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A one-machine strategy that puts all variables on a single device.
Inherits From: Strategy
tf.compat.v2.distribute.experimental.CentralStorageStrategy(
compute_devices=None, parameter_device=None
)
Variables are assigned to local CPU or the only GPU. If there is more than one GPU, compute operations (other than variable update operations) will be replicated across all GPUs.
For Example:
strategy = tf.distribute.experimental.CentralStorageStrategy()
# Create a dataset
ds = tf.data.Dataset.range(5).batch(2)
# Distribute that dataset
dist_dataset = strategy.experimental_distribute_dataset(ds)
with strategy.scope():
@tf.function
def train_step(val):
return val + 1
# Iterate over the distributed dataset
for x in dist_dataset:
# process dataset elements
strategy.experimental_run_v2(train_step, args=(x,))
Attributes | |
---|---|
extended
|
tf.distribute.StrategyExtended with additional methods.
|
num_replicas_in_sync
|
Returns number of replicas over which gradients are aggregated. |
Methods
experimental_distribute_dataset
experimental_distribute_dataset(
dataset
)
Distributes a tf.data.Dataset instance provided via dataset.
The returned dataset is a wrapped strategy dataset which creates a multidevice iterator under the hood. It prefetches the input data to the specified devices on the worker. The returned distributed dataset can be iterated over similar to how regular datasets can.
For Example:
strategy = tf.distribute.CentralStorageStrategy() # with 1 CPU and 1 GPU
dataset = tf.data.Dataset.range(10).batch(2)
dist_dataset = strategy.experimental_distribute_dataset(dataset)
for x in dist_dataset:
print(x) # Prints PerReplica values [0, 1], [2, 3],...
Args:
dataset: tf.data.Dataset
to be prefetched to device.
Returns | |
---|---|
A "distributed Dataset " that the caller can iterate over.
|
experimental_distribute_datasets_from_function
experimental_distribute_datasets_from_function(
dataset_fn
)
Distributes tf.data.Dataset
instances created by calls to dataset_fn
.
dataset_fn
will be called once for each worker in the strategy. In this
case, we only have one worker so dataset_fn
is called once. Each replica
on this worker will then dequeue a batch of elements from this local
dataset.
The dataset_fn
should take an tf.distribute.InputContext
instance where
information about batching and input replication can be accessed.
For Example:
def dataset_fn(input_context):
batch_size = input_context.get_per_replica_batch_size(global_batch_size)
d = tf.data.Dataset.from_tensors([[1.]]).repeat().batch(batch_size)
return d.shard(
input_context.num_input_pipelines, input_context.input_pipeline_id)
inputs = strategy.experimental_distribute_datasets_from_function(dataset_fn)
for batch in inputs:
replica_results = strategy.experimental_run_v2(replica_fn, args=(batch,))
Args | |
---|---|
dataset_fn
|
A function taking a tf.distribute.InputContext instance and
returning a tf.data.Dataset .
|
Returns | |
---|---|
A "distributed Dataset ", which the caller can iterate over like regular
datasets.
|
experimental_local_results
experimental_local_results(
value
)
Returns the list of all local per-replica values contained in value
.
In CentralStorageStrategy
there is a single worker so the value returned
will be all the values on that worker.
Args | |
---|---|
value
|
A value returned by experimental_run() , experimental_run_v2() ,
extended.call_for_each_replica() , or a variable created in scope .
|
Returns | |
---|---|
A tuple of values contained in value . If value represents a single
value, this returns (value,).
|
experimental_make_numpy_dataset
experimental_make_numpy_dataset(
numpy_input
)
Makes a tf.data.Dataset
for input provided via a numpy array.
This avoids adding numpy_input
as a large constant in the graph,
and copies the data to the machine or machines that will be processing
the input.
Note that you will likely need to use experimental_distribute_dataset
with the returned dataset to further distribute it with the strategy.
Example:
numpy_input = np.ones([10], dtype=np.float32)
dataset = strategy.experimental_make_numpy_dataset(numpy_input)
dist_dataset = strategy.experimental_distribute_dataset(dataset)
Args | |
---|---|
numpy_input
|
A nest of NumPy input arrays that will be converted into a
dataset. Note that lists of Numpy arrays are stacked, as that is normal
tf.data.Dataset behavior.
|
Returns | |
---|---|
A tf.data.Dataset representing numpy_input .
|
experimental_run_v2
experimental_run_v2(
fn, args=(), kwargs=None
)
Run fn
on each replica, with the given arguments.
In CentralStorageStrategy
, fn
is called on each of the compute
replicas, with the provided "per replica" arguments specific to that device.
Args | |
---|---|
fn
|
The function to run. The output must be a tf.nest of Tensor s.
|
args
|
(Optional) Positional arguments to fn .
|
kwargs
|
(Optional) Keyword arguments to fn .
|
Returns | |
---|---|
Return value from running fn .
|
reduce
reduce(
reduce_op, value, axis
)
Reduce value
across replicas.
Given a per-replica value returned by experimental_run_v2
, say a
per-example loss, the batch will be divided across all the replicas. This
function allows you to aggregate across replicas and optionally also across
batch elements. For example, if you have a global batch size of 8 and 2
replicas, values for examples [0, 1, 2, 3]
will be on replica 0 and
[4, 5, 6, 7]
will be on replica 1. By default, reduce
will just
aggregate across replicas, returning [0+4, 1+5, 2+6, 3+7]
. This is useful
when each replica is computing a scalar or some other value that doesn't
have a "batch" dimension (like a gradient). More often you will want to
aggregate across the global batch, which you can get by specifying the batch
dimension as the axis
, typically axis=0
. In this case it would return a
scalar 0+1+2+3+4+5+6+7
.
If there is a last partial batch, you will need to specify an axis so
that the resulting shape is consistent across replicas. So if the last
batch has size 6 and it is divided into [0, 1, 2, 3] and [4, 5], you
would get a shape mismatch unless you specify axis=0
. If you specify
tf.distribute.ReduceOp.MEAN
, using axis=0
will use the correct
denominator of 6. Contrast this with computing reduce_mean
to get a
scalar value on each replica and this function to average those means,
which will weigh some values 1/8
and others 1/4
.
For Example:
strategy = tf.distribute.experimental.CentralStorageStrategy(
compute_devices=['CPU:0', 'GPU:0'], parameter_device='CPU:0')
ds = tf.data.Dataset.range(10)
# Distribute that dataset
dist_dataset = strategy.experimental_distribute_dataset(ds)
with strategy.scope():
@tf.function
def train_step(val):
# pass through
return val
# Iterate over the distributed dataset
for x in dist_dataset:
result = strategy.experimental_run_v2(train_step, args=(x,))
result = strategy.reduce(tf.distribute.ReduceOp.SUM, result,
axis=None).numpy()
# result: array([ 4, 6, 8, 10])
result = strategy.reduce(tf.distribute.ReduceOp.SUM, result, axis=0).numpy()
# result: 28
Args | |
---|---|
reduce_op
|
A tf.distribute.ReduceOp value specifying how values should
be combined.
|
value
|
A "per replica" value, e.g. returned by experimental_run_v2 to
be combined into a single tensor.
|
axis
|
Specifies the dimension to reduce along within each
replica's tensor. Should typically be set to the batch dimension, or
None to only reduce across replicas (e.g. if the tensor has no batch
dimension).
|
Returns | |
---|---|
A Tensor .
|
scope
scope()
Returns a context manager selecting this Strategy as current.
Inside a with strategy.scope():
code block, this thread
will use a variable creator set by strategy
, and will
enter its "cross-replica context".
Returns | |
---|---|
A context manager. |