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[tree] warn when forking multiprocess #21240

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Merged
dpiparo merged 1 commit into from
Feb 13, 2026
Merged

[tree] warn when forking multiprocess #21240

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11 changes: 11 additions & 0 deletions tree/treeplayer/src/TTreeProcessorMP.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,9 @@
#include "ROOT/TTreeProcessorMP.hxx"
#include "TMPWorkerTree.h"

#include <thread>
static const std::thread::id MAIN_THREAD_ID = std::this_thread::get_id();

//////////////////////////////////////////////////////////////////////////
///
/// \class ROOT::TTreeProcessorMP
Expand Down Expand Up @@ -104,6 +107,14 @@ TList *TTreeProcessorMP::Process(TTree &tree, TSelector &selector, TEntryList &e
jFirst = 0;
}

// Warn about forking of already threaded program
if (std::this_thread::get_id() != MAIN_THREAD_ID) {
Warning("Process",
"multiprocessing from an already multi-threaded program "
"(first thread '%zu' vs current thread id '%zu') might lead to inefficient forking",
std::hash<std::thread::id>{}(MAIN_THREAD_ID), std::hash<std::thread::id>{}(std::this_thread::get_id()));
}

//prepare environment
Reset();
UInt_t nWorkers = GetNWorkers();
Expand Down
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