Restart workers when worker-ttl expires (dask#8538)

distributed/nanny.py

@@ -390,17 +390,14 @@ async def start_unsafe(self):
 
         return self
 
-    async def kill(self, timeout: float = 2, reason: str = "nanny-kill") -> None:
+    async def kill(self, timeout: float = 5, reason: str = "nanny-kill") -> None:
         """Kill the local worker process
 
         Blocks until both the process is down and the scheduler is properly
         informed
         """
-        if self.process is None:
-            return
-
-        deadline = time() + timeout
-        await self.process.kill(reason=reason, timeout=0.8 * (deadline - time()))
+        if self.process is not None:
+            await self.process.kill(reason=reason, timeout=timeout)
 
     async def instantiate(self) -> Status:
         """Start a local worker process
@@ -822,7 +819,7 @@ def mark_stopped(self):
 
     async def kill(
         self,
-        timeout: float = 2,
+        timeout: float = 5,
         executor_wait: bool = True,
         reason: str = "workerprocess-kill",
     ) -> None:
@@ -876,7 +873,7 @@ async def kill(
                 pass
 
             logger.warning(
-                f"Worker process still alive after {wait_timeout} seconds, killing"
+                f"Worker process still alive after {wait_timeout:.1f} seconds, killing"
             )
             await process.kill()
             await process.join(max(0, deadline - time()))

distributed/scheduler.py

@@ -547,7 +547,7 @@ def __init__(
         self._memory_unmanaged_old = 0
         self._memory_unmanaged_history = deque()
         self.metrics = {}
-        self.last_seen = 0
+        self.last_seen = time()
         self.time_delay = 0
         self.bandwidth = parse_bytes(dask.config.get("distributed.scheduler.bandwidth"))
         self.actors = set()
@@ -6335,7 +6335,10 @@ async def restart_workers(
                         # FIXME does not raise if the process fails to shut down,
                         # see https://github.com/dask/distributed/pull/6427/files#r894917424
                         # NOTE: Nanny will automatically restart worker process when it's killed
-                        nanny.kill(reason=stimulus_id, timeout=timeout),
+                        # NOTE: Don't propagate timeout to kill(): we don't want to
+                        # spend (.8*.8)=64% of our end-to-end timeout waiting for a hung
+                        # process to restart.
+                        nanny.kill(reason=stimulus_id),
                         timeout,
                     )
                     for nanny in nannies
@@ -8406,19 +8409,29 @@ async def get_worker_monitor_info(self, recent=False, starts=None):
     # Cleanup #
     ###########
 
-    async def check_worker_ttl(self):
+    @log_errors
+    async def check_worker_ttl(self) -> None:
         now = time()
         stimulus_id = f"check-worker-ttl-{now}"
+        assert self.worker_ttl
+        ttl = max(self.worker_ttl, 10 * heartbeat_interval(len(self.workers)))
+        to_restart = []
+
         for ws in self.workers.values():
-            if (ws.last_seen < now - self.worker_ttl) and (
-                ws.last_seen < now - 10 * heartbeat_interval(len(self.workers))
-            ):
+            last_seen = now - ws.last_seen
+            if last_seen > ttl:
+                to_restart.append(ws.address)
                 logger.warning(
-                    "Worker failed to heartbeat within %s seconds. Closing: %s",
-                    self.worker_ttl,
-                    ws,
+                    f"Worker failed to heartbeat for {last_seen:.0f}s; "
+                    f"{'attempting restart' if ws.nanny else 'removing'}: {ws}"
                 )
-                await self.remove_worker(address=ws.address, stimulus_id=stimulus_id)
+
+        if to_restart:
+            await self.restart_workers(
+                to_restart,
+                wait_for_workers=False,
+                stimulus_id=stimulus_id,
+            )
 
     def check_idle(self) -> float | None:
         if self.status in (Status.closing, Status.closed):

distributed/tests/test_active_memory_manager.py

@@ -1084,7 +1084,6 @@ async def test_RetireWorker_new_keys_arrive_after_all_keys_moved_away(c, s, a, b
 @gen_cluster(
     client=True,
     config={
-        "distributed.scheduler.worker-ttl": "500ms",
         "distributed.scheduler.active-memory-manager.start": True,
         "distributed.scheduler.active-memory-manager.interval": 0.05,
         "distributed.scheduler.active-memory-manager.measure": "managed",

distributed/tests/test_failed_workers.py

@@ -14,9 +14,10 @@
 from dask import delayed
 from dask.utils import parse_bytes
 
-from distributed import Client, Nanny, profile, wait
+from distributed import Client, KilledWorker, Nanny, get_worker, profile, wait
 from distributed.comm import CommClosedError
 from distributed.compatibility import MACOS
+from distributed.core import Status
 from distributed.metrics import time
 from distributed.utils import CancelledError, sync
 from distributed.utils_test import (
@@ -450,10 +451,10 @@ async def test_restart_timeout_on_long_running_task(c, s, a):
 
 
 @pytest.mark.slow
-@gen_cluster(client=True, scheduler_kwargs={"worker_ttl": "500ms"})
+@gen_cluster(client=True, config={"distributed.scheduler.worker-ttl": "500ms"})
 async def test_worker_time_to_live(c, s, a, b):
-    from distributed.scheduler import heartbeat_interval
-
+    # Note that this value is ignored because is less than 10x heartbeat_interval
+    assert s.worker_ttl == 0.5
     assert set(s.workers) == {a.address, b.address}
 
     a.periodic_callbacks["heartbeat"].stop()
@@ -465,10 +466,84 @@ async def test_worker_time_to_live(c, s, a, b):
 
     # Worker removal is triggered after 10 * heartbeat
     # This is 10 * 0.5s at the moment of writing.
-    interval = 10 * heartbeat_interval(len(s.workers))
     # Currently observing an extra 0.3~0.6s on top of the interval.
     # Adding some padding to prevent flakiness.
-    assert time() - start < interval + 2.0
+    assert time() - start < 7
+
+
+@pytest.mark.slow
+@pytest.mark.parametrize("block_evloop", [False, True])
+@gen_cluster(
+    client=True,
+    Worker=Nanny,
+    nthreads=[("", 1)],
+    scheduler_kwargs={"worker_ttl": "500ms", "allowed_failures": 0},
+)
+async def test_worker_ttl_restarts_worker(c, s, a, block_evloop):
+    """If the event loop of a worker becomes completely unresponsive, the scheduler will
+    restart it through the nanny.
+    """
+    ws = s.workers[a.worker_address]
+
+    async def f():
+        w = get_worker()
+        w.periodic_callbacks["heartbeat"].stop()
+        if block_evloop:
+            sleep(9999)  # Block event loop indefinitely
+        else:
+            await asyncio.sleep(9999)
+
+    fut = c.submit(f, key="x")
+
+    while not s.workers or (
+        (new_ws := next(iter(s.workers.values()))) is ws
+        or new_ws.status != Status.running
+    ):
+        await asyncio.sleep(0.01)
+
+    if block_evloop:
+        # The nanny killed the worker with SIGKILL.
+        # The restart has increased the suspicious count.
+        with pytest.raises(KilledWorker):
+            await fut
+        assert s.tasks["x"].state == "erred"
+        assert s.tasks["x"].suspicious == 1
+    else:
+        # The nanny sent to the WorkerProcess a {op: stop} through IPC, which in turn
+        # successfully invoked Worker.close(nanny=False).
+        # This behaviour makes sense as the worker-ttl timeout was most likely caused
+        # by a failure in networking, rather than a hung process.
+        assert s.tasks["x"].state == "processing"
+        assert s.tasks["x"].suspicious == 0
+
+
+@pytest.mark.slow
+@gen_cluster(
+    client=True,
+    Worker=Nanny,
+    nthreads=[("", 2)],
+    scheduler_kwargs={"allowed_failures": 0},
+)
+async def test_restart_hung_worker(c, s, a):
+    """Test restart_workers() to restart a worker whose event loop has become completely
+    unresponsive.
+    """
+    ws = s.workers[a.worker_address]
+
+    async def f():
+        w = get_worker()
+        w.periodic_callbacks["heartbeat"].stop()
+        sleep(9999)  # Block event loop indefinitely
+
+    fut = c.submit(f)
+    # Wait for worker to hang
+    with pytest.raises(asyncio.TimeoutError):
+        while True:
+            await wait(c.submit(inc, 1, pure=False), timeout=0.2)
+
+    await c.restart_workers([a.worker_address])
+    assert len(s.workers) == 1
+    assert next(iter(s.workers.values())) is not ws
 
 
 @gen_cluster(client=True, nthreads=[("", 1)])