[Halld-offline] subtle bug in ROOT

Paul Mattione pmatt at jlab.org
Sat Feb 25 08:59:20 EST 2017

How are you determining this tagger weight?  I would be interested to 
know how that works.

  - Paul

On 02/25/2017 08:56 AM, Richard Jones wrote:
> Hello Paul,
> Thank you, that is a good suggestion. I use weighted spectra for just 
> about all spectra I create, so for that TH1I is not so useful. However 
> in some of the use cases I covered in my message, a TH1I would do just 
> as well.
> For example, simply using a tagger_weight in filling your histograms 
> would immediately give you automatic tagger accidentals subtraction. 
> That is how I do it in my analyses. The non-Poisson errors are 
> automatically accounted for by the TH1D. If you think that would be of 
> general interest, I can offer to add a method to the DBeamPhoton that 
> returns a tagger weight. It would be part of the calibration cycle to 
> establish the correct bounds for the tagger trues and accidentals for 
> each run.
> -Richard Jones
> On Sat, Feb 25, 2017 at 8:33 AM, Paul Mattione <pmatt at jlab.org 
> <mailto:pmatt at jlab.org>> wrote:
>     I recommend using TH1I, since it uses half the memory of TH1D,
>     unless of course you need to fill a bin more than 2 billion times
>     (or the content could be that large after merging, or if you
>     anticipate dividing histograms, etc.).
>      - Paul
>     On 02/25/2017 08:23 AM, Richard Jones wrote:
>>     correction: "That, plus the fact that TH1F::Fill() is a couple
>>     orders of magnitude slower than TH1F::Fill()" should be
>>     That, plus the fact that TH1F::Fill() is a couple orders of
>>     magnitude slower than TH1D::Fill().
>>     On Sat, Feb 25, 2017 at 8:14 AM, Richard Jones
>>     <richard.t.jones at uconn.edu <mailto:richard.t.jones at uconn.edu>> wrote:
>>         Dear colleagues,
>>         Maybe this is common knowledge to people more versed in ROOT
>>         than I am, but this was a surprise to me, that took me quite
>>         some effort to discover why my TTree analysis was producing
>>         nonsense.
>>         >>  TH1F histograms silently truncate their bin contents at
>>         16,777,216 (2^28) <<
>>         unless you fill with non-unity weights, in which case they
>>         truncate at OTHER SMALL VALUES. To be specific, if you create
>>         a TH1F histogram h1 and then do h1.Fill(0.) repeatedly then
>>         this bin will silently stop incrementing when the bin content
>>         reaches 1.677216e+7. If you fill it with a non-unity weight
>>         then it will gradually lose precision as the number of Fill
>>         calls on that bin exceeds some threshold like a few M, and
>>         then silently stop incrementing altogether when the bin
>>         content reaches some limit. For w=100 I found this limit to
>>         be 2.147e+9. This was unexpected because the letter F in TH1F
>>         implies "float" which has a max value about 3.4e+38. What use
>>         is a histogram object that silently discards entries as soon
>>         as the count reaches some small value that we expect to
>>         commonly hit in high-statistics analysis? They must be doing
>>         some kind of range-truncating-compression in the storage of
>>         TH1F bin contents. Personally, I would rather get the right
>>         answer, even if it means using more memory, but that's just me.
>>         A workaround would be never to use TH1F, always TH1D. I have
>>         not been able to discover a similar silent truncation in
>>         TH1D. That, plus the fact that TH1F::Fill() is a couple
>>         orders of magnitude slower than TH1F::Fill(). Apparently it
>>         takes a lot of cpu time to generate bugs of this kind?
>>         Meanwhile, beware. This is especially insidious because the
>>         command tree.Draw("px") in your interactive ROOT session
>>         implicitly creates and fills a TH1F, not a TH1D, even if px
>>         is declared double in your tree. In my present analysis, my
>>         tree has 200M rows, but in principle that will bite you even
>>         if you have only 20M rows in your tree.
>>         -Richard Jones
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