Hello,
in G4.9.3 (and before), the function G4PrimaryTransformer::GimmePrimaries use the following loop:

  for( G4int i=0; i<n_vertex; i++ )
  { GenerateTracks( anEvent->GetPrimaryVertex(i) ); }

with 
      inline G4PrimaryVertex* GetPrimaryVertex(G4int i=0)  const
      {
        if( i == 0 )
        { return thePrimaryVertex; }
        else if( i > 0 && i < numberOfPrimaryVertex )
        {
          G4PrimaryVertex* primaryVertex = thePrimaryVertex;
          for( G4int j=0; j<i; j++ )
          {
            if( primaryVertex == 0 ) return 0;
            primaryVertex = primaryVertex->GetNext();
          }
          return primaryVertex;
        }
        else
        { return 0; }
      }

So the loop inside GetPrimaryVertex goes through every vertex each time it is called, up to the requested one. It is not that bad with a low mulitplicity, but when it gets high, the time grows too fast.

I propose the following modification, in order to not have to use two loops :


G4TrackVector* G4PrimaryTransformer::GimmePrimaries(G4Event* anEvent,G4int trackIDCounter)
{
  trackID = trackIDCounter;

  //TV.clearAndDestroy();
  for( size_t ii=0; ii<TV.size();ii++)
  { delete TV[ii]; }
  TV.clear();
  G4int n_vertex = anEvent->GetNumberOfPrimaryVertex();
  if(n_vertex==0) return 0;
  G4PrimaryVertex *previousVertex=0, *newVertex=0;
  for( G4int i=0; i<n_vertex; i++ )
  {
    newVertex= anEvent->GetPrimaryVertex(i, previousVertex);
    GenerateTracks( newVertex );
    previousVertex = newVertex;
  }
  return &TV;
}

and 

inline G4PrimaryVertex* GetPrimaryVertex(G4int i=0, G4PrimaryVertex* lastVertex=0)  const
{
 if( i == 0)
 { return thePrimaryVertex; }
 else if( i > 0 && i < numberOfPrimaryVertex && lastVertex != 0)
 {
   return lastVertex->GetNext();
 }
 else
 { return 0; }
}

I checked that it gives the same result (hopefully, I did not oversee anything). For a multiplicity of 100000 photons, it increased the speed of computation by a factor 15 !

Regards,
Pierre