Hi Rene,
Actually my case is the access to the tree entries via Index so it is really
arbitrary from the entry point of view. In script is only the simpliest hack to
illustrate the problem.
The reality I fased to is that index access is much less effective then I
expected, especially in the case of million of entries and it is due to the
GetEntry slowing down. May be I'm mistaken but my impression that with 2.25 it
was much faster?
I do not see how 2 Trees could help in case of abitrary (index) access.
Regards
Valeri
On Fri, 10 May 2002, Rene Brun wrote:
> Hi Valeri,
>
> Your access type is not arbitrary but the most masochist way
> to destroy the ROOT cache ::)
>
> Recently, we had a discussion on roottalk on this subject.
> I suggested for this kind of applications, to open the same Tree twice.
> Each Tree will have its own cache and you will see a dramatic improvement
> in performance.
>
> Rene Brun
>
>
> Valeri Tioukov wrote:
> >
> > Hi rooters,
> >
> > I noted that the arbitrary access to the tree entries is significantly slower
> > then the sequential ones.
> >
> > May be it is an important feature of reading algorithm and it not simple to
> > optimise, overwise it cold be useful to speed up.
> >
> > In the applied script I tested 2 cases of access to the tree entries:
> >
> > 1) sequential
> > 2) "arbitrary" (not really arbitrary in my test)
> >
> > Test was done on the file Event.root with 20000 events generated with the
> > standard settings.
> >
> > In case of reading only one branch fNtrack the output is:
> > 20000 events: Real time 0:0:3, CP time 3.380
> > 20000 events: Real time 0:0:40, CP time 40.330
> >
> > so the second case is 12 times slower. In principle one could expect some
> > encrease of access time due to memory (disk) pages lists or similar effects.
> > But this could behave like a constant: 40-3 = 37.
> > Instead when I read more branches the ratio is even more havy:
> >
> > 20000 events: Real time 0:0:6, CP time 6.110
> > 20000 events: Real time 0:2:19, CP time 139.130
> >
> > ratio = 23, difference = 133
> >
> > Regards,
> > Valeri
> >
> > //--test1.C-------------------------------------------------------------------
> >
> > // To start the test do:
> > //
> > // root [0] .L libEvent.so
> > // root [1] .L test1.C++
> > // root [2] init()
> > // root [3] sel()
> >
> > #include "TTree.h"
> > #include "TFile.h"
> > #include "Event.h"
> > #include "TStopwatch.h"
> > #include "iostream.h"
> >
> > TFile *f = 0;
> > TTree *chain =0;
> >
> > void init()
> > {
> > f = new TFile("Event.root");
> > chain = (TTree*)f->Get("T");
> > }
> >
> > void sel()
> > {
> > Event *event = 0;
> >
> > //chain->SetBranchStatus("*",0); // ratio 12
> > //chain->SetBranchStatus("fNtrack",1); //
> >
> > chain->SetBranchStatus("*",1); // ratio 23
> > chain->SetBranchStatus("fTracks*",0); //
> >
> > chain->SetBranchAddress("event",&event);
> >
> > int entries = (int)(chain->GetEntries());
> > int entries2 = entries/2;
> > int counter;
> > printf("chain has %d entries\n",entries);
> >
> > TStopwatch timer1;
> > timer1.Start();
> > counter=0;
> > for(int i=0; i<entries; i++ ) {
> > chain->GetEntry(i);
> > counter++;
> > }
> >
> > timer1.Stop();
> > cout << counter<< " events: \t"; timer1.Print();
> >
> > int e1 = 200;
> > int e2 = entries-200;
> >
> > TStopwatch timer2;
> > timer2.Start();
> > counter=0;
> > for(int i=0; i<entries2; i++ ) {
> > chain->GetEntry(e1);
> > counter++;
> > chain->GetEntry(e2);
> > counter++;
> > }
> > timer2.Stop();
> > cout << counter<< " events: \t"; timer2.Print();
> > }
> > //----------------------------------------------------------------
>
>
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