uasyncio.core: Switch to separate run and wait queues.

Instead of using single priority queue for all tasks, split into using
"run queue", which represents tasks not waiting until specific time,
which should be run on every (well, next) loop iteration, and wait queue,
still a priority queue. Run queue is a simple FIFO, implemented by
ucollections.deque, recently introduced in pfalcon/micropython. Thus,
there's minimal storage overhead and intrinsic scheduling fairness.
Generally, run queue should hold both a callback/coro and its arguments,
but as we don't feed any send args into coros still, it's optimized to
hold just 1 items for coros, while 2 for callbacks.

Introducing run queue will also allow to get rid of tie-breaking counter
in utimeq implementation, which was introduced to enforce fair scheduling.
It's no longer needed, as all tasks which should be run at given time
are batch-removed from wait queue and batch-inserted into run queue. So,
they may be executed not in the order scheduled (due to non-stable order
of heap), but the whole batch will be executed "atomically", and any new
schedulings from will be processed no earlier than next loop iteration.

Author: pfalcon

uasyncio.core/uasyncio/core.py

@@ -1,5 +1,6 @@
 import utime as time
 import utimeq
+import ucollections
 
 
 type_gen = type((lambda: (yield))())
@@ -25,8 +26,9 @@ class TimeoutError(CancelledError):
 
 class EventLoop:
 
-    def __init__(self, len=42):
-        self.q = utimeq.utimeq(len)
+    def __init__(self, runq_len=16, waitq_len=16):
+        self.runq = ucollections.deque((), runq_len, True)
+        self.waitq = utimeq.utimeq(waitq_len)
         # Current task being run. Task is a top-level coroutine scheduled
         # in the event loop (sub-coroutines executed transparently by
         # yield from/await, event loop "doesn't see" them).
@@ -41,18 +43,24 @@ def create_task(self, coro):
         # CPython asyncio incompatibility: we don't return Task object
 
     def call_soon(self, callback, *args):
-        self.call_at_(self.time(), callback, args)
+        if __debug__ and DEBUG:
+            log.debug("Scheduling in runq: %s", (callback, args))
+        self.runq.append(callback)
+        if not isinstance(callback, type_gen):
+            self.runq.append(args)
 
     def call_later(self, delay, callback, *args):
         self.call_at_(time.ticks_add(self.time(), int(delay * 1000)), callback, args)
 
     def call_later_ms(self, delay, callback, *args):
+        if not delay:
+            return self.call_soon(callback, *args)
         self.call_at_(time.ticks_add(self.time(), delay), callback, args)
 
     def call_at_(self, time, callback, args=()):
         if __debug__ and DEBUG:
-            log.debug("Scheduling %s", (time, callback, args))
-        self.q.push(time, callback, args)
+            log.debug("Scheduling in waitq: %s", (time, callback, args))
+        self.waitq.push(time, callback, args)
 
     def wait(self, delay):
         # Default wait implementation, to be overriden in subclasses
@@ -64,45 +72,45 @@ def wait(self, delay):
     def run_forever(self):
         cur_task = [0, 0, 0]
         while True:
-            if self.q:
-                # wait() may finish prematurely due to I/O completion,
-                # and schedule new, earlier than before tasks to run.
-                while 1:
-                    t = self.q.peektime()
-                    tnow = self.time()
-                    delay = time.ticks_diff(t, tnow)
-                    if delay < 0:
-                        delay = 0
-                    # Always call wait(), to give a chance to I/O scheduling
-                    self.wait(delay)
-                    if delay == 0:
-                        break
-
-                self.q.pop(cur_task)
-                t = cur_task[0]
-                cb = cur_task[1]
-                args = cur_task[2]
+            # Expire entries in waitq and move them to runq
+            tnow = self.time()
+            while self.waitq:
+                t = self.waitq.peektime()
+                delay = time.ticks_diff(t, tnow)
+                if delay > 0:
+                    break
+                self.waitq.pop(cur_task)
+                if __debug__ and DEBUG:
+                    log.debug("Moving from waitq to runq: %s", cur_task[1])
+                self.call_soon(cur_task[1], *cur_task[2])
+
+            # Process runq
+            l = len(self.runq)
+            if __debug__ and DEBUG:
+                log.debug("Entries in runq: %d", l)
+            while l:
+                cb = self.runq.popleft()
+                l -= 1
+                args = ()
+                if not isinstance(cb, type_gen):
+                    args = self.runq.popleft()
+                    l -= 1
+                    if __debug__ and DEBUG:
+                        log.info("Next callback to run: %s", (cb, args))
+                    cb(*args)
+                    continue
+
                 if __debug__ and DEBUG:
-                    log.debug("Next coroutine to run: %s", (t, cb, args))
+                    log.info("Next coroutine to run: %s", (cb, args))
                 self.cur_task = cb
-#                __main__.mem_info()
-            else:
-                self.wait(-1)
-                # Assuming IO completion scheduled some tasks
-                continue
-            if callable(cb):
-                cb(*args)
-            else:
                 delay = 0
                 try:
-                    if __debug__ and DEBUG:
-                        log.debug("Coroutine %s send args: %s", cb, args)
-                    if args == ():
+                    if args is ():
                         ret = next(cb)
                     else:
                         ret = cb.send(*args)
                     if __debug__ and DEBUG:
-                        log.debug("Coroutine %s yield result: %s", cb, ret)
+                        log.info("Coroutine %s yield result: %s", cb, ret)
                     if isinstance(ret, SysCall1):
                         arg = ret.arg
                         if isinstance(ret, SleepMs):
@@ -147,7 +155,22 @@ def run_forever(self):
                 # Currently all syscalls don't return anything, so we don't
                 # need to feed anything to the next invocation of coroutine.
                 # If that changes, need to pass that value below.
-                self.call_later_ms(delay, cb)
+                if delay:
+                    self.call_later_ms(delay, cb)
+                else:
+                    self.call_soon(cb)
+
+            # Wait until next waitq task or I/O availability
+            delay = 0
+            if not self.runq:
+                delay = -1
+                if self.waitq:
+                    tnow = self.time()
+                    t = self.waitq.peektime()
+                    delay = time.ticks_diff(t, tnow)
+                    if delay < 0:
+                        delay = 0
+            self.wait(delay)
 
     def run_until_complete(self, coro):
         def _run_and_stop():
@@ -195,10 +218,10 @@ class IOWriteDone(SysCall1):
 
 _event_loop = None
 _event_loop_class = EventLoop
-def get_event_loop(len=42):
+def get_event_loop(runq_len=16, waitq_len=16):
     global _event_loop
     if _event_loop is None:
-        _event_loop = _event_loop_class(len)
+        _event_loop = _event_loop_class(runq_len, waitq_len)
     return _event_loop
 
 def sleep(secs):