hard-code a finer grained chunk size.

src/engine.c

@@ -3640,7 +3640,8 @@ void engine_unskip(struct engine *e) {
 
   /* Activate all the regular tasks */
   threadpool_map(&e->threadpool, runner_do_unskip_mapper, e->s->cells_top,
-                 e->s->nr_cells, sizeof(struct cell), 0, e);
+                 e->s->nr_cells, sizeof(struct cell),
+                 e->s->nr_cells / (e->threadpool.num_threads * 20), e);
 
   /* And the top level gravity FFT one */
   if (e->s->periodic && (e->policy & engine_policy_self_gravity))