Remove contention on runtime

See merge request !2

cython/runtime/runtime.pxd

@@ -21,7 +21,6 @@ cdef cypclass Worker
 cdef inline void * worker_function(void * arg) nogil:
     worker = <lock Worker> arg
     sch = <Scheduler> <void*> worker.scheduler
-    cdef int num_remaining_queues
     # Wait until all the workers are ready.
     pthread_barrier_wait(&sch.barrier)
     while 1:
@@ -75,24 +74,23 @@ cdef cypclass Worker:
     lock SequentialMailBox steal_queue(lock self):
         # Steal a queue from another worker:
         # - inspect each worker in order starting at a random offset
-        # - skip this worker and any worker with an empty queue list
+        # - skip any worker with an empty queue list
         # - return the last queue of the first worker with a non-empty list
+        # - continue looping until a queue is found
         cdef int i, index, num_workers, random_offset
         sch = <Scheduler> <void*> self.scheduler
         num_workers = <int> sch.workers.size()
-        random_offset = rand() % num_workers
-        for i in range(num_workers):
-            index = (i + random_offset) % num_workers
+        index = rand() % num_workers
+        while True:
             victim = sch.workers[index]
-            if victim is self:
-                continue
             with wlocked victim:
                 if not victim.queues.empty():
                     stolen_queue = victim.queues.back()
                     victim.queues.pop_back()
-                    stolen_queue.has_worker = True
                     return stolen_queue
-        return NULL
+            index += 1
+            if index >= num_workers:
+                index = 0
 
     int join(self):
         # Join the worker thread.
@@ -106,10 +104,12 @@ cdef cypclass Scheduler:
     atomic[int] num_pending_queues
     sem_t done
     volatile bint is_done
+    int num_workers
 
     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
         sem_init(&self.num_free_queues, 0, 0)
         sem_init(&self.done, 0, 0)
         self.num_pending_queues.store(0)
@@ -136,16 +136,20 @@ cdef cypclass Scheduler:
         sem_destroy(&self.num_free_queues)
         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:
+    void post_queue(lock self, lock SequentialMailBox queue):
+        cdef int num_workers, random_offset
+        sch = <Scheduler> <void*> self
+        # Add a queue to a random worker.
+        num_workers = <int> sch.workers.size()
+        random_offset = rand() % num_workers
+        receiver = sch.workers[random_offset]
+        with wlocked receiver:
             queue.has_worker = True
-          main_worker.queues.push_back(queue)
+            receiver.queues.push_back(queue)
         # Increment the number of non-completed queues.
-        self.num_pending_queues.fetch_add(1)
+        sch.num_pending_queues.fetch_add(1)
         # Signal that a queue is available.
-        sem_post(&self.num_free_queues)
+        sem_post(&sch.num_free_queues)
 
     void finish(lock self):
         # Wait until there is no more work.