Minimal total branch size for compression

ROOT
1

Hi everyone,

I have found that in all my ROOT files compression is always 1.0 when a branch has “Total Size” less than ~20k. What is a reason for such a limit to exist? Can I compress smaller than 20kbytes branches and is there significant performance loss?

*............................................................................*
*Br  236 :MB_beamSpot_z : MB_beamSpot_z/F                                    *
*Entries :     1184 : Total  Size=      17190 bytes  One basket in memory    *
*Baskets :        0 : Basket Size=       8000 bytes  Compression=   1.00     *
*............................................................................*
*Br  237 :MB_R2All  : MB_R2All/F                                             *
*Entries :     1184 : Total  Size=      17160 bytes  One basket in memory    *
*Baskets :        0 : Basket Size=       8000 bytes  Compression=   1.00     *
*............................................................................*
*Br  328 :MB_cms_e  : MB_cms_e/D                                             *
*Entries :     1184 : Total  Size=      21975 bytes  File Size  =        233 *
*Baskets :        1 : Basket Size=       8000 bytes  Compression=  34.31     *
*............................................................................*
*Br  329 :MB_cms_px : MB_cms_px/D                                            *
*Entries :     1184 : Total  Size=      21982 bytes  File Size  =        262 *
*Baskets :        1 : Basket Size=       8000 bytes  Compression=  30.52     *
*............................................................................*

Thanks,
Dmitriy

2

Why do you draw this conclusion? Your baskets are perfectly compressed once they are written to teh file (2 baskets in your example).
Baskets in memory are never compressed. They are compressed only
when they get full and written to the file or when they are written
together with the Tree header when closing the file.

Rene

3

Thanks Rene, I think I just miss interpreted what this compression factor refers to.

Dmitriy