Avoid recursion in the tasks if no particle is active.

Does exactly what it says. We don't want to go through all the cells recursively just to find the handful that need this task to act upon.

Big improvements when added to the kill_drift branch on the EAGLE_25 case.

src/runner.c

@@ -113,8 +113,12 @@ void runner_do_grav_external(struct runner *r, struct cell *c, int timer) {
   const struct external_potential *potential = r->e->external_potential;
   const struct phys_const *constants = r->e->physical_constants;
   const double time = r->e->time;
+
   TIMER_TIC;
 
+  /* Anything to do here? */
+  if (c->ti_end_min > ti_current) return;
+
   /* Recurse? */
   if (c->split) {
     for (int k = 0; k < 8; k++)
@@ -391,6 +395,9 @@ void runner_do_init(struct runner *r, struct cell *c, int timer) {
 
   TIMER_TIC;
 
+  /* Anything to do here? */
+  if (c->ti_end_min > ti_current) return;
+
   /* Recurse? */
   if (c->split) {
     for (int k = 0; k < 8; k++)
@@ -443,6 +450,9 @@ void runner_do_extra_ghost(struct runner *r, struct cell *c) {
   const int count = c->count;
   const int ti_current = r->e->ti_current;
 
+  /* Anything to do here? */
+  if (c->ti_end_min > ti_current) return;
+
   /* Recurse? */
   if (c->split) {
     for (int k = 0; k < 8; k++)
@@ -493,6 +503,9 @@ void runner_do_ghost(struct runner *r, struct cell *c) {
 
   TIMER_TIC;
 
+  /* Anything to do here? */
+  if (c->ti_end_min > ti_current) return;
+
   /* Recurse? */
   if (c->split) {
     for (int k = 0; k < 8; k++)
@@ -928,15 +941,22 @@ void runner_do_kick(struct runner *r, struct cell *c, int timer) {
   struct gpart *restrict gparts = c->gparts;
   const double const_G = r->e->physical_constants->const_newton_G;
 
-  int updated = 0, g_updated = 0;
-  int ti_end_min = max_nr_timesteps, ti_end_max = 0;
+  TIMER_TIC;
 
-  TIMER_TIC
+  /* Anything to do here? */
+  if (c->ti_end_min > ti_current) {
+    c->updated = 0;
+    c->g_updated = 0;
+    return;
+  }
 
 #ifdef TASK_VERBOSE
   OUT;
 #endif
 
+  int updated = 0, g_updated = 0;
+  int ti_end_min = max_nr_timesteps, ti_end_max = 0;
+
   /* No children? */
   if (!c->split) {