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Create N-1 threads in a thread pool when configured with intra-op parallelism of N. This ensures we have N active threads, given that the main thread also runs work. To avoid ambiguity on the value returned, rename ThreadPool::NumThreads method to ThreadPool::DegreeOfParallelism, and make corresponding updates in MLAS and operators.
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Notes on Threading in ORT
This document is intended for ORT developers.
ORT allows the usage of either OpenMP or non-OpenMP (ORT) threads for execution. Threadpool management is abstracted behind: (1) ThreadPool class in threadpool.h and (2) functions in thread_utils.h.
When developing an op, please use these abstractions to parallelize your code. These abstractions centralize 2 things. When OpenMP is enabled, they resort to using OpenMP. When OpenMP is disabled they resort to sequential execution if the threadpool ptr is NULL or schedule the tasks on the threadpool otherwise.
Examples of these abstractions are: (threadpool.h has more documentation for these)
- TryBatchParallelFor
- TryParallelFor
- TrySimpleParallelFor
- DegreeOfParallelism
Please do not write #ifdef pragma omp in operator code.
For intra op parallelism ORT users can use either OpenMP or ORT threadpool. The choice of using OpenMP is indicated by building ORT with --use_openmp switch. For inter op parallelism, however, we always use the ORT threadpool.