Description: This change makes three changes to the ThreadPool class to clean up issues identified during performance analysis and optimization. (1) It uses mm_pause intrinsics in spin loops, helping avoid consuming pipeline resources while waiting. (2) It re-organizes the spin-then-steal loop for work distribution to start out spinning as intended, rather than to start out trying to steal. (3) It updates the ThreadPool class's API to be consistent in the use of static methods for public functions. The PR includes minor doc updates and corresponding changes to test cases.
Motivation and Context
The change helps ensure consistency in behavior between the OpenMP and Eigen-based implementations. Unlike the instance methods, the static methods abstract over the different ways in which threading can be implemented; they will map onto the OpenMP or Eigen-based implementations when threading is used. When threading is not used they will run work sequentially.
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.
* Add TrySimpleParallerFor so that there's a path with OpenMP awareness for SimpleParallelFor. Makes it consistent with [Try]BatchParallelFor and [Try]ParallelFor.
Update TopK to check for the number of threads better, and to use TrySimpleParallelFor.
* Update doco to mention TrySimpleParallelFor
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>
