Hello All,

I have been analyzing a rather large file (~6 Gb, 57m events) that consists of a TTree.

Inside, I have some int’s, some bool’s, and also some std::vectors.

Now, I do most of my analysis inside an event loop in code so I did not run into this problem until recently. I needed to check something really quick (number of triggers fired), so I did a quick Draw on my tree. For reference, I provide a segment of the tree structure.

```
*............................................................................*
*Br 31 :passed_HLT_mb_sptrk_L1MBTS_1 : passed_HLT_mb_sptrk_L1MBTS_1/O *
*Entries : 57277967 : Total Size= 61428237 bytes File Size = 13081503 *
*Baskets : 36086 : Basket Size= 32000 bytes Compression= 4.64 *
*............................................................................*
*Br 32 :prescale_HLT_mb_sptrk_L1MBTS_1 : prescale_HLT_mb_sptrk_L1MBTS_1/F *
*Entries : 57277967 : Total Size= 233334324 bytes File Size = 19137777 *
*Baskets : 36086 : Basket Size= 32000 bytes Compression= 12.15 *
*............................................................................*
```

When I draw passed_HLT_mb_sptrk_L1MBTS_1, which is just a bool, I see a histogram with two bins, 0->1 and 1->2. I see the number of entries is 5.72780e+07, which is fine. However, the integral of that histogram is 2.26782e+07. This can’t be right. However, when I draw the prescale_HLT_mb_sptrk_L1MBTS_1, I get 5.72780e+07 for the integral and number of entries. Note how the prescale_HLT_mb_sptrk_L1MBTS_1 variable has higher compression than the passed_HLT_mb_sptrk_L1MBTS_1 one.

Now, when I try to Draw the passed_HLT_mb_sptrk_L1MBTS_1 for 15M events, it works fine. Number of entries is 15M, and integral is 15M. Whene I draw 20M events, number of entries is 20M, but integral is 1.92218e+07. Now, what if there is something wrong in memory somewhere between 19M and 20M event. Ok… I draw 15M from the 5M event. This should cover the same as drawing 20M from 0. When I do this, there is no problem. Integral is 15M and number of entries is 15M.

Now, I have some other variables (note the compression).

```
*............................................................................*
*Br 1 :LBN : LBN/I *
*Entries : 57277967 : Total Size= 232359894 bytes File Size = 4988100 *
*Baskets : 36086 : Basket Size= 32000 bytes Compression= 46.44 *
*............................................................................*
*Br 2 :runNumber : runNumber/I *
*Entries : 57277967 : Total Size= 232576434 bytes File Size = 5275057 *
*Baskets : 36086 : Basket Size= 32000 bytes Compression= 43.95 *
*............................................................................*
```

When I do draw on these, for some reason, the LBN has Integral = Number of Events.

However, the integral of the histogram after drawing runNumber gives me 5.44075e+07. This is close, but not 57M.

Why do some of these variables get filled correctly and some don’t? Also, why does it work below ~19M but not with 20M+ events? I do not have this problem with another file with ~8M events and size ~3Gb. Also, I do not have this problem filling my own histograms in an event loop. Also, I tried this on a couple different computers both with Root5 and Root6.

I usually only use tree->Draw() to do some quick things so this does not cause a problem for me, but I am curious why this isnt working. Hope someone can explain this to me.

Cheers,

-Yakov