[Scip] Same SCIP, same ILP on different machines -> "different" results
Gordon Royle
gordon.royle at uwa.edu.au
Thu Oct 27 04:41:08 MEST 2011
I am running SCIP both on a Linux machine and a MacOS machine.
I am using the binaries
scip-2.0.2.darwin.x86_64.gnu.opt.spx
scip-2.0.2.linux.x86_64.gnu.opt.spx
When I run the same LP file (which is a binary pure integer linear program) on the two platforms, I get different computation paths and hence solve times. (One machine is faster than the other so a difference is expected there, but I did not expect different outcomes).
The values returned are correct - they are just different feasible solutions - but the variation can be quite large, which makes it different to perform comparative work.
Here are examples of the solve nodes for 12 ILPs of similar "type" - all have 273 variables and 274 constraints.
Each line has the format: Nodes_on_mac ( Time_on_mac) Nodes_on_linux ( Time_on_linux )
40 ( 5.00 ) 40 ( 7.92 )
728 ( 7.55 ) 1098 ( 13.24 )
10 ( 3.82 ) 10 ( 5.79 )
96 ( 5.37 ) 1298 ( 15.57 )
12008 ( 32.50 ) 887 ( 18.30 )
510 ( 6.79 ) 648 ( 11.25 )
1560 ( 9.77 ) 1418 ( 14.50 )
654573 ( 1200.07 ) 223861 ( 663.25 )
28295 ( 65.48 ) 321577 ( 1033.71 )
28204 ( 63.66 ) 14652 ( 83.94 )
84891 ( 163.91 ) 1752 ( 16.07 )
248229 ( 426.26 ) 13963 ( 50.55 )
Why does this occur? Is it some sort of floating point issue or is there some arbitrariness in "tie-breaking" etc.
(I do not have a deep knowledge of the internal process of solving a pure ILP.)
Professor Gordon Royle
School of Mathematics and Statistics
University of Western Australia
Gordon.Royle at uwa.edu.au<mailto:Gordon.Royle at uwa.edu.au>
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