1. Zooming on ROOT files and Containers
CMS-ROOT meeting
CERN- December 5
René Brun
ftp://root.cern.ch/root/ZoomFiles.ppt
ZOOM on ROOT files & Containers

2. Keys in a TDirectory/TFiletr
Keys in a TDirectory/TFile
Key in directory
in memory
Key on file
record header
Obj.Write(“key”)
File
Streamed data of obj
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

3. Memory <--> Tree The Tree entry serial number
T.GetEntry(6) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
T.Fill() 18 T
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

4. T.BuildIndex(“Run”,”Event”)tr
Entry number and Index
T.BuildIndex(“Run”,”Event”) 1 2 3
Index(45,12) = 0 4 5 6 7
Index(45,78) = 8 8 9 10 11 12
Index(45,95) = 18 13 14 15 16 17 18
T.GetEntry(8)
T.GetEntryWithIndex(45,78) T
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

5. Entry number and EventList
TEventList el;
el.Enter(3); el.Enter(9); el.Enter(15); 1 2 3 4
T.GetEntry(0); returns entry 0
T.SetEventList(el);
T.GetEntry(0); returns entry 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 T
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

6. ZOOM on ROOT files & Containerstr
Tree Friends
Entry # 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Public
read
User
Write
Public
read
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

7. Binary search in table above ch.GetEntryWithIndex(12,567);
Chains of Trees f0 f1 f2 f3 f4 f5 f6 f7 2 4 4 5 5 4 7 11 12 17 23 26 32 37 45 49 1 2 3 4 5 6 7
TChain ch(“T”);
ch.Add(“f0.root”);
ch.Add(“f1.root”);
ch.Add(“f2.root”);
ch.Add(“f3.root”);
ch.Add(“f4.root”);
ch.Add(“f5.root”);
ch.Add(“f6.root”);
ch.Add(“f7.root”);
Binary search in table above
find slot 4, local entry 2
T.GetEntry(2) in f4.root
ch.GetEntry(28);
ch.GetEntryWithIndex(12,567);
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

8. Binary search in table above ch.GetEntryWithIndex(12,567);
Chains of Trees f0 f1 f2 f3 f4 f5 f6 f7
This chain definition
should be generated
on the fly from the
catalog 2 4 4 5 5 4 7 11 12 17 23 26 32 37 45 49 1 2 3 4 5 6 7
TChain ch(“T”);
ch.Add(“f0.root”);
ch.Add(“f1.root”);
ch.Add(“f2.root”);
ch.Add(“f3.root”);
ch.Add(“f4.root”);
ch.Add(“f5.root”);
ch.Add(“f6.root”);
ch.Add(“f7.root”);
Binary search in table above
find slot 4, local entry 2
T.GetEntry(2) in f4.root
ch.GetEntry(28);
ch.GetEntryWithIndex(12,567);
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

9. Status with Persistent pointers
C++ persistent pointers
In all versions of ROOT since day1
TRef, TRefArray
implemented in dev version 3.02/00
TRef with Action on Demand
implemented in 3.02/04
TRef simplification & extensions
implemented in 3.02/05
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

10. Normal Streaming mode References using TRef pointers
TBuffer b1;
A.Streamer(b1)
TBuffer b2;
B.Streamer(b2)
Objects in blue
are only in b1
C++ pointer
TRef A B Bz z
Set pointer to z with: TRef Bz = z;
Get pointer to z with: z = Bz.GetObject()
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

11. ZOOM on ROOT files & Containers
Setting a TRef pointer
Assuming obj = pointer to a TObject*
TRef ref = obj;
TRef is itself a TObject
If the obj::kIsReferenced bit is not yet set, the obj::fUniqueID is set to the CurrentNumber+1 and obj::kIsReferenced is set to 1.
Its fUniqueID is set to obj::fUniqueID
CurrentNumber is managed by TProcessID
fObjs[CurrentNumber] is set to obj in fPID
Class TObject {
unsigned int fBits;
unsigned int fUniqueID;
Class TRef : public TObject {
TProcessID *fPID; //!pointer to process id
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

12. ZOOM on ROOT files & Containers
TRef example: Event.h
class Event : public TObject {
private:
char fType[20]; //event type
char *fEventName; //run+event number in character format
int fNtrack; //Number of tracks
int fNseg; //Number of track segments
int fNvertex;
int fMeasures[10];
float fMatrix[4][4];
float *fClosestDistance; //[fNvertex]
EventHeader fEvtHdr;
TClonesArray *fTracks; //->array with all tracks
TRefArray *fHighPt; //array of High Pt tracks only
TRefArray *fMuons; //array of Muon tracks only
TRef fLastTrack; //reference pointer to last track
TRef fWebHistogram; //EXEC:GetWebHistogram
TH1F *fH; //->
public:
...
TH1F *GetHistogram() const {return fH;}
TH1F *GetWebHistogram(Bool_t reload=kFALSE) const {
return (TH1F*)fWebHistogram.GetObject(reload);}
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

13. Writing Referenced objects
A referenced object is written by obj->Streamer
This Streamer at some point calls its TObject::Streamer
In TObject::Streamer, if the kIsReferenced bit is set in fBits, the following additional info is also written:
pid (4 bytes) = TProcessID of current process
Average overhead in memory = 0 bytes
Average overhead on disk (no comp) = 2 bytes
Average overhead on disk (comp) = 2.4 bytes (0.4)
A Referenced object may be written multiple times
in the same file as the TRef or in other files
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

14. Writing TRefs to a buffer
A TRef is written by TRef::Streamer
Writes uid(4 bytes) + pid(4 bytes)
uid = object unique identifier
uid = ref::fUniqueID = obj::fUniqueID = current object nr
pid = Process identifier
Each process has a unique pid (TProcessID)
A file contains the pids of all processes that have written objects to it.
TRef memory size = 16 bytes
TRef size on disk (no comp) = 8 bytes
TRef size on disk (comp) = 2.4 bytes
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

15. Reading Referenced objects
A referenced object is read by obj->Streamer
This Streamer at some point calls its TObject::Streamer
In TObject::Streamer, if the kIsReferenced bit is set in fBits, the following additional info is also read:
pid (4 bytes) = TProcessID of current process
In TProcessID::fObjs fObjs[fUniqueID] = obj
When obj is deleted, fObjs[fUniqueID] = 0;
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

16. Reading TRefs from a buffer
A TRef object is read by TRef::Streamer
The pair uid,pid is read
the fUniqueID of TRef is set to uid
The transient pointer fPID is set to the TProcessID corresponding to pid
The bit 1 of fBits is set
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

17. ZOOM on ROOT files & Containers
A TRef use case
Session 3
Session 1
Session 2
TFile f3(“f3.root”);
TRef *r2;
r2 = (TRef*)f3.Get(“r2”);
r2->GetObject(); = 0
TFile f1(“f1.root”);
Myclass *obj1;
obj1 = (Myclass*)f1.Get(“obj1”);
r2->GetObject(); --> obj1
Myclass *obj1;
TRef r1 = obj1;
TFile f1(“f1.root”,”new”);
obj1->Write(“obj1”);
TFile f2(“f2.root”,”new”);
r1.Write(“r1”);
TFile f2(“f2.root”);
TRef *r1;
r1 = (TRef*)f2.Get(“r1”);
TFile f3(“f3.root”,”new”);
TRef r2 = *r1;
r2.Write(“r2”)
obj1 r1 r1 r2 r2
obj1
obj1 r1 r2
f1.root
f3.root
f2.root
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

18. Returns obj = fUniqueID + gSystem
Using a TRef
To get a pointer to the referenced object, do:
Myclass *obj = (Myclass*)ref.GetObject() =0
Returns obj = fUniqueID + gSystem
GetObject
fBits[1]
fPID = 0 or
fPID>GetObjectWithUniqueID() = 0 =1
execid = fBits[8/8]
YES NO
Set fBits[1]=0
return obj
YES
return null
execid = 0 NO
Execute TExec
with execid
Obj = 0
YES
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

19. TRef & Action on Demand
When the keyword “EXEC:” is found in the comments of the data member as in:
TRef fWebHistogram; //EXEC:GetWebHistogram
The information in the comment field is kept in the dictionary. Execid is saved in TStreamerElement
When the TRef object is read, the execid is stored in the fBits on one byte (from TStreamerElement).
When TRef::GetObject is called,
TObjArray *lexecs = gROOT->GetListOfExecs();
TExec *exec = (TExec*)lexecs[execid];
exec->Exec();
Action on Demand
is Persistent
fWebHistogram.GetObject() executes the action GetWebHistogram
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

20. ZOOM on ROOT files & Containers
What a TExec can do
TExec is a CORE ROOT class that can be used to execute:
a call to a compiled or interpreted function
example: Exec:LoadHits()
an interpreted script
example: Exec:GetWebHistogram
If GetWebHistogram is not a function (compiled or interpreted), then TExec::Exec will try to execute the script GetWebHistogram.C
void GetWebHistogram(){
// example of script called from an Action on Demand when a TRef object
// is dereferenced. See Event.h, member fWebHistogram
TFile *f= TFile::Open("
TH1 *h6 = (TH1*)f->Get("h6");
h6->SetDirectory(0);
delete f;
Tref::SetObject(h6); }
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers

21. ZOOM on ROOT files & Containers
A Working Example
Action.C {
gSystem.Load("libEvent");
TFile f("Event.root");
Event *event=0;
T.SetBranchAddress("event",&event);
T.GetEntry(45);
event->GetWebHistogram()->Draw(); }
Root > .x Action.C
GetWebHistogram.C
void GetWebHistogram(){
// example of script called from an Action on Demand when a TRef object
// is dereferenced. See Event.h, member fWebHistogram
TFile *f= TFile::Open("
TH1 *h6 = (TH1*)f->Get("h6");
h6->SetDirectory(0);
delete f;
TRef::SetObject(h6); }
CMS 5 Dec Rene Brun
ZOOM on ROOT files & Containers