improve dataloader signals and messages (apache#16114)

* improve dataloader signals and messages

* address comments

* fix spawn tests on windows

python/mxnet/gluon/data/dataloader.py

@@ -24,6 +24,7 @@
 import pickle
 import io
 import sys
+import signal
 import multiprocessing
 import multiprocessing.queues
 from multiprocessing.reduction import ForkingPickler
@@ -426,7 +427,8 @@ def _thread_worker_fn(samples, batchify_fn, dataset):
 class _MultiWorkerIter(object):
     """Internal multi-worker iterator for DataLoader."""
     def __init__(self, worker_pool, batchify_fn, batch_sampler, pin_memory=False,
-                 pin_device_id=0, worker_fn=_worker_fn, prefetch=0, dataset=None, data_loader=None):
+                 pin_device_id=0, worker_fn=_worker_fn, prefetch=0, dataset=None,
+                 data_loader=None, timeout=120):
         self._worker_pool = worker_pool
         self._batchify_fn = batchify_fn
         self._batch_sampler = batch_sampler
@@ -439,6 +441,7 @@ def __init__(self, worker_pool, batchify_fn, batch_sampler, pin_memory=False,
         self._pin_device_id = pin_device_id
         self._dataset = dataset
         self._data_loader = data_loader
+        self._timeout = timeout
         # pre-fetch
         for _ in range(prefetch):
             self._push_next()
@@ -465,12 +468,29 @@ def __next__(self):
         assert self._rcvd_idx < self._sent_idx, "rcvd_idx must be smaller than sent_idx"
         assert self._rcvd_idx in self._data_buffer, "fatal error with _push_next, rcvd_idx missing"
         ret = self._data_buffer.pop(self._rcvd_idx)
-        batch = pickle.loads(ret.get()) if self._dataset is None else ret.get()
-        if self._pin_memory:
-            batch = _as_in_context(batch, context.cpu_pinned(self._pin_device_id))
-        batch = batch[0] if len(batch) == 1 else batch
-        self._rcvd_idx += 1
-        return batch
+        try:
+            if self._dataset is None:
+                batch = pickle.loads(ret.get(self._timeout))
+            else:
+                batch = ret.get(self._timeout)
+            if self._pin_memory:
+                batch = _as_in_context(batch, context.cpu_pinned(self._pin_device_id))
+            batch = batch[0] if len(batch) == 1 else batch
+            self._rcvd_idx += 1
+            return batch
+        except multiprocessing.context.TimeoutError:
+            msg = '''Worker timed out after {} seconds. This might be caused by \n
+            - Slow transform. Please increase timeout to allow slower data loading in each worker.
+            '''.format(self._timeout)
+            if not isinstance(self._worker_pool, multiprocessing.pool.ThreadPool):
+                msg += '''- Insufficient shared_memory if `timeout` is large enough.
+            Please consider reduce `num_workers` or increase shared_memory in system.
+            '''
+            print(msg)
+            raise
+        except Exception:
+            self._worker_pool.terminate()
+            raise
 
     def next(self):
         return self.__next__()
@@ -537,16 +557,22 @@ def default_batchify_fn(data):
         If ``True``, use threading pool instead of multiprocessing pool. Using threadpool
         can avoid shared memory usage. If `DataLoader` is more IO bounded or GIL is not a killing
         problem, threadpool version may achieve better performance than multiprocessing.
-
+    timeout : int, default is 120
+        The timeout in seconds for each worker to fetch a batch data. Only modify this number
+        unless you are experiencing timeout and you know it's due to slow data loading.
+        Sometimes full `shared_memory` will cause all workers to hang and causes timeout. In these
+        cases please reduce `num_workers` or increase system `shared_memory` size instead.
     """
     def __init__(self, dataset, batch_size=None, shuffle=False, sampler=None,
                  last_batch=None, batch_sampler=None, batchify_fn=None,
                  num_workers=0, pin_memory=False, pin_device_id=0,
-                 prefetch=None, thread_pool=False):
+                 prefetch=None, thread_pool=False, timeout=120):
         self._dataset = dataset
         self._pin_memory = pin_memory
         self._pin_device_id = pin_device_id
         self._thread_pool = thread_pool
+        self._timeout = timeout
+        assert timeout > 0, "timeout must be positive, given {}".format(timeout)
 
         if batch_sampler is None:
             if batch_size is None:
@@ -577,9 +603,13 @@ def __init__(self, dataset, batch_size=None, shuffle=False, sampler=None,
                                                initializer=_thread_worker_initializer,
                                                initargs=(is_np_shape(), is_np_array()))
             else:
+                # set ignore keyboard interupt signal before forking processes
+                original_sigint_handler = signal.signal(signal.SIGINT, signal.SIG_IGN)
                 self._worker_pool = multiprocessing.Pool(
                     self._num_workers, initializer=_worker_initializer,
                     initargs=[self._dataset, is_np_shape(), is_np_array()])
+                # resume keyboard interupt signal in main process
+                signal.signal(signal.SIGINT, original_sigint_handler)
         if batchify_fn is None:
             if num_workers > 0:
                 self._batchify_fn = default_mp_batchify_fn
@@ -604,7 +634,7 @@ def same_process_iter():
                                 worker_fn=_thread_worker_fn if self._thread_pool else _worker_fn,
                                 prefetch=self._prefetch,
                                 dataset=self._dataset if self._thread_pool else None,
-                                data_loader=self)
+                                data_loader=self, timeout=self._timeout)
 
     def __len__(self):
         return len(self._batch_sampler)

tests/python/unittest/test_gluon_data.py

@@ -249,7 +249,10 @@ def test_multi_worker_forked_data_loader():
 @with_seed()
 def test_multi_worker_dataloader_release_pool():
     # will trigger too many open file if pool is not released properly
-    for _ in range(100):
+    if os.name == 'nt':
+        print('Skip for windows since spawn on windows is too expensive.')
+        return
+    for _ in range(10):
         A = np.random.rand(999, 2000)
         D = mx.gluon.data.DataLoader(A, batch_size=8, num_workers=8)
         the_iter = iter(D)