Add queue to workers in round robin

kdtree/runtime/runtime.pxd

@@ -104,12 +104,16 @@ cdef cypclass Scheduler:
     pthread_barrier_t barrier
     sem_t num_free_queues
     atomic[int] num_pending_queues
+    int next_worker
+    int num_workers
     sem_t done
     volatile bint is_done
 
     lock Scheduler __new__(alloc, int num_workers=0):
         self = <lock Scheduler> consume alloc()
         if num_workers == 0: num_workers = sysconf(_SC_NPROCESSORS_ONLN)
+        self.num_workers = num_workers
+        self.next_worker = 0
         sem_init(&self.num_free_queues, 0, 0)
         sem_init(&self.done, 0, 0)
         self.num_pending_queues.store(0)
@@ -137,11 +141,12 @@ cdef cypclass Scheduler:
         sem_destroy(&self.done)
 
     void post_queue(self, lock SequentialMailBox queue):
-        # Add a queue to the first worker.
-        main_worker = self.workers[0]
-        with wlocked main_worker:
+        # Add a queue to workers in round robin
+        worker = self.workers[<int> self.next_worker]
+        self.next_worker = (self.next_worker + 1) % self.num_workers
+        with wlocked worker:
           queue.has_worker = True
-          main_worker.queues.push_back(queue)
+          worker.queues.push_back(queue)
         # Increment the number of non-completed queues.
         self.num_pending_queues.fetch_add(1)
         # Signal that a queue is available.