[io] Implement TFile performance metrics (Sparseness, Randomness)

[JasMehta08]

This Pull request:

Changes or fixes:

This PR implements new I/O performance metrics in TFile to assist users in analyzing data access patterns, particularly for optimizing jobs on high-latency storage systems (e.g., EOS, XRootD).

• Added protected members fSumSkip and fLastReadEnd to TFile.h (transient) to track seek distances.

• Instrumented TFile::ReadBuffer in TFile.cxx to calculate the "skip distance" (absolute difference between the current position and the end of the previous read).

• Implemented public getters: GetSparseness() and GetRandomness().

• Metrics Definitions:

  • Sparseness: Total Bytes Read / Total File Size. Indicates how much of the file was actually consumed.

  • Randomness: Sum(Skip Distances) / Total Bytes Read. A higher value indicates more seeking per byte read (inefficient access).

  • Transactions: Uses existing GetReadCalls().

• Updated TFile::Print() to display these three metrics (Transactions, Sparseness, Randomness) when the file is open.

Verification :

void test_io_metrics() {
   const char* filename = "io_metrics_test.root";
   auto f = TFile::Open(filename, "RECREATE");
   TTree *t = new TTree("t", "test tree");
   int val;
   t->Branch("val", &val, "val/I");
   for(int i=0; i<1000; i++) { val = i; t->Fill(); }
   t->Write();
   f->Close();
   delete f;

   std::cout << "\n Test 1: Sequential Read" << std::endl;
   auto fSeq = TFile::Open(filename);
   TTree *tSeq = (TTree*)fSeq->Get("t");
   
   tSeq->SetCacheSize(1000000); 
   tSeq->AddBranchToCache("*");
   
   int count = 0;
   TTreeReader rSeq(tSeq);
   TTreeReaderValue<int> vSeq(rSeq, "val");
   while(rSeq.Next()) { count++; }
   
   fSeq->Print();
   
   if (fSeq->IsA()->GetMethodAllAny("GetRandomness")) {
       double randSeq = fSeq->GetRandomness();
       std::cout << "Sequential Randomness: " << randSeq << std::endl;
       
       std::cout << "\n Test 2: Random Jump Read" << std::endl;
       auto fRand = TFile::Open(filename);
       TTree *tRand = (TTree*)fRand->Get("t");
       
       tRand->SetCacheSize(0); 
       
       tRand->GetEntry(0);
       tRand->GetEntry(900);
       tRand->GetEntry(50);
       
       fRand->Print();

       double randRand = fRand->GetRandomness();
       std::cout << "Random Randomness: " << randRand << std::endl;
       
       if (randRand > randSeq) {
          std::cout << "SUCCESS" << std::endl;
       } else {
          std::cout << "FAILURE" << std::endl;
       }
   }
}

Output :

Processing ../test_io_metrics.C...

 Test 1: Sequential Read
TFile: name=io_metrics_test.root, title=, option=READ
  IO Performance : Transactions=4     , Sparseness=0.7817, Randomness=0.0000
******************************************************************************
*Tree    :t         : test tree                                              *
*Entries :     1000 : Total =            4885 bytes  File  Size =       1869 *
*        :          : Tree compression factor =   2.69                       *
******************************************************************************
*Br    0 :val       : val/I                                                  *
*Entries :     1000 : Total  Size=       4544 bytes  File Size  =       1510 *
*Baskets :        1 : Basket Size=      32000 bytes  Compression=   2.69     *
*............................................................................*
Sequential Randomness: 0

 Test 2: Random Jump Read
TFile: name=io_metrics_test.root, title=, option=READ
  IO Performance : Transactions=5     , Sparseness=1.0000, Randomness=0.0341
******************************************************************************
*Tree    :t         : test tree                                              *
*Entries :     1000 : Total =            4889 bytes  File  Size =       1869 *
*        :          : Tree compression factor =   2.69                       *
******************************************************************************
*Br    0 :val       : val/I                                                  *
*Entries :     1000 : Total  Size=       4548 bytes  File Size  =       1510 *
*Baskets :        1 : Basket Size=      32000 bytes  Compression=   2.69     *
*............................................................................*
Random Randomness: 0.0341188
SUCCESS

Checklist:

• tested changes locally

This PR fixes #20853