TensorFlow 1 version | View source on GitHub |
A state & compute distribution policy on a list of devices.
tf.distribute.Strategy(
extended
)
See the guide for overview and examples.
In short:
- To use it with Keras
compile
/fit
, please read. - You may pass descendant of
tf.distribute.Strategy
totf.estimator.RunConfig
to specify how atf.estimator.Estimator
should distribute its computation. See guide. - Otherwise, use
tf.distribute.Strategy.scope
to specify that a strategy should be used when building an executing your model. (This puts you in the "cross-replica context" for this strategy, which means the strategy is put in control of things like variable placement.) If you are writing a custom training loop, you will need to call a few more methods, see the guide:
Start by either creating a
tf.data.Dataset
normally or usingtf.distribute.experimental_make_numpy_dataset
to make a dataset out of anumpy
array.Use
tf.distribute.Strategy.experimental_distribute_dataset
to convert atf.data.Dataset
to something that produces "per-replica" values. If you want to manually specify how the dataset should be partitioned across replicas, usetf.distribute.Strategy.experimental_distribute_datasets_from_function
instead.Use
tf.distribute.Strategy.experimental_run_v2
to run a function once per replica, taking values that may be "per-replica" (e.g. from a distributed dataset) and returning "per-replica" values. This function is executed in "replica context", which means each operation is performed separately on each replica.Finally use a method (such as
tf.distribute.Strategy.reduce
) to convert the resulting "per-replica" values into ordinaryTensor
s.
A custom training loop can be as simple as:
with my_strategy.scope():
@tf.function
def distribute_train_epoch(dataset):
def replica_fn(input):
# process input and return result
return result
total_result = 0
for x in dataset:
per_replica_result = my_strategy.experimental_run_v2(replica_fn,
args=(x,))
total_result += my_strategy.reduce(tf.distribute.ReduceOp.SUM,
per_replica_result, axis=None)
return total_result
dist_dataset = my_strategy.experimental_distribute_dataset(dataset)
for _ in range(EPOCHS):
train_result = distribute_train_epoch(dist_dataset)
This takes an ordinary dataset
and replica_fn
and runs it
distributed using a particular tf.distribute.Strategy
named
my_strategy
above. Any variables created in replica_fn
are created
using my_strategy
's policy, and library functions called by
replica_fn
can use the get_replica_context()
API to implement
distributed-specific behavior.
You can use the reduce
API to aggregate results across replicas and use
this as a return value from one iteration over the distributed dataset. Or
you can use tf.keras.metrics
(such as loss, accuracy, etc.) to
accumulate metrics across steps in a given epoch.
See the custom training loop tutorial for a more detailed example.
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 distributed dataset can be iterated over similar to how regular datasets can. NOTE: Currently, the user cannot add any more transformations to a distributed dataset.
The following is an example:
strategy = tf.distribute.MirroredStrategy()
# Create a dataset
dataset = dataset_ops.Dataset.TFRecordDataset([
"/a/1.tfr", "/a/2.tfr", "/a/3.tfr", "/a/4.tfr"])
# Distribute that dataset
dist_dataset = strategy.experimental_distribute_dataset(dataset)
# Iterate over the distributed dataset
for x in dist_dataset:
# process dataset elements
strategy.experimental_run_v2(train_step, args=(x,))
We will assume that the input dataset is batched by the global batch size. With this assumption, we will make a best effort to divide each batch across all the replicas (one or more workers).
In a multi-worker setting, we will first attempt to distribute the dataset by attempting to detect whether the dataset is being created out of ReaderDatasets (e.g. TFRecordDataset, TextLineDataset, etc.) and if so, attempting to shard the input files. Note that there has to be at least one input file per worker. If you have less than one input file per worker, we suggest that you should disable distributing your dataset using the method below.
If that attempt is unsuccessful (e.g. the dataset is created from a
Dataset.range), we will shard the dataset evenly at the end by appending a
.shard
operation to the end of the processing pipeline. This will cause
the entire preprocessing pipeline for all the data to be run on every
worker, and each worker will do redundant work. We will print a warning
if this method of sharding is selected. In this case, consider using
experimental_distribute_datasets_from_function
instead.
You can disable dataset sharding across workers using the auto_shard
option in tf.data.experimental.DistributeOptions
.
Within each worker, we will also split the data among all the worker devices (if more than one a present), and this will happen even if multi-worker sharding is disabled using the method above.
If the above batch splitting and dataset sharding logic is undesirable,
please use experimental_distribute_datasets_from_function
instead, which
does not do any automatic splitting or sharding.
Args | |
---|---|
dataset
|
tf.data.Dataset that will be sharded across all replicas using
the rules stated above.
|
Returns | |
---|---|
A "distributed Dataset ", which acts like a tf.data.Dataset except
it produces "per-replica" values.
|
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. Each
replica on that worker will dequeue one batch of inputs from the local
Dataset
(i.e. if a worker has two replicas, two batches will be dequeued
from the Dataset
every step).
This method can be used for several purposes. For example, where
experimental_distribute_dataset
is unable to shard the input files, this
method might be used to manually shard the dataset (avoiding the slow
fallback behavior in experimental_distribute_dataset
). In cases where the
dataset is infinite, this sharding can be done by creating dataset replicas
that differ only in their random seed.
experimental_distribute_dataset
may also sometimes fail to split the
batch across replicas on a worker. In that case, this method can be used
where that limitation does not exist.
The dataset_fn
should take an tf.distribute.InputContext
instance where
information about batching and input replication can be accessed:
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 acts like a tf.data.Dataset except
it produces "per-replica" values.
|
experimental_local_results
experimental_local_results(
value
)
Returns the list of all local per-replica values contained in value
.
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.
Executes ops specified by fn
on each replica. If args
or kwargs
have
"per-replica" values, such as those produced by a "distributed Dataset
",
when fn
is executed on a particular replica, it will be executed with the
component of those "per-replica" values that correspond to that replica.
fn
may call tf.distribute.get_replica_context()
to access members such
as all_reduce
.
All arguments in args
or kwargs
should either be nest of tensors or
per-replica objects containing tensors or composite tensors.
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 | |
---|---|
Merged return value of fn across replicas. The structure of the return
value is the same as the return value from fn . Each element in the
structure can either be "per-replica" Tensor objects or Tensor s
(for example, if running on a single replica).
|
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
.
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. |