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Clamp GetThreadLimit by the live OpenMP thread limit #1224

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Clamp GetThreadLimit by the live OpenMP thread limit #1224

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17 changes: 15 additions & 2 deletions src/core/include/utils/parallel.h
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Original file line number Diff line number Diff line change
Expand Up @@ -108,10 +108,23 @@ class ParallelControls {
#endif
}

// @Brief returns min of int n and machineThreads
// @Brief returns the thread count a parallel region should use: the
// minimum of the requested n, machineThreads, and the live OpenMP limit
// omp_get_max_threads().
//
// The live-limit clamp is what makes runtime thread control effective.
// machineThreads is latched once from omp_get_max_threads() at static
// init and never refreshed, yet every hot region emits
// num_threads(GetThreadLimit(...)) -- a clause that overrides the runtime
// ICV. Clamping only by machineThreads therefore ignores a subsequent
// omp_set_num_threads() (including one made through SetNumThreads() below),
// so the library cannot be capped in-process. Also clamping by the live
// omp_get_max_threads() restores that.
int GetThreadLimit(int n) const {
#ifdef PARALLEL
return n > machineThreads ? machineThreads : n;
int lim = n > machineThreads ? machineThreads : n;
int live = omp_get_max_threads();
return lim > live ? live : lim;
#else
return 1;
#endif
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