warn on tbb::global_control interferences

warn when the number of threads set by the user is limited at runtime by tbb::global_control. Fix for github issue root-project#6363: root-project#6363
xvallspl · Jan 12, 2021 · ee2c8c2 · ee2c8c2
1 parent 361704b
commit ee2c8c2
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diff --git a/core/imt/src/RTaskArena.cxx b/core/imt/src/RTaskArena.cxx
@@ -6,6 +6,7 @@
 #include <mutex>
 #include <thread>
 #include "tbb/task_arena.h"
+#include "tbb/global_control.h"
 
 //////////////////////////////////////////////////////////////////////////
 ///
@@ -75,6 +76,10 @@ RTaskArenaWrapper::RTaskArenaWrapper(unsigned maxConcurrency) : fTBBArena(new tb
       Warning("RTaskArenaWrapper", "CPU Bandwith Control Active. Proceeding with %d threads accordingly", bcCpus);
       maxConcurrency = bcCpus;
    }
+   if (maxConcurrency > tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism)) {
+      Warning("RTaskArenaWrapper", "tbb::global_control is active, limiting the number of parallel workers"
+                                   "from this task arena available for execution.");
+   }
    fTBBArena->initialize(maxConcurrency);
    fNWorkers = maxConcurrency;
    ROOT::EnableThreadSafety();

diff --git a/core/imt/src/TThreadExecutor.cxx b/core/imt/src/TThreadExecutor.cxx
@@ -155,6 +155,12 @@ TThreadExecutor::TThreadExecutor(UInt_t nThreads)
 void TThreadExecutor::ParallelFor(unsigned int start, unsigned int end, unsigned step,
                                   const std::function<void(unsigned int i)> &f)
 {
+   if (GetPoolSize() > tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism)) {
+      Warning("TThreadExecutor::ParallelFor",
+              "tbb::global_control is limiting the number of parallel workers."
+              " Proceeding with %zu threads this time",
+              tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism));
+   }
    fTaskArenaW->Access().execute([&] {
       tbb::this_task_arena::isolate([&] {
          tbb::parallel_for(start, end, step, f);
@@ -171,6 +177,12 @@ void TThreadExecutor::ParallelFor(unsigned int start, unsigned int end, unsigned
 double TThreadExecutor::ParallelReduce(const std::vector<double> &objs,
                                        const std::function<double(double a, double b)> &redfunc)
 {
+   if (GetPoolSize() > tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism)) {
+      Warning("TThreadExecutor::ParallelReduce",
+              "tbb::global_control is limiting the number of parallel workers."
+              " Proceeding with %zu threads this time",
+              tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism));
+   }
    return fTaskArenaW->Access().execute([&] { return ROOT::Internal::ParallelReduceHelper<double>(objs, redfunc); });
 }