Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis (SC '20). IEEE Press, Article 91, 1–12. arXiv:2007.05218 (2020).
• Ann Almgren, Maria Barrios Sazo, John Bell, Alice Harpole, Max Katz, Jean Sexton, Donald Willcox, Weiqun Zhang, and Michael Zingale. “CASTRO: A Massively Parallel Compressible Astrophysics Simulation Code.” Journal of Open Source Software, 5(54), 2513, https://doi.org/10.21105/joss.02513
• J. M. Sexton, D. R. Reynolds. "Relaxed Multirate Infinitesimal Step Methods: an extension of Multirate Infinitesimal Step Methods and Recursive Flux-Splitting Multirate Methods." [arxiv]
• W. Thompson, S. McGinnis, D. McDaniel, J.M. Sexton, R. Pettit, S. Anderson, M. C. Jackson, K. Sellers. "A Geographical and Statistical Analysis of Childhood Leukemia Deaths Relating to the Locations of Nuclear Power Plants." Advances and Applications in Statistical Sciences 6.5 pp 313-328, 2011. [link]

Presentations

• J. M. Sexton. “Sundials User Breakout: Nyx.” Presentation at the Exascale Computing Project Annual Meeting 2021.
• J. M. Sexton. “A Deferred Correction Coupling Strategy for Cosmological Simulations.” Presentation at the SIAM Conference on Computational Science and Engineering 2021. 
• Zingale, M., Harpole, A., Katz, M., Nonaka, A., Sexton, J.,  Willcox, D. (2021). “Improved Coupling of Hydro and Reactions Using Spectral Deferred Corrections”. Bulletin of the AAS, 53(1). Presentation at American Astronomical Society Meeting 2021.
• J. M. Sexton. “AMReX Update” Virtual ExaSky F2F Meeting 2020.
• J. M. Sexton. “A Deferred Correction Coupling Strategy for Cosmological Simulations.” AMS Western Sectional Meeting 2020. (Cancelled)
• J. M. Sexton, D. R. Reynolds. Efficient Multirate Methods from High Order. Minisymposium talk at the SIAM Conference on Computational Science and Engineering 2019.
• J. M. Sexton. A Deferred Correction Coupling Strategy for Cosmological Simulations. Contributed talk at the IEEE WIE International Leadership Summit 2018.
• J. M. Sexton, D. R. Reynolds. "High-order Relaxed Multirate Infinitesimal Step Methods for Multiphysics Applications." Contributed talk at the Texas Applied Mathematics and Engineering Symposium 2017.
• D. R. Reynolds, C. S. Woodward, D. J. Gardner, J. M. Sexton. "Flexible and Accurate Multiphysics Time Integration with ARKode." Presentation at the SIAM Conference on Computational Science and Engineering 2017.
• J. M. Sexton,D. R. Reynolds. "An Optimal Multirate Method for Climate Applications." Poster Presentation at the SIAM Conference on Mathematics of Planet Earth 2016