Performance and TSelectorDraw

ROOT
1

Hello,
we would appreciate some help to improve the performance of our code. A short description of the problem:

We are using root version 5.22 and TTrees containing several std:vector of variable size (typically of size less than 10).

We create histograms using TSelectorDraw in the following way:

TSelectorDraw* thisSelector = new TSelectorDraw();
TList* input = new TList();
input->Add(new TNamed(“varexp”,varexp));
input->Add(new TNamed(“selection”,selection));
thisSelector->SetInputList(input);

where “varexp” denotes the variable in the tree and histogram to be filled, such as:
“jetPt+>>+histogramName”

“selection” is a string containing cut statements such as
jetPt>30 && TMath::Abs(jetEta)<2.4 && triggerHLTJet15U==1 && 3dDistance>-0.5

where jetPt, jetEta, etc. are again std::vectors<> of same length.

The TTree is then processed with something like:

TFile* thisFile = TFile::Open( … );
TTree* thisTree = (TTree*)thisFile->Get(“Events”);
Long64_t nentries = thisTree->GetEntries();
thisTree->Process(thisSelector,“goff”,nentries,0);

Trees typically contain several millions of events and the program runs several hours at 100% CPU which is not satisfactory.

The profiler (gprof) tells us that most of the CPU time is spent in TSelectorDraw::Process.

Do you have any hints or recommendations how to increase performance? Would a manual event loop or a different data structure (fixed size arrays instead of vectors) improve anything?

Thanks for your help in advance,
best,
alex

2

Simply create a TSelector class with the standard functions Begin, Process, Terminate and implement your selection as pure C++ code. Run your selector with ACLIC.
Concerning the data model, if your std::vector are in general less than 10 entries, I suggest to replace them with simple C style arrays like
int fN; //vector length
float *x; //[fN]
float *y; //[fN]

etc
In this way you will likely gain at least a factor 2 compared to std::vector and a better compression factor.

Rene

3

Dear Rene,
thanks for your suggestion. I will try switching to C style arrays.

Do I understand correctly that you are suggesting to implement our cuts as pure C++ code? Unfortunately this is not an option, we sometimes need quite complex cut queries and need high flexibility. Implementing a separate TSelector for all use cases would be too much.

We could implement a TSelector and compile the cuts using TTreeFormula (or similar), but as I understand this would not be too different from using TSelectorDraw, which does the same thing, right?

thanks again for your help.
best,
alex

4

up to you, anything going via TTreeFormula will be much slower than direct C++ code in your selector.
And in your selector you can easily branch to different algorithms/selections when required.

Rene