A performance comparison between row wise and column wise PAW ntuples

This page presents a comparison between the performances of Row Wise Ntuple and Column Wise Ntuple during the first access to the data. After the first access, variables are cached in memory and the performances are the same.

The test has been done on two chains (one with RWN and an other with CWN) of 9 hbook files. In total this represented 81060 events each having 302 simple floating point variables. The data were exactly the same.

A serie of loops over the whole chain has been done with various COMIS functions. Each function used more variables than the previous one. The results are summarized in the following plot:

We can see that CWN are more efficient only if a small fraction of the variables is used ( < 10%) . Over 10% of variables used the CWN access time increase whereas the RWN access time remains stable. When a few variables are used (less than 3%) the speed up provided by the CWN is important but it decrease quickly when the number of variables used increase.

• CWN should be used when typed variables are mandatory (character, integer etc ...) or when array variables are needed.
• CWN should NOT be used to speed up the data analysis (on the first pass) when more than 10% of the total number of variables are used in one query.

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