Hi Valeri
The branches of TTree objects are written with so called "index-sequential" method.
This doesn't provide the direct access to an arbitrary event rather to the block of the
several events.
This means when you try to access you events sequentially then everything fine.
When you try to access your events in the random order your program are forced
to read the entire block of the events into memory even you need only one single event
from that block.
This may lead to the performance degradation by two reason:
1. With one single I/O operation your code reads an extra information (events)
that is no use at the time
2. When you will require some particular event then very likely it is to be read in
again (see point 1)
This is what Rene called "masochist way"
If this is what you need really you may regard the "normal" ROOT I/O rather
TTree I/O.
That does provide the direct access to the objects.
However the number of events you may handle may be less those you can handle with
TTree. You may be required to write some code to get the service that TTree class provides.
Hope this helps, Valeri
-----
Dr.Valeri Fine
STAR/US Atlas E-mail: fine@bnl.gov
Brookhaven National Lab Phone: +1 631 344 7806
Upton, NY 11973-5000 FAX: +1 631 344 4206
USA
----- Original Message -----
From: "Valeri Tioukov" <Valeri.Tioukov@na.infn.it>
To: "Rene Brun" <Rene.Brun@cern.ch>
Cc: "roottalk" <roottalk@pcroot.cern.ch>
Sent: Friday, May 10, 2002 11:36 AM
Subject: Re: [ROOT] Arbitrary access to tree entries VS sequential one
> 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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