Hi Karl,
On Wed, 16 Jan 2002 18:07:57 +0100 (MET)
"K. Hauschild" <karlhaus@hep.saclay.cea.fr> wrote
concerning "[ROOT] reduction":
> Hi All,
>
> I want to make my root Data files smaller : here is what I do, but I
> would like to know if the is a more space efficient way of storing
> the data. I have the following class to store/access data for a
> particular detector type.
>
> //Class for CsI Detectors
> class DetectorCsI : public TObject {
>
> private:
> UShort_t fId; //Detector Id
> UShort_t fPId; //Detector Particle Id
> UShort_t fChan; //Detector chan
> UShort_t fTac; //Detector TAC
>
> public:
> DetectorCsI() {;}
> DetectorCsI(UShort_t id, UShort_t pid, UShort_t chan, UShort_t tac);
> virtual ~DetectorCsI() {;}
>
> //setters and getters
> inline UShort_t GetId() {return fId; }
> inline UShort_t GetPId() {return fPId; }
> inline UShort_t GetChan() {return fChan;}
> inline UShort_t GetTac() {return fTac; }
>
> ClassDef (DetectorCsI,1) //CsI Detector class
> };
>
> So, in this case "fCsIs_" is a large overhead. I presume this is
> where the number of CsI elements hit is stored per event entry. From
> the numbers above it would seem this is 8 bytes per event entry. In
> my particular case this need only be 8 bits since I have less than
> 255 CsI detectors. Why is the overhead needed by ROOT so large
> ?. Is there any way to define the size of "fCsIs_" ?
>
> Or, am I barking up the wrong tree and there is a more space
> efficient way of handling this ?
>
The branch fCsIs_ is from the TClonesArray, and I guess it can't be
changed, and probably shouldn't.
However, I you only have 255 detector elements, I guess that
DetectorCsI::fId needs only take values form 0 - 254, you can store
the make DetectorCsI::fId of type Byte_t (1 byte = 8 bits, giving a
range of 0 - 255):
class DetectorCsI : public TObject {
private:
Byte_t fId; //Detector Id
UShort_t fPId; //Detector Particle Id
UShort_t fChan; //Detector chan
UShort_t fTac; //Detector TAC
public:
DetectorCsI() {}
DetectorCsI(UShort_t id, UShort_t pid, UShort_t chan, UShort_t tac) {
fId = Byte_t(id > 255 ? 255 : id); fPid=pid; fChan=1; fTac=tac; }
virtual ~DetectorCsI() {}
inline UShort_t GetId() { return (UShort_t)fId; }
inline UShort_t GetPId() { return fPId; }
inline UShort_t GetChan() { return fChan;}
inline UShort_t GetTac() { return fTac; }
ClassDef (DetectorCsI,1) //CsI Detector class
};
That saves you 1 byte per entry in the TClonesArray, which comes to
255 bytes if all detectors fire, much more then the possible 7 bytes
from making fCsIs_ a Byte_t instead of a Double_t. And if possible,
you can apply this to fPid, fChan, and fTac.
If you only store one DetectorCsI object per tree fill, don't put it
in a TClonesArray (doesn't make sense really) but put it on a seperate
branch.
Also, if you don't use TRef or TRefArray, you can use
TObject::fUniqueID to hold fPid, fTac, or fChan, giving you another 2
bytes.
Finally, ROOT does compress integer types per default, so you're
probably already as low as you can get. Check out the manual for more
on how to set different compression schemes.
Hope that helps.
Yours,
Christian Holm Christensen -------------------------------------------
Address: Sankt Hansgade 23, 1. th. Phone: (+45) 35 35 96 91
DK-2200 Copenhagen N Cell: (+45) 28 82 16 23
Denmark Office: (+45) 353 25 305
Email: cholm@nbi.dk Web: www.nbi.dk/~cholm
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