Re: [ROOT] Hbook/PAW files now readable from ROOT

From: Rene Brun (Rene.Brun@cern.ch)
Date: Wed Feb 20 2002 - 22:35:12 MET


 Hi Mandeep,

I cc my reply to roottalk. I had a few questions similar to yours.

The conversion of histograms from Hbook format to the Root classes
is extremely fast (overhead is negligible).
The THbookTree class (via THbookBranch/THbookFile) converts
only the ntuple header to a Root class. All the data buffers
are read directly by the Hbook/Zebra routines.

I have benchmarked some queries with:
 A- PAW itself on myfile.hbook
 B- THbookFile/THbookTree on myfile.hbook
 C- Root on myfile.root created via h2root
 
In average the myfile.root files that I have used are two times smaller
than the original myfile.hbook.
Concerning the time, one has to distinguish the first pass and the
following
passes through the data.
 -First pass:
   C is always the best (because less physical I/O)
	B is slightly faster than A
 - Second pass:
   C is still the best in general (unless one uses a small ntuple
	that the PAW query processor caches in memory)
	A is faster than B (for the reason above).
	
It would be possible to develop a cache mechanism for Hbook files
within Root, but I would prefer to not start this exercise.
I am interested by additional benchmarks.

These  new classes were developed on request from some experiments
having a large collection of PAW ntuples (I was told multi Terabytes)
who cannot convert all their applications to Root immediatly.

Also note that THbookFile supports read only. No plans to support
a Write option.

In fact, my main motivation in implementing these classes was not so much
the direct support for Hbook files, but mainly to show a concrete example
how to interface to foreigh formats, still using the standard
ROOT Tree query processor.

Rene Brun



On Wed, 20 Feb 2002, Mandeep S. Gill wrote:

> 
> hi Rene:  this sounds very intriguing, do you have any feeling for how
> much overhead there is though for loading and converting files on the fly,
> vs. pre-converting them with h2root?
> 
> thanks-
> -M
> 



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