For what it's worth: Today I helped Jens track down a problem under FreeBSD where Zope was crashing rendering a page which rendered successfully on other systems. It turns out that the default stack size for a thread under FreeBSD is 64K, and that's not enough stack space to render this particular page of his (a Tracker page) -- most likely due to changes with how the RestrictedPython compiler is used/invoked by DTML processing, since this represents a big change from Zope 2.3. The solution we used was modifying Python/thread_pthread.h and the PyThread_start_new_thread function; specifically, instead of using a (pthread_attr_t *) NULL, we made a new attribute instead: pthread_attr_t pta; pthread_attr_init(&pta); pthread_attr_setstacksize(&pta, (1<<17)); and using &pta in the thread creation function rather than NULL. Recompiling Python 2.1 with changes gives it a 128K stack per thread, which seemed sufficient to render the Tracker page in question. Does anyone know of a better way to tune thread stack sizes on FreeBSD?