They are more likely due to properties of the puzzles themselves than operating system behaviour, especially on a modern operating system on a multi-core machin

They are more likely due to properties of the puzzles themselves than operating system behaviour, especially on a modern operating system on a multi-core machine. Plus, in the original Norvig article (http://norvig.com/sudoku.html http://norvig.com/sudoku.html) he talks about several puzzles which take 10s and 100s of seconds to complete, which is extremely unlikely to be due to scheduler behaviour. With languages like Python and Clojure several background processes (like GC pauses) could also contribute to some drops in throughput. Of course, it would be easy to tell these cases apart by counting operations (backtracks, etc) rather than time.