[Openmp-commits] [PATCH] D158416: Adding a throttling threshold to bound dependent tasking memory footprint

[Romain PEREIRA via Phabricator via Openmp-commits]

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>From "5. Discussions - Task Throttling" of this paper <https://hal.science/hal-04136674v1/>

Task Throttling is a runtime mechanism to reduce tasking operational and memory overheads. 
Once at threshold is reached, producer threads stop producing and start consuming tasks instead.

LLVM runtime implements a threshold on **(1) the number of ready-tasks per thread.**
It was implemented in the context of independent tasking (OpenMP-3.0, 2008), with a motivation being to provide a memory consumption bound.
In the context of dependent tasking (OpenMP-4.0, 2013), this threshold is not sufficient to bound memory consumption: as many (not ready) successor tasks may be created.

This patch introduces a new threshold: **(2) the total number of tasks** and an environment variable `KMP_TASK_MAXIMUM` to configure its value (set to 65,536); providing strong warranties on memory consumption.
The patch also adds the `KMP_TASK_MAXIMUM_READY` to configure the threshold **(1)** (set to 256).

Few points that came to my mind that may need discussions:

- a) thresholds default values
- b) induced overheads: an atomic variable is written by any threads on each task allocation/deallocation - maybe a compile-time option could be added to disabled **(2)**-throttling entirely preserving the current behavior

If other points come to your mind, please let me know


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D158416

Files:
  openmp/runtime/src/kmp.h
  openmp/runtime/src/kmp_global.cpp
  openmp/runtime/src/kmp_settings.cpp
  openmp/runtime/src/kmp_tasking.cpp
  openmp/runtime/test/tasking/omp_throttling_max.c
  openmp/runtime/test/tasking/omp_throttling_max_ready.c

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