[nialo]

I don't work on Solvespace and therefore don't know for sure, but I suspect they're not linear. In particular, a significant fraction of all constraints in a CAD system like that are of the form "the distance between these two points must equal x", which is a not linear in any of the positions

I'd love to be wrong about this, because this general problem is one I'd be very interested in being able to solve more efficiently.

[istvan_csanady]

2d geometric constraints solvers are usually based on BFGS algorithm (https://en.wikipedia.org/wiki/Broyden–Fletcher–Goldfarb–Shan...). This is what we use in Shapr3D for example (http://www.shapr3d.com)