With SWI-Prolog 5.1.0 Prolog threads have become portable and the performance gap between the single and multi-threaded version has become acceptable. (59) Most of the basic machinery and API have been tested on some large applications and appear to be stable. Especially as we do not anticipate problems as long as multi-threading is not used or threads do not interact much we believe it is time to move towards making the multi-threaded version the default distribution.