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On the Limits of Performance Portability in Directive-Based GPU Programming

Alessandro Romeo, Nitin Shukla, Stefano Truzzi, Alessio Suriano 2026-06-14
Exascale OpenMP Performance Portability HPC Compiler Runtime × GPU

The problem is that directive-based GPU programming faces fundamental trade-offs between performance, portability, and productivity when transitioning scientific applications to exascale systems. The method involved porting the production-grade magnetohydrodynamics code gPLUTO from OpenACC to OpenMP and evaluating its performance on NVIDIA A100 and AMD MI250X devices. Experimental evidence shows that while OpenACC and OpenMP achieve comparable performance on NVIDIA platforms, the same OpenMP implementation is approximately three times slower at the application level on AMD MI250X, with kernel-level slowdowns reaching up to 47x due to strided memory-access patterns, compiler limitations, and register pressure from C++ abstractions. This matters because it demonstrates that achieving portable performance across GPU architectures requires not only application-level changes but also continued advances in compiler backends and architecture-aware optimization strategies.

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GF-DiT: Scheduling Parallelism for Diffusion Transformer Serving

Xinwei Qiang, Yifan Hu, Shixuan Sun, Jing Yang 2026-06-14
Transformer GPU Runtime × Scheduling Parallelism × Serving

The problem is that existing Diffusion Transformer (DiT) serving systems use static parallelism for each request, which is inefficient due to heterogeneity across requests, execution stages, and system conditions. GF-DiT introduces a policy-programmable runtime that dynamically adapts parallelism via an asynchronous execution abstraction and group-free collectives for low-overhead online GPU reallocation. Experimental evaluation in vLLM-Omni shows GF-DiT improves throughput by up to 6.01×, reduces mean latency by up to 95%, and lowers SLO violation rates by up to 90% compared to fixed-pipeline execution. This matters because it enables efficient, elastic DiT serving that treats GPU parallelism as a schedulable resource, significantly improving performance and service quality for image and video generation workloads.

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