I came across mathgrrl and was intrigued by the rolling knot http://makerhome.blogspot.co.uk/2013/12/day-110-rocking-knot.html. I wondered if I could define it with Openscad.
The challenge is to create a tube which is smooth enough and will fully render in an acceptable time. I tried a tube of simple cylinders oriented along the path but this proved desparately slow and rough. At nophead's suggestion I then moved to segments made from hull() ing successive pairs of disks or spheres. The script for spheres turned out to be not only simpler but also faster. However nophead suggested a cone instead of a disk, and with the addition of explicit render()s this provide the best performance so far.
The best script is rolling-knot-hull-cone which renders the 2 degree version (180 segments) in 12 minutes on my Acer Aspire.
Many thanks to mathgrrl and the helpful OpenScad community especially nop head.