[Cherry-pick]Add multi process reader (#18116)

* add multi process reader
test=release/1.5

paddle/fluid/API.spec

@@ -405,7 +405,7 @@ paddle.fluid.contrib.QuantizeTranspiler.training_transpile (ArgSpec(args=['self'
 paddle.fluid.contrib.Calibrator.__init__ (ArgSpec(args=['self'], varargs='args', keywords='kwargs', defaults=None), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
 paddle.fluid.contrib.Calibrator.sample_data (ArgSpec(args=['self'], varargs=None, keywords=None, defaults=None), ('document', '3b8c85ca1e2cf753cc8c90a6c6992958'))
 paddle.fluid.contrib.Calibrator.save_int8_model (ArgSpec(args=['self'], varargs=None, keywords=None, defaults=None), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
-paddle.fluid.contrib.distributed_sampler (ArgSpec(args=['reader', 'batch_size'], varargs=None, keywords=None, defaults=None), ('document', '9a271cd9700deb6d837ed724ba094315'))
+paddle.fluid.contrib.distributed_batch_reader (ArgSpec(args=['batch_reader'], varargs=None, keywords=None, defaults=None), ('document', 'b60796eb0a481484dd34e345f0eaa4d5'))
 paddle.fluid.contrib.reader.ctr_reader.ctr_reader (ArgSpec(args=['feed_dict', 'file_type', 'file_format', 'dense_slot_index', 'sparse_slot_index', 'capacity', 'thread_num', 'batch_size', 'file_list', 'slots', 'name'], varargs=None, keywords=None, defaults=(None,)), ('document', 'b2ebf3de2a6ef1af2c3b88d2db7591ab'))
 paddle.fluid.contrib.Compressor.__init__ (ArgSpec(args=['self', 'place', 'scope', 'train_program', 'train_reader', 'train_feed_list', 'train_fetch_list', 'eval_program', 'eval_reader', 'eval_feed_list', 'eval_fetch_list', 'teacher_programs', 'checkpoint_path', 'train_optimizer', 'distiller_optimizer', 'search_space'], varargs=None, keywords=None, defaults=(None, None, None, None, None, None, None, [], None, None, None, None)), ('document', 'c195b3bba26169cff9439e8c467557c0'))
 paddle.fluid.contrib.Compressor.config (ArgSpec(args=['self', 'config_file'], varargs=None, keywords=None, defaults=None), ('document', '780d9c007276ccbb95b292400d7807b0'))

python/paddle/fluid/contrib/reader/README.md

@@ -16,10 +16,10 @@ and two types of data format:
 
 ## Distributed reader
 
-The distributed reader is mainly used by multi-process tasks, it splits the origin batch samples to N sub-batch samples, and the N is equal to the number of processes. The usage is similar to `paddle.batch`.
+The distributed reader is mainly used by multi-process tasks, and the input must be a batch reader.
 
 Cons:
   - It can be operated conveniently so that different processes can read different data.
 
 Pros:
-  - Because each process reads the original batch data and then divides the data, the performance may be poor.
+  - If batch_reader produces training data, and batch_reader loads or preprocesses data for a long time, this data reading method may be slower.

python/paddle/fluid/contrib/reader/distributed_reader.py

@@ -15,52 +15,51 @@
 from __future__ import print_function
 import os
 
-__all__ = ["distributed_sampler"]
+__all__ = ["distributed_batch_reader"]
 
 
-def distributed_sampler(reader, batch_size):
+def distributed_batch_reader(batch_reader):
     """
-    Create a distributed reader.
+    Create a reader for multi-process training. The input must be a batch reader.
 
-    :param reader: the data reader to read from.
-    :type reader: callable
-    :param batch_size: the size of the batch
-    :type batch_size: int
-    """
-
-    def batch_reader():
-        if not os.getenv('PADDLE_TRAINER_ID'):
-            raise RuntimeError(
-                "The current program is not in distributed mode.")
+    Args:
+        batch_reader (callable): The input reader should be a batch reader.
 
-        trainer_id = int(os.getenv("PADDLE_TRAINER_ID", "0"))
-        trainer_count = int(os.getenv("PADDLE_TRAINERS_NUM", "1"))
+    Examples:
 
-        def _slice_data(size):
-            per_node_lines = size // trainer_count
-            return [
-                trainer_id * per_node_lines, (trainer_id + 1) * per_node_lines
-            ]
+    .. code-block:: python
+           import paddle
+           import paddle.fluid as fluid
 
-        r = reader()
-        b = []
+           train_reader = paddle.batch(paddle.dataset.mnist.train(),
+                    batch_size=32,drop_last=True)
+           train_reader = fluid.contrib.reader.distributed_batch_reader(
+                    train_reader)
 
-        for instance in r:
-            b.append(instance)
-            if len(b) == batch_size:
-                if len(b) >= trainer_count:
-                    begin, end = _slice_data(len(b))
-                    yield b[begin:end]
-                b = []
+    """
+    trainers_num = int(os.environ.get('PADDLE_TRAINERS_NUM', 1))
+    trainer_id = int(os.getenv("PADDLE_TRAINER_ID", 0))
+    assert trainer_id < trainers_num
 
-        if len(b) >= trainer_count:
-            begin, end = _slice_data(len(b))
-            yield b[begin:end]
+    def decorate_for_multi_process():
+        if trainers_num > 1:
+            print("start data reader (trainers_num: {}, trainer_id: {})".format(
+                trainers_num, trainer_id))
 
-    # Batch size check
-    batch_size = int(batch_size)
-    if batch_size <= 0:
-        raise ValueError("batch_size should be a positive integeral value, "
-                         "but got batch_size={}".format(batch_size))
+        train_data, idx = None, 1
+        for batch_id, data in enumerate(batch_reader()):
+            if trainers_num > 1:
+                if idx < trainers_num:
+                    if idx == trainer_id + 1:
+                        train_data = data
+                    idx += 1
+                else:
+                    if idx == trainer_id + 1:
+                        train_data = data
+                    assert train_data is not None, "train data should not be None."
+                    yield train_data
+                    train_data, idx = None, 1
+            else:
+                yield data
 
-    return batch_reader
+    return decorate_for_multi_process

python/paddle/fluid/contrib/tests/test_distributed_reader.py

@@ -20,26 +20,21 @@ import paddle.fluid as fluid
 import os
 
 
-def data_generator(input_shape=(1, 32, 32), label_range=9):
-    while True:
-        img = np.random.random(size=input_shape).astype(np.float32)
-        label = np.array(np.random.randint(0, label_range)).astype("int64")
-        yield img, label
+def data_generator():
+    data = [0, 1, 2, 3]
+    for val in data:
+        yield val
 
 
 class TestDistributedReader(unittest.TestCase):
     def test_distributed_reader(self):
-        batch_size = 32
         trainer_num = 4
-        os.environ['PADDLE_TRAINER_ID'] = str(0)
+        os.environ['PADDLE_TRAINER_ID'] = str(1)
         os.environ['PADDLE_TRAINERS_NUM'] = str(trainer_num)
 
-        reader = fluid.contrib.reader.distributed_sampler(
-            data_generator, batch_size=batch_size)
+        reader = fluid.contrib.reader.distributed_batch_reader(data_generator)
         data = next(reader())
-        assert len(data) == batch_size // trainer_num,\
-            "sub batch size should be {}, but the returned size is {}".format(
-            batch_size // trainer_num, len(data))
+        assert data == 1
 
         os.unsetenv('PADDLE_TRAINER_ID')
         os.unsetenv('PADDLE_TRAINERS_NUM')

python/paddle/reader/tests/decorator_test.py

@@ -198,7 +198,7 @@ class TestMultiProcessReader(unittest.TestCase):
             results.append(data)
         self.assertEqual(sorted(self.samples), sorted(results))
 
-    def test_multi_process_reader(self):
+    def test_distributed_batch_reader(self):
         self.reader_test(use_pipe=False)
         self.reader_test(use_pipe=True)