class TTreePerfStats: public TVirtualPerfStats

                       TTreePerfStats

        TTree I/O performance measurement. see example of use below.

 The function FileReadEvent is called from TFile::ReadBuffer.
 For each call the following information is stored in fGraphIO
     - x[i]  = Tree entry number
     - y[i]  = 1e-6*(file position)
     - ey[i] = 1e-9*number of bytes read
 For each call the following information is stored in fGraphTime
     - x[i]  = Tree entry number
     - y[i]  = Time now
     - ey[i] = readtime, eg timenow - start
 The TTreePerfStats object can be saved in a ROOT file in such a way that
 its inspection can be done outside the job that generated it.

       Example of use
 {
   TFile *f = TFile::Open("RelValMinBias-GEN-SIM-RECO.root");
   T = (TTree*)f->Get("Events");
   Long64_t nentries = T->GetEntries();
   T->SetCacheSize(10000000);
   T->SetCacheEntryRange(0,nentries);
   T->AddBranchToCache("*");

   TTreePerfStats *ps= new TTreePerfStats("ioperf",T);

   for (Int_t i=0;i<nentries;i++) {
      T->GetEntry(i);
   }
   ps->SaveAs("cmsperf.root");
 }

 then, in a root interactive session, one can do:
    root > TFile f("cmsperf.root");
    root > ioperf->Draw();
    root > ioperf->Print();

 The Draw or Print functions print the following information:
   TreeCache = TTree cache size in MBytes
   N leaves  = Number of leaves in the TTree
   ReadTotal = Total number of zipped bytes read
   ReadUnZip = Total number of unzipped bytes read
   ReadCalls = Total number of disk reads
   ReadSize  = Average read size in KBytes
   Readahead = Readahead size in KBytes
   Readextra = Readahead overhead in percent
   Real Time = Real Time in seconds
   CPU  Time = CPU Time in seconds
   Disk Time = Real Time spent in pure raw disk IO
   Disk IO   = Raw disk IO speed in MBytes/second
   ReadUZRT  = Unzipped MBytes per RT second
   ReadUZCP  = Unipped MBytes per CP second
   ReadRT    = Zipped MBytes per RT second
   ReadCP    = Zipped MBytes per CP second

   NOTE1 : The ReadTotal value indicates the effective number of zipped bytes
           returned to the application. The physical number of bytes read
           from the device (as measured for example with strace) is
           ReadTotal +ReadTotal*Readextra/100. Same for ReadSize.

   NOTE2 : A consequence of NOTE1, the Disk I/O speed corresponds to the effective
           number of bytes returned to the application per second.
           The Physical disk speed is DiskIO + DiskIO*ReadExtra/100.