What kinds of error compensation are supported by linuxcnc

there is no compensation effect,why? I added comp_file as it is descripted in linuxcnc.org/docs/devel/html/config/ini-...t__lt_num_gt_section

I tried these data as figure, there is no one has a offset.

 I expect the G0 x70 can move to position 50mm and G0 x0 can move to position 10mm. 

First, I draw a line 0mm to 70mm  with a ruler. Second, i home x and open a laser on the machine and the light is on the 0mm position.  Third, send a command G0 x70. Four, watch the light position, and it is on the position 70mm.

I change the offset from 20 to 10000 in screwcomp0.data and  there is also no offset.
 
Tips, home x is OK. Linear units = mm. I think the unit of every value is mm  in screwcomp0.data.