Re: Cloning trees with TChain: drastic performance loss

Hi Alexander,

Instead of

     Long64_t ientry = collChain->LoadTree(jentry);
     collChain->GetEntry(ientry);

you meant:

     Long64_t ientry = collChain->LoadTree(jentry);
     collChain->GetEntry(jentry);

or

     Long64_t ientry = collChain->LoadTree(jentry);
     collChain->GetTree()->GetEntry(ientry);

The value returned by LoadTree is the entry number within the current TTree and must be used only on the TBranch object or on the underlying TTree.

The code as-is is not only slow but also only look at the data in the first TTree.

Cheers,
Philippe.

On 5/16/11 10:58 AM, Alexander Mann wrote:
>
> Hi Philippe,
>
> I don't see anything that could trigger such a behaviour, but maybe I am missing something. This is the main loop of my code:
>
> // connect branches
> TBranch *b_runno;
> collChain->SetBranchAddress("RunNumber", &runno, &b_runno);
> TBranch *b_evno;
> collChain->SetBranchAddress("EventNumber", &evno, &b_evno);
>
> //Create a new file + a clone of old tree in new file
> TFile *newfile = new TFile(outFile.c_str(), "recreate");
> TTree *newtree = collChain->CloneTree(0);
>
> // do cloning
> time_t stimee = time(NULL);
> cout << "Start cloning tree at " << stimee << "..." << endl;
> long stepsize = nentries / 100;
> long stepleft = stepsize;
> long evwritten = 0;
> for (Long64_t jentry = 0; jentry < nentries; ++jentry) {
> stepleft = 1;
> if (--stepleft == 0) {
> stepleft = stepsize;
> cout << jentry << "/" << nentries << " (" << evwritten << ", " << currtime() << ")\r";
> fflush(stdout);
> }
> Long64_t ientry = collChain->LoadTree(jentry);
> collChain->GetEntry(ientry);
> if (vec_evnos[runno].find(evno) != vec_evnos[runno].end()) {
> newtree->Fill();
> ++evwritten;
> }
> }
> cout << endl;
>
> I am using ROOT 5.26.00e and running over quasi-local files. If you prefer I can send you a test case including the complete macro.
>
> cu,
> Alexander
>
>
> On 05/16/2011 05:46 PM, Philippe Canal wrote:
>> Hi Alexander,
>>
>> LoadTree should be most of the time 'immediate'. The behavior you
>> describe usually indicates that some other portion of the code is
>> directly or indirectly doing another call to LoadTree with a totally
>> different entry number
>> ( 0 or GetEntries()-1 are the most typical). This result is having to
>> open the file and load the TTree object every entry (and this is a
>> 'slow' operation).
>>
>> Cheers,
>> Philippe.
>>
>> On 5/16/11 10:41 AM, Alexander Mann wrote:
>>>
>>> Hi,
>>>
>>> I am using a slightly modified version of some standard ROOT example
>>> code from
>>>
>>> http://root.cern.ch/root/html/tutorials/tree/copytree3.C.html
>>>
>>> to skim TTrees. The main difference is that I am using TChain instead
>>> of TTree, and therefore I am using
>>>
>>> Long64_t ientry = collChain->LoadTree(jentry);
>>> collChain->GetEntry(ientry);
>>>
>>> This works fine, but after ~500 events (which is probably all from the
>>> first file in the TChain) the speed abruptly decreases from about 27
>>> events/s to 0.05 events/s, spent in equal parts in LoadTree and
>>> GetEntry as it seems.
>>>
>>> Is there something I can do about this?
>>>
>>> cu,
>>> Alexander
>>>
>
Received on Mon May 16 2011 - 18:08:30 CEST