Issue #20311: asyncio: Add a granularity attribute to BaseEventLoop: maximum

between the resolution of the BaseEventLoop.time() method and the resolution of
the selector. The granuarility is used in the scheduler to round time and
deadline.

Doc/library/asyncio-eventloop.rst

@@ -144,6 +144,12 @@ a different clock than :func:`time.time`.
    Return the current time, as a :class:`float` value, according to the
    event loop's internal clock.
 
+.. attribute:: BaseEventLoop.granularity
+
+   Granularity of the time: maximum between the resolution of the
+   :meth:`BaseEventLoop.time` method and the resolution of the selector (see
+   :attr:`selectors.BaseSelector.resolution`).
+
 .. seealso::
 
    The :func:`asyncio.sleep` function.

Lib/asyncio/base_events.py

@@ -18,6 +18,7 @@ import collections
 import concurrent.futures
 import heapq
 import logging
+import math
 import socket
 import subprocess
 import time
@@ -96,6 +97,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         self._default_executor = None
         self._internal_fds = 0
         self._running = False
+        self.granularity = time.get_clock_info('monotonic').resolution
 
     def _make_socket_transport(self, sock, protocol, waiter=None, *,
                                extra=None, server=None):
@@ -603,6 +605,8 @@ class BaseEventLoop(events.AbstractEventLoop):
         elif self._scheduled:
             # Compute the desired timeout.
             when = self._scheduled[0]._when
+            # round deadline aways from zero
+            when = math.ceil(when / self.granularity) * self.granularity
             deadline = max(0, when - self.time())
             if timeout is None:
                 timeout = deadline
@@ -629,6 +633,8 @@ class BaseEventLoop(events.AbstractEventLoop):
 
         # Handle 'later' callbacks that are ready.
         now = self.time()
+        # round current time aways from zero
+        now = math.ceil(now / self.granularity) * self.granularity
         while self._scheduled:
             handle = self._scheduled[0]
             if handle._when > now:

Lib/asyncio/selector_events.py

@@ -34,6 +34,7 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
             selector = selectors.DefaultSelector()
         logger.debug('Using selector: %s', selector.__class__.__name__)
         self._selector = selector
+        self.granularity = max(selector.resolution, self.granularity)
         self._make_self_pipe()
 
     def _make_socket_transport(self, sock, protocol, waiter=None, *,

Lib/test/test_asyncio/test_events.py

@@ -1116,6 +1116,29 @@ class EventLoopTestsMixin:
         r.close()
         w.close()
 
+    def test_timeout_rounding(self):
+        def _run_once():
+            self.loop._run_once_counter += 1
+            orig_run_once()
+
+        orig_run_once = self.loop._run_once
+        self.loop._run_once_counter = 0
+        self.loop._run_once = _run_once
+        calls = []
+
+        @tasks.coroutine
+        def wait():
+            loop = self.loop
+            calls.append(loop._run_once_counter)
+            yield from tasks.sleep(loop.granularity * 10, loop=loop)
+            calls.append(loop._run_once_counter)
+            yield from tasks.sleep(loop.granularity / 10, loop=loop)
+            calls.append(loop._run_once_counter)
+
+        self.loop.run_until_complete(wait())
+        calls.append(self.loop._run_once_counter)
+        self.assertEqual(calls, [1, 3, 5, 6])
+
 
 class SubprocessTestsMixin:
 

Lib/test/test_asyncio/test_selector_events.py

@@ -39,7 +39,9 @@ def list_to_buffer(l=()):
 class BaseSelectorEventLoopTests(unittest.TestCase):
 
     def setUp(self):
-        self.loop = TestBaseSelectorEventLoop(unittest.mock.Mock())
+        selector = unittest.mock.Mock()
+        selector.resolution = 1e-3
+        self.loop = TestBaseSelectorEventLoop(selector)
 
     def test_make_socket_transport(self):
         m = unittest.mock.Mock()

Misc/NEWS

@@ -36,6 +36,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #20311: asyncio: Add a granularity attribute to BaseEventLoop: maximum
+  between the resolution of the BaseEventLoop.time() method and the resolution
+  of the selector. The granuarility is used in the scheduler to round time and
+  deadline.
+
 - Issue #20311: selectors: Add a resolution attribute to BaseSelector.
 
 - Issue #20189: unittest.mock now no longer assumes that any object for