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`TChain::GetEntries` internal caching ineffective?

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#23,738 2 commenti 0 reazioni 1 assegnatario Vedi su GitHub

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@pcanal ci sta già lavorando.

Dal 16/10/2026.

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Descrizione

improvement in:TTree
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  • Checked for duplicates
Description

We experienced some longer-than-expected delays when using Podio (internally using a TChain) when switching between the files of a chain. Claude traced this to our, maybe too paranoid, check in the event loop to see if the current entry is in the range of the chain before we access it, e.g. something like:

for (Long64_t i = 0; i < nEntries; ++i)
         if (i < ch.GetEntries())
            ch.GetEntry(i);

which causes us to spend up to 50% of the total runtime in TChain::GetEntries for essentially empty event loops.

Indeed, it looks like the internal caching machinery for fEntries in GetEntries is reset by TChain::LoadTree (called by GetEntry) here:
https://github.com/root-project/root/blob/fbcf83fe9a58e65f36ba55bdc3821a7983f4799a/tree/tree/src/TChain.cxx#L1559

This is causing the event loop to open many more files than expected, which can be seen when running the reproducer below as root -q repro_chain_getentries.C, yielding

Processing repro_chain_getentries.C...
files in chain: 10   files opened in loop: 55
files in chain: 100   files opened in loop: 5050
files in chain: 1000   files opened in loop: 500500

This becomes especially painful when the files are on EOS...

Reproducer
#include <TChain.h>
#include <TFile.h>
#include <TTree.h>
#include <iostream>

void repro_chain_getentries()
{
   {
      TFile file("chain_ge.root", "RECREATE");
      TTree t("t", "t");
      t.Fill();
      t.Write();
   }

   for (int nFiles : {10, 100, 1000}) {
      TChain ch("t");
      for (int f = 0; f < nFiles; ++f)
         ch.Add("chain_ge.root");
      const auto nEntries = ch.GetEntries();

      const auto opensBefore = TFile::GetFileCounter();
      for (Long64_t i = 0; i < nEntries; ++i)
         // Some (paranoid?) bounds check
         if (i < ch.GetEntries())
            ch.GetEntry(i);
      std::cout << "files in chain: " << nFiles
                << "   files opened in loop: " << TFile::GetFileCounter() - opensBefore << '\n';
   }
}
ROOT version
   ------------------------------------------------------------------
  | Welcome to ROOT 6.40.04                        https://root.cern |
  | (c) 1995-2025, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Oct 07 2026, 01:16:29                 |
  | From tags/6-40-04@6-40-04                                        |
  | With g++ (GCC) 16.1.0 std202302                                  |
  | Try '.help'/'.?', '.demo', '.license', '.credits', '.quit'/'.q'  |
   ------------------------------------------------------------------
Installation method

source /cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh

Operating system

Alma9

Additional context

No response

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