BOLT: Optimizing OpenMP Parallel Regions with User-Level Threads

OpenMP is widely used by a number of applications, computational libraries, and runtime systems. As a result, multiple levels of the software stack use OpenMP independently of one another, often leading to nested parallel regions. Although exploiting such nested parallelism is a potential opportunit...

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Bibliographic Details
Published in:Proceedings / International Conference on Parallel Architectures and Compilation Techniques pp. 29 - 42
Main Authors: Iwasaki, Shintaro, Amer, Abdelhalim, Taura, Kenjiro, Seo, Sangmin, Balaji, Pavan
Format: Conference Proceeding
Language:English
Published: IEEE 01.09.2019
Subjects:
Concurrency control
Fasteners
Libraries
Multithreading
OpenMP
Parallel processing
Runtime
Runtime Systems
Software
Task analysis
User Level Threads
ISSN:2641-7936
Online Access:Get full text
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Summary:OpenMP is widely used by a number of applications, computational libraries, and runtime systems. As a result, multiple levels of the software stack use OpenMP independently of one another, often leading to nested parallel regions. Although exploiting such nested parallelism is a potential opportunity for performance improvement, it often causes destructive performance with leading OpenMP runtimes because of their reliance on heavyweight OS-level threads. User-level threads (ULTs) are more lightweight alternatives but existing ULT-based runtimes suffer from several shortcomings: 1) thread management costs remain significant and outweigh the benefits from additional parallelism; 2) the shift to ULTs often hurts the more common flat parallelism case; and 3) absence of user control over thread-to-CPU binding, a critical feature on modern systems. This paper presents BOLT, a practical ULT-based OpenMP runtime system that efficiently supports both flat and nested parallelism. This is accomplished on three fronts: 1) advanced data reuse and thread synchronization strategies; 2) thread coordination that adapts to the level of oversubscription; and 3) an implementation of the modern OpenMP thread-to-CPU binding interface tailored to ULT-based runtimes. The result is a highly optimized runtime that transparently achieves similar performance compared with leading state-of-the-art widely used OpenMP runtimes under flat parallelism, while outperforming all existing runtimes under nested parallelism.
ISSN:2641-7936
DOI:10.1109/PACT.2019.00011