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An Investigation of using FleCSI for Monte Carlo Radiation Transport

This document details the attempt to use FleCSI to provide MPI parallelization and domain decomposition for a Monte Carlo radiation transport application. FleCSI is a framework designed to support multi-physics application development with a focus on task-based parallelism and domain decomposition [1]. FleCSI also supports performance portability via a wrapper around a back-end performance portability layer. The goal of this work is to use FleCSI for domain decomposition and MPI parallelization inside of a Monte Carlo radiation transport application and document any pain points and shortcomings. The following section introduce the nomenclature used in FleCSI and its potential benefit for physics code developers, detail the code used to explore using FleCSI in a Monte Carlo radiation transport solver, and list the issues and concerns discovered during the work.

36 MATERIALS SCIENCE↗

User-Defined Types as Task Parameters and Arguments in FleCSI

FleCSI is a C++ framework for writing multi-physics simulations. FleCSI abstracts away complicated backends, enabling high performance while eliminating the need for users to directly manage memory and execution. This is done via FleCSI’s data and execution model.

97 MATHEMATICS AND COMPUTING↗

A view into FleCSI [Slides]

FleCSI is largely based around sharing of memory spaces. FleCSI facilitates parallel execution and sharing of memory across memory spaces. Execute() transforms sender-side data structures into receiver-side data structures.

97 MATHEMATICS AND COMPUTING↗

FleCSI Intro 2.0 [Slides]

FleCSI is a programming system or framework for developing mulitphysics application codes. FleCSI provides two primary capabilities--runtime abstraction layer and topology data structures.

97 MATHEMATICS AND COMPUTING↗

HARD: A performance portable radiation hydrodynamics code based on FleCSI framework

Hydrodynamics And Radiation Diffusion (HARD) is an open-source application for high-performance simulations of compressible hydrodynamics with radiation-diffusion coupling. Built on the FleCSI (Bergen et al., 2021 [1]) (Flexible Computational Science Infrastructure) framework, HARD expresses its computational units as tasks whose execution can be orchestrated by multiple back-end runtimes, including Legion (Bauer et al., 2012 [2]), MPI (Forum, 1994 [3]), and HPX (Kaiser et al., 2020 [4]). Node-level parallelism is handled through Kokkos (Edwards et al., 2014 [5]), providing a single-source, portable code base that runs efficiently on laptops, small homogeneous clusters, and the largest heterogeneous supercomputers currently available. To ensure scientific reliability, HARD includes a regression test suite that automatically reproduces canonical verification problems such as the Sod and LeBlanc shock tubes, and the Sedov blast wave, comparing numerical solutions against known analytical results. The project is distributed under an OSI-approved license, hosted on GitHub, and accompanied by reproducible build scripts and continuous integration workflows. This combination of performance portability, verification infrastructure, and community-focused development makes HARD a sustainable platform for advancing radiation hydrodynamics research across multiple domains.

97 MATHEMATICS AND COMPUTING↗

FleCSI status

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Computer Science↗

FleCSI Developments

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Computer Science↗

LANL ASC FY23 L2 Milestone Description and Completion Criteria [Slides]

In early 2021, the ASC Program decided that Ristra should provide a production code (Moya) for LANL’s mission in the Low Energy-Density Physics application space. At the same time, Ristra should design an environment for rapid development of new codes that can answer new questions on new hardware. We started FY22 by expanding upon our R&D efforts of high-order DG methods for hydrodynamics; Technical & staffing challenges caused us to abandon this for a traditional FV SGH approach, but leveraging modern techniques. Significant changes in FleCSI (v1.4→2.x) have us writing code anew, in a co-design effort with the FleCSI and FleCSI specialization projects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

(U) Radiation Hydrodynamics in the Ristra Project’s Symphony code: Initial release

One of the recent focuses in the next-generation code (NGC/Ristra)project at LANL is to develop a cell-centered Lagrangian radiation-hydrodynamics code under FleCSI framework. The FleCSI frame-work provides a unified data and execution model in order to insulate production code development from the uncertainty of evolving runtimeand architectures. We have developed the code, Symphony, which in-corporates the hydrodynamics code, FleCSALE, and the radiation transport code, Puno.This memo is intended to describe the coupling of radiation and hydrodynamics within the Ristra Project’s code Symphony.

97 MATHEMATICS AND COMPUTING↗

Summer Internship Project: Set Topology [Slides]

Why a Set Topology? 1. Expand the array of tools given to physicists, educators, and students. 2. Through usages of PIC, MPM, and Monte Carlo. 3. My work at LANL was specified to handle requirements for the usage of MPM with FleCSI. 4. What we want it to do - phase change, simulating plasma, and other Multiphysics applications. 5. In FleCSI we already have mesh structures, we needed a way to represent those particles and certain behavior's of these particles that clients desire.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

HARD

Hydrodynamics And Radiative Diffusion Solver based on FleCSI framework

Loiseau, Julien↗

flecsolve

FleCSI Linear Algebra Package

Reisner, Andrew↗

Ristra Project FY23 L2 Milestone Report, Rev.1: MRT #8541: Multiphysics Scaling on EAS-3

The findings of this report were used to close out the ATDM milestone MRT# 8541, which was designed to demonstrate readiness of ATDM multiphysics codes for mission-relevant work on ATS-4, El Capitan. To this end, the closure criteria were to run a 3D shaped charge problem at scale up to 50% of the El Capitan early-access system, RZVernal (AMD Trento CPUs and AMD MI-250X GPUs), demonstrate scalability, and document challenges with the software stack and environment. LANL’s approach to this milestone was to test our modular software capability by developing an entirely new code, Moya, built upon our FleCSI framework. The physics capability and the GPU infrastructure needed for the shaped charge problem on GPUs was added to Moya, and the required calculations were performed at scale. Moya showed good scaling without any fine-tuning of GPU kernels; there is still significant room for performance enhancements, especially for the Legion backend. Tied up in this L2 milestone was a closeout of KPP-3s for the ECP ST Projects at LANL; this material will be covered in a separate document.

97 MATHEMATICS AND COMPUTING↗