onnxruntime/docs/NotesOnThreading.md

# 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
* static version of NumThreads

**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.
Threadpool related changes. (#3564) Threadpool related changes. Don't create ORT threadpool if openmp is enabled (except for inter op threadpool). Created a new static function ThreadPool::NumThreads to account for openmp settings and null threadpool ptr. Log a warning when using SetIntraOpNumThreads when openmp is enabled. Added a document for ORT devs. Fix LSTM to use the new threadpool abstractions. Rename GetNumCpuCores to GetThreadAffinityMasks and move it to the Env class. Co-authored-by: Tracy Sharpe <tracysh@microsoft.com> 2020-04-21 16:57:39 +00:00			`# 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`
			`* static version of NumThreads`

			`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.