High Description
It seems there is a memory usage issue when creating an uncompressed ROOT file that contains a TTree in a multi-thread application.
Here is a short TDataFrame snippet that writes 4 million events from multiple threads. Events contain one single branch of type std::vector<double> with 100 values (0 to 99).
#include <ROOT/TDataFrame.hxx>
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int main() |
{
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ROOT::EnableImplicitMT();
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ROOT::Experimental::TDataFrame d(4000000);
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auto gencol = []() {
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std::vector<double> v; |
v.reserve(100);
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for (int i = 0; i < 100; ++i) |
v.emplace_back(i);
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return v; |
};
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ROOT::Experimental::TDF::TSnapshotOptions opts;
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opts.fCompressionLevel = 0;
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d.Define("col0", gencol).Snapshot<std::vector<double>>("t", "ofile.root", {"col0"}, opts); |
return 0; |
}
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Multi-thread (8 threads) and uncompressed file:
32 seconds, ~4GB max RAM usage
Single-thread and uncompressed file:
22 seconds, 200MB max RAM usage
Multi-thread (8 threads) and compressed file:
4 seconds, 400MB max RAM usage
Single-thread and compressed file:
10 seconds, 250MB max RAM usage
I was able to somehow replicate the issue without TDataFrame. Here is the snippet:
#include <ROOT/TBufferMerger.hxx>
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#include <TFile.h>
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#include <TROOT.h>
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#include <TTree.h>
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static void Fill(TTree *tree, int count) |
{
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std::vector<double> col0; |
tree->Branch("col0", &col0); |
for (int i = 0; i < count; ++i) { |
col0 = []() {
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std::vector<double> v; |
v.reserve(100);
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for (int i = 0; i < 100; ++i) |
v.emplace_back(i);
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return v; |
}();
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tree->Fill();
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}
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tree->ResetBranchAddresses();
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}
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int main() |
{
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constexpr int nThreads = 8; |
constexpr int evtsPerThread = 4000000 / 8; |
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ROOT::EnableImplicitMT();
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ROOT::Experimental::TBufferMerger merger("testwrite.root", "RECREATE", /*compress=*/0); |
std::vector<std::thread> threads; |
for (int i = 0; i < nThreads; ++i) { |
threads.emplace_back([&merger]() {
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auto myfile = merger.GetFile();
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auto mytree = new TTree("t", "t"); |
mytree->ResetBit(kMustCleanup);
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Fill(mytree, evtsPerThread);
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myfile->Write();
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});
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}
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for (auto &t : threads) |
t.join();
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return 0; |
}
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Multi-thread (8 threads) and uncompressed file:
26 seconds, ~8GB max RAM usage
Multi-thread (8 threads) and compressed file:
4 seconds, 570MB max RAM usage
Single-thread execution does not make sense with the second snippet, but I tried calling EnableThreadSafety instead of EnableImplicitMT, so that TTree::Fill is not executed in IMT mode.
noIMT, 8 threads, uncompressed file:
36 seconds, ~9GB max RAM usage
noIMT, 8 threads, compressed file:
4 seconds, 490MB max RAM usage
