actively remove hydro tasks that have zero hydro particles in either cell.

src/scheduler.c

@@ -127,7 +127,8 @@ static void scheduler_splittask_hydro(struct task *t, struct scheduler *s) {
     redo = 0;
 
     /* Non-splittable task? */
-    if ((t->ci == NULL) || (t->type == task_type_pair && t->cj == NULL)) {
+    if ((t->ci == NULL) || (t->type == task_type_pair && t->cj == NULL) ||
+        t->ci->count == 0 || (t->cj != NULL && t->cj->count == 0)) {
       t->type = task_type_none;
       t->subtype = task_subtype_none;
       t->cj = NULL;
@@ -167,7 +168,7 @@ static void scheduler_splittask_hydro(struct task *t, struct scheduler *s) {
           while (ci->progeny[first_child] == NULL) first_child++;
           t->ci = ci->progeny[first_child];
           for (int k = first_child + 1; k < 8; k++)
-            if (ci->progeny[k] != NULL)
+            if (ci->progeny[k] != NULL && ci->progeny[k]->count)
               scheduler_splittask_hydro(
                   scheduler_addtask(s, task_type_self, t->subtype, 0, 0,
                                     ci->progeny[k], NULL),
@@ -175,9 +176,9 @@ static void scheduler_splittask_hydro(struct task *t, struct scheduler *s) {
 
           /* Make a task for each pair of progeny */
           for (int j = 0; j < 8; j++)
-            if (ci->progeny[j] != NULL)
+            if (ci->progeny[j] != NULL && ci->progeny[j]->count)
               for (int k = j + 1; k < 8; k++)
-                if (ci->progeny[k] != NULL)
+                if (ci->progeny[k] != NULL && ci->progeny[k]->count)
                   scheduler_splittask_hydro(
                       scheduler_addtask(s, task_type_pair, t->subtype,
                                         sub_sid_flag[j][k], 0, ci->progeny[j],
@@ -210,8 +211,7 @@ static void scheduler_splittask_hydro(struct task *t, struct scheduler *s) {
       if (cell_can_split_pair_task(ci) && cell_can_split_pair_task(cj)) {
 
         /* Replace by a single sub-task? */
-        if (scheduler_dosub && ci->count &&
-            cj->count && /* Use division to avoid integer overflow. */
+        if (scheduler_dosub && /* Use division to avoid integer overflow. */
             ci->count * sid_scale[sid] < space_subsize_pair / cj->count &&
             !sort_is_corner(sid)) {
 
@@ -561,7 +561,6 @@ static void scheduler_splittask_hydro(struct task *t, struct scheduler *s) {
 
         /* Otherwise, break it up if it is too large? */
       } else if (scheduler_doforcesplit && ci->split && cj->split &&
-                 ci->count && cj->count &&
                  (ci->count > space_maxsize / cj->count)) {
 
         // message( "force splitting pair with %i and %i parts." , ci->count ,
@@ -571,9 +570,9 @@ static void scheduler_splittask_hydro(struct task *t, struct scheduler *s) {
         t->type = task_type_none;
 
         for (int j = 0; j < 8; j++)
-          if (ci->progeny[j] != NULL)
+          if (ci->progeny[j] != NULL && ci->progeny[j]->count)
             for (int k = 0; k < 8; k++)
-              if (cj->progeny[k] != NULL) {
+              if (cj->progeny[k] != NULL && cj->progeny[k]->count) {
                 struct task *tl =
                     scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                       ci->progeny[j], cj->progeny[k]);