Throttle OpenMP stress tests to 2 threads for CRAN (#3, part 2/2)

NEWS.md

@@ -6,6 +6,10 @@
   is now stored by value and re-thrown afresh instead of going through
   `std::exception_ptr` (#3).
 
+* The OpenMP thread-safety tests use at most 2 threads and respect
+  `OMP_THREAD_LIMIT`, so CRAN no longer sees "OMP: Warning #96: Cannot
+  form a team" (#3).
+
 # RcppTrust 0.2.0
 
 * The package license is now GPL-2, because the BOBYQA/NEWUOA ports and

src/solver_test_objfuns.cpp

@@ -132,7 +132,8 @@ List solver_openmp_stress_test(std::string solver, int n, int nStarts,
 
   std::vector<int> threadIds(nStarts, 0);
 #ifdef _OPENMP
-  int usedThreads = std::max(1, std::min(nThreadsRequested, omp_get_max_threads()));
+  int usedThreads = std::max(1, std::min({nThreadsRequested, omp_get_max_threads(),
+                                          omp_get_thread_limit()}));
 #pragma omp parallel for num_threads(usedThreads) schedule(static)
 #endif
   for (int i = 0; i < nStarts; i++) {

src/trust_test_objfuns.cpp

@@ -124,7 +124,10 @@ List trust_openmp_stress_test(int nStarts, int nThreadsRequested) {
   std::vector<int> threadIds(nStarts, -1);
 
 #ifdef _OPENMP
-  int usedThreads = std::min(nThreadsRequested, omp_get_max_threads());
+  // Also respect OMP_THREAD_LIMIT (CRAN allows 2 threads), so the runtime
+  // never has to shrink the team and warn.
+  int usedThreads = std::min({nThreadsRequested, omp_get_max_threads(),
+                              omp_get_thread_limit()});
   if (usedThreads < 1) usedThreads = 1;
 #pragma omp parallel for num_threads(usedThreads) schedule(static)
 #endif

tests/testthat/test-c-objfun-threadsafety.R

@@ -6,7 +6,11 @@
 # (not thread-safe) and keeps no shared mutable state across calls.
 
 test_that("trust_solve_c() gives identical results run sequentially vs. across OpenMP threads", {
-  res <- RcppTrust:::trust_openmp_stress_test(nStarts = 64L, nThreadsRequested = 8L)
+  # Two threads: the most CRAN allows, and enough to exercise concurrency.
+  res <- RcppTrust:::trust_openmp_stress_test(
+    nStarts = 64L,
+    nThreadsRequested = 2L
+  )
 
   expect_equal(res$maxArgDiff, 0)
   expect_equal(res$maxValDiff, 0)