Modern HPC systems are placing increasing demands on job schedulers due to their scale and novel hardware. El Capitan’s Rabbit nodes exemplify this challenge: unlike traditional systems where storage is remote and shared, Rabbit nodes wire local NVMe SSDs directly to compute nodes via PCIe, forcing schedulers to actively track storage topology, capacity, and cross-job persistence, concerns they were never designed to handle. We introduce Flux Fiction, a fully plugin-based HPC system emulator built on top of Flux that replays historical job traces to evaluate scheduling policies in Flux. We validate Flux Fiction against the LLNL Tuolumne cluster using two workloads across four queueing policies, achieving a P99-bounded slowdown error below 1 in 7 of 8 experiments and a maximum utilization error of 1.2%. We then use Flux Fiction to explore Rabbit storage scheduling, demonstrating its ability to explore novel scheduling scenarios.

GEOtiled enables efficient computation of high-resolution terrain parameters from digital elevation models (DEMs) by decomposing large regions into smaller, parallelizable tiles. Originally developed as GEOtiled-G with support for three parameters (Slope, Aspect, Hillshade) via GDAL, we present GEOtiled-SG, an enhanced version that integrates the SAGA GIS library to compute over 15 parameters, expanding its utility in Earth science. To offset SAGA’s computational overhead, GEOtiled-SG introduces three optimizations: concurrent DEM cropping, buffer-aware mosaicking, and unified concurrency across workflow stages. Evaluations show that GEOtiled-SG maintains GEOtiled-G’s performance on the original parameters and offers consistent speedups across the expanded set. The framework is open source on GitHub, with data hosted on Dataverse, supporting reproducible, scalable terrain analysis.

We present performance scaling and optimization of iPIC3D - an exascale-class, GPU-enabled implicit particle-in-cell code for planetary-scale magnetosphere modeling and plasma simulation - on El Capitan. Our strong and weak scaling studies demonstrate near-linear scaling up to 8,000 nodes (32,000 APUs) with a parallel efficiency of nearly 100%. We optimize iPIC3D to leverage AMD’s MI300A APUs, the Merced Lustre filesystem, and Rabbit nodes for high-bandwidth I/O. Optimizations reduce memory usage by 97% and runtime by 74%, enabling simulations that are 1.8 times larger than before. Rabbit further improve checkpointing bandwidth by 2 times, ensuring scalable fault- tolerant simulations on exascale architectures.

El Capitan, currently the world's largest supercomputer at 1.742 Ex-aflop/s, introduces challenges in scheduling due to its scale and innovative rabbit nodes, which traditional schedulers cannot efficiently handle. Flux, a resource and job management system, handles dynamic resource allocation tailored for exascale systems through its graph-based scheduler, Fluxion. This work introduces the Flux Emulator, a tool designed to test scheduling policies in Fluxion without impacting production systems. The emulator plugs into the real components of Flux and Fluxion to mimic job execution, emulate resource usage, and collect information on how the job behaves. Preliminary tests show negligible overhead introduced by the emulator and demonstrate its effectiveness in evaluating scheduli ng policies, like conservative backfilling, in a fraction of the time required with a real system.

Efficient checkpoint restoration is critical in high-performance computing (HPC) and AI applications, where slow recovery times disrupt workflows, waste resources, and hinder reproducibility. This work introduces a Merkle tree checkpoint restoration method to accelerate failure recovery and improve explainability. Our method integrates asynchronous I/O via the Liburing library to optimize scattered reads in HPC applications. Tested on the Polaris system at Argonne National Laboratory, it exhibits lower restoration time and memory consumption than state-of-the-art checkpoint restoration methods, reaching near-full efficiency with duplicated data. Our work advances scalable and efficient checkpointing solutions for HPC, ensuring reliable and fast failure recovery for large-scale simulations.

Modern supercomputer architectures are diverse and becoming increasingly complex. Scientists are constantly porting code and re-optimizing it for the new architecture, but achieving good performance is challenging. Performance portability programming models such as RAJA, Kokkos, and OpenMP enable codes to maintain a single-source code rather than rewriting for each target architecture. However, portability models alone will not result in optimal performance as hardware has varying specifications (e.g., cache sizes and speeds) and parallel algorithms may use varying amounts of memory and compute resources. We present a systematic analysis of application behaviors across a diverse set of CPU and GPU hardware. We leverage the RAJA Performance Suite, which contains a curated set of kernels commonly found in HPC applications, to perform an in-depth GPU and memory analysis as well as a quantitative performance portability evaluation across different compute platforms. In analyzing the performance portability scores, we identify gaps and opportunities to achieve consistent performance across platforms. We provide a comprehensive analysis across seven architectures, including the most recent GPU systems with new physical memory layouts, where kernels demonstrate a runtime speedup of up to 44 ×. Although the speedup highlights the baseline improvements of newer hardware, the performance portability scores calculated, ranging from 0% to 92%, showcase where opportunities remain for scientists to increase utilization of the newer systems.

Deep Learning (DL) is increasingly applied across various fields to solve complex scientific challenges in modern high-performance computing (HPC) systems that are beyond the reach of traditional algorithms. Training DL models for scientific applications involves processing multi-terabyte datasets in each epoch. The data access behavior during DL training exposes optimization opportunities to cache these datasets in near-compute storage accelerators in HPC systems, enhancing I/O throughput. However, current middleware solutions employ near-compute storage accelerators primarily as exclusive caches, which limits the effectiveness of cache access locality. To address this problem, we introduce DYAD, a system designed to maximize sample locality in the cache, thereby significantly increasing I/O throughput in HPC systems.DYAD optimizes I/O for DL training based on three key features. First, DYAD boosts inter-node access speeds by using a novel streaming RPC with RDMA protocol, achieving a 1.25x performance gain over state-of-the-art solutions. Second, DYAD further enhances inter-node access by coordinating data movement, which mitigates network congestion and increases throughput for inter-node accesses by up to 8.78x. Last, DYAD uses smart metadata caching that outperforms traditional global metadata access methods by several orders of magnitude in terms of lookup throughput. We demonstrate how DYAD accelerates large-scale DL training on a high-end HPC cluster with 512 GPUs by up to 10.82x faster epochs compared to UnifyFS by performing locality-aware caching on near-compute storage accelerators.

Interactive visualization can support fluid exploration but is often limited to predetermined tasks. Scripting can support a vast range of queries but may be more cumbersome for free-form exploration. Embedding interactive visualization in scripting environments, such as computational notebooks, provides an opportunity to leverage the strengths of both direct manipulation and scripting. We investigate interactive visualization design methodology, choices, and strategies under this paradigm through a design study of calling context trees used in performance analysis, a field which exemplifies typical exploratory data analysis workflows with Big Data and hard to define problems. We first produce a formal task analysis assigning tasks to graphical or scripting contexts based on their specificity, frequency, and suitability. We then design a notebook-embedded interactive visualization and validate it with intended users. In a follow-up study, we present participants with multiple graphical and scripting interaction modes to elicit feedback about notebook-embedded visualization design, finding consensus in support of the interaction model. We report and reflect on observations regarding the process and design implications for combining visualization and scripting in notebooks.

Thicket is an open-source Python toolkit for Exploratory Data Analysis (EDA) of multi-run performance experiments. It enables an understanding of optimal performance configuration for large-scale application codes. Most performance tools focus on a single execution (e.g., single platform, single measurement tool, single scale). Thicket bridges the gap to convenient analysis in multi-dimensional, multi-scale, multi-architecture, and multi-tool performance datasets by providing an interface for interacting with the performance data. Thicket has a modular structure composed of three components. The first component is a data structure for multi-dimensional performance data, which is composed automatically on the portable basis of call trees, and accommodates any subset of dimensions present in the dataset. The second is the metadata, enabling distinction and sub-selection of dimensions in performance data. The third is a dimensionality reduction mechanism, enabling analysis such as computing aggregated statistics on a given data dimension. Extensible mechanisms are available for applying analyses (e.g., top-down on Intel CPUs), data science techniques (e.g., K-means clustering from scikit-learn), modeling performance (e.g., Extra-P), and interactive visualization. We demonstrate the power and flexibility of Thicket through two case studies, first with the open-source RAJA Performance Suite on CPU and GPU clusters and another with a large physics simulation run on both a traditional HPC cluster and an AWS Parallel Cluster instance.

As computational science applications benefit from larger-scale, more heterogeneous high performance computing (HPC) systems, the process of studying their performance becomes increasingly complex. The performance data analysis library Hatchet provides some insights into this complexity, but is currently limited in its analysis capabilities. Missing capabilities include the handling of relational caller-callee data captured by HPC profilers. To address this shortcoming, we augment Hatchet with a Call Path Query Language that leverages relational data in the performance analysis of scientific applications. Specifically, our Query Language enables data reduction using call path pattern matching. We demonstrate the effectiveness of our Query Language in identifying performance bottlenecks and enhancing Hatchet's analysis capabilities through three case studies. In the first case study, we compare the performance of sequential and multi-threaded versions of the graph alignment application Fido. In doing so, we identify the existence of large memory inefficiencies in both versions. In the second case study, we examine the performance of MPI calls in the linear algebra mini-application AMG2013 when using MVAPICH and Spectrum-MPI. In doing so, we identify hidden performance losses in specific MPI functions. In the third case study, we illustrate the use of our Query Language in Hatchet's interactive visualization. In doing so, we show that our Query Language enables a simple and intuitive way to massively reduce profiling data.

Exploiting task parallelism is getting increasingly difficult for diverse and complex scientific workflows running on High Performance Computing (HPC) systems. In this paper, we argue that the difficulty rises from a void in the spectrum of existing data-transfer models for resolving inter-task data dependence within a workflow and propose a novel model to fill that gap: Ubique. The Ubique model combines the best from in-transit and in situ models in order for loosely coupled producer and consumer tasks to run concurrently and to resolve their data dependencies efficiently with little or no modifications to their codes, striking a balance between transparent optimization, productivity, and performance. Our preliminary evaluation suggests that Ubique can significantly outperform the parallel file system (PFS)-based model while offering automatic data transfer and synchronization which are the features lacking in many traditional models. It also identifies the performance characteristics of its key depending subsystems, which must be understood for further broadening its benefits.