This PR updates the ThreadPool API to support multi-loop parallel sections. As with the OpenMP "parallel" construct, this allows per-loop work to be amortized over a series of loops. For ORT, it also promotes locality between successive loops in the sense that iteration X of one loop will tend to run on the same worker thread as iteration X of preceding loops.
The change was developed while optimizing the implementation of a model that performed better with OpenMP. Profiling indicated that OpenMP was providing lower loop entry/exit costs and that, via OpenMP's static scheduling, it was leading to a lower L2 miss rate in the series of parallel loops used in GRU.
The main changes are:
- Addition of ThreadPool::ParallelSection and underlying support in the modified Eigen thread pool.
- In EigenNonBlockingThreadPool.h, refactoring the RunInParallel method to support two variants: one that takes an existing parallel section object created by the caller, and another (used by default) that creates its own parallel section.
- Simplify ThreadPool::LoopCounter (used by worker threads to claim loop iterations), basing it an ID supplied by the underlying Eigen thread pool for affinity in a series of loops.
- Fix a possible perf issue where a loop with iterations scheduled in batches would have more threads than batches available.
- Use of parallel sections in the GRU operator.
- Additional test cases in threadpool_test.h.
- Additional comments at the top of threadpool.h and EigenNonBlockingThreadPool.h.
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>
