OMPNUMTHREADS

OMPNUMTHREADS is commonly encountered as a shorthand reference to the OpenMP environment variable that controls the default number of threads used in parallel regions. The standard and portable name is OMP_NUM_THREADS; OMPNUMTHREADS is not part of the OpenMP specification and is typically not recognized by conforming implementations. Users should rely on OMP_NUM_THREADS for predictable behavior.

What it does: OMP_NUM_THREADS sets the default thread count for parallel regions that do not specify a num_threads clause. If it is not set, the runtime selects a value, often related to the system’s hardware concurrency. The value can affect the performance of parallel regions, particularly on systems with a high ratio of cores to memory bandwidth. The effect can be altered by dynamic adjustment settings or by nested parallelism if enabled.

How to set: The variable can be defined before launching a program, for example by exporting OMP_NUM_THREADS=4 in a Unix-like shell or setting OMP_NUM_THREADS=4 in Windows. It can also be changed from within code by calling omp_set_num_threads(4) in C/C++ or by using equivalent interfaces in other languages. A specific parallel region can override the default by providing a num_threads clause, such as #pragma omp parallel num_threads(2).

Scope and interactions: OMP_NUM_THREADS applies to the current process and its children unless overridden. It interacts with other OpenMP controls, such as OMP_DYNAMIC and OMP_NESTED, and can be overridden by per-region specifications. Nested parallelism, when enabled, can introduce additional teams with their own thread counts.

Compatibility: Supported by major OpenMP implementations (GCC/LLVM, Intel, IBM, MSVC, and others). Users should ensure the correct variable name and understand how region-level directives interact with the global setting to achieve reproducible performance.

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