Petastorm
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Petastorm is an open source data access library developed at Uber ATG. This library enables single machine or
distributed training and evaluation of deep learning models directly from datasets in Apache Parquet
format. Petastorm supports popular Python-based machine learning (ML) frameworks such as
Tensorflow <http://www.tensorflow.org/>
, PyTorch <https://pytorch.org/>
, and
PySpark <http://spark.apache.org/docs/latest/api/python/pyspark.html>
_. It can also be used from pure Python code.
Documentation web site: <https://petastorm.readthedocs.io>
_
Installation
.. code-block:: bash
pip install petastorm
There are several extra dependencies that are defined by the petastorm
package that are not installed automatically.
The extras are: tf
, tf_gpu
, torch
, opencv
, docs
, test
.
For example to trigger installation of GPU version of tensorflow and opencv, use the following pip command:
.. code-block:: bash
pip install petastorm[opencv,tf_gpu]
Generating a dataset
A dataset created using Petastorm is stored in Apache Parquet <https://parquet.apache.org/>
_ format.
On top of a Parquet schema, petastorm also stores higher-level schema information that makes multidimensional arrays into a native part of a petastorm dataset.
Petastorm supports extensible data codecs. These enable a user to use one of the standard data compressions (jpeg, png) or implement her own.
Generating a dataset is done using PySpark.
PySpark natively supports Parquet format, making it easy to run on a single machine or on a Spark compute cluster.
Here is a minimalistic example writing out a table with some random data.
.. code-block:: python
HelloWorldSchema = Unischema('HelloWorldSchema', [
UnischemaField('id', np.int32, (), ScalarCodec(IntegerType()), False),
UnischemaField('image1', np.uint8, (128, 256, 3), CompressedImageCodec('png'), False),
UnischemaField('other_data', np.uint8, (None, 128, 30, None), NdarrayCodec(), False),
])
def row_generator(x):
"""Returns a single entry in the generated dataset. Return a bunch of random values as an example."""
return {'id': x,
'image1': np.random.randint(0, 255, dtype=np.uint8, size=(128, 256, 3)),
'other_data': np.random.randint(0, 255, dtype=np.uint8, size=(4, 128, 30, 3))}
def generate_hello_world_dataset(output_url='file:///tmp/hello_world_dataset'):
rows_count = 10
rowgroup_size_mb = 256
spark = SparkSession.builder.config('spark.driver.memory', '2g').master('local[2]').getOrCreate()
sc = spark.sparkContext
# Wrap dataset materialization portion. Will take care of setting up spark environment variables as
# well as save petastorm specific metadata
with materialize_dataset(spark, output_url, HelloWorldSchema, rowgroup_size_mb):
rows_rdd = sc.parallelize(range(rows_count))\
.map(row_generator)\
.map(lambda x: dict_to_spark_row(HelloWorldSchema, x))
spark.createDataFrame(rows_rdd, HelloWorldSchema.as_spark_schema()) \
.coalesce(10) \
.write \
.mode('overwrite') \
.parquet(output_url)
HelloWorldSchema
is an instance of aUnischema
object.
Unischema
is capable of rendering types of its fields into different
framework specific formats, such as: SparkStructType
, Tensorflow
tf.DType
and numpynumpy.dtype
.- To define a dataset field, you need to specify a
type
,shape
, a
codec
instance and whether the field is nullable for each field of the
Unischema
. - We use PySpark for writing output Parquet files. In this example, we launch
PySpark on a local box (.master('local[2]')
). Of course for a larger
scale dataset generation we would need a real compute cluster. - We wrap spark dataset generation code with the
materialize_dataset
context manager. The context manager is responsible for configuring row
group size at the beginning and write out petastorm specific metadata at the
end. - The row generating code is expected to return a Python dictionary indexed by
a field name. We userow_generator
function for that. dict_to_spark_row
converts the dictionary into apyspark.Row
object while ensuring schemaHelloWorldSchema
compliance (shape,
type and is-nullable condition are tested).- Once we have a
pyspark.DataFrame
we write it out to a parquet
storage. The parquet schema is automatically derived from
HelloWorldSchema
.
Plain Python API
The petastorm.reader.Reader
class is the main entry point for user
code that accesses the data from an ML framework such as Tensorflow or Pytorch.
The reader has multiple features such as:
- Selective column readout
- Multiple parallelism strategies: thread, process, single-threaded (for debug)
- N-grams readout support
- Row filtering (row predicates)
- Shuffling
- Partitioning for multi-GPU training
- Local caching
Reading a dataset is simple using the petastorm.reader.Reader
class which can be created using the
petastorm.make_reader
factory method:
.. code-block:: python
from petastorm import make_reader
with make_reader('hdfs://myhadoop/some_dataset') as reader:
for row in reader:
print(row)
hdfs://...
and file://...
are supported URL protocols.
Once a Reader
is instantiated, you can use it as an iterator.
Tensorflow API
To hookup the reader into a tensorflow graph, you can use the tf_tensors
function:
.. code-block:: python
with make_reader('file:///some/localpath/a_dataset') as reader:
row_tensors = tf_tensors(reader)
with tf.Session() as session:
for _ in range(3):
print(session.run(row_tensors))
Alternatively, you can use new tf.data.Dataset
API;
.. code-block:: python
with make_reader('file:///some/localpath/a_dataset') as reader:
dataset = make_petastorm_dataset(reader)
iterator = dataset.make_one_shot_iterator()
tensor = iterator.get_next()
with tf.Session() as sess:
sample = sess.run(tensor)
print(sample.id)
Pytorch API
As illustrated in
pytorch_example.py <https://github.com/uber/petastorm/blob/master/examples/mnist/pytorch_example.py>
_,
reading a petastorm dataset from pytorch
can be done via the adapter class petastorm.pytorch.DataLoader
,
which allows custom pytorch collating function and transforms to be supplied.
Be sure you have torch
and torchvision
installed:
.. code-block:: bash
pip install torchvision
The minimalist example below assumes the definition of a Net
class and
train
and test
functions, included in pytorch_example
:
.. code-block:: python
import torch
from petastorm.pytorch import DataLoader
torch.manual_seed(1)
device = torch.device('cpu')
model = Net().to(device)
optimizer = torch.optim.SGD(model.parameters(), lr=0.01, momentum=0.5)
def _transform_row(mnist_row):
transform = transforms.Compose([
transforms.ToTensor(),
transforms.Normalize((0.1307,), (0.3081,))
])
return (transform(mnist_row['image']), mnist_row['digit'])
transform = TransformSpec(_transform_row, removed_fields=['idx'])
with DataLoader(make_reader('file:///localpath/mnist/train', num_epochs=10,
transform_spec=transform), batch_size=64) as train_loader:
train(model, device, train_loader, 10, optimizer, 1)
with DataLoader(make_reader('file:///localpath/mnist/test', num_epochs=10,
transform_spec=transform), batch_size=1000) as test_loader:
test(model, device, test_loader)
PySpark and SQL
Using the Parquet data format, which is natively supported by Spark, makes it possible to use a wide range of Spark
tools to analyze and manipulate the dataset. The example below shows how to read a Petastorm dataset
as a Spark RDD object:
.. code-block:: python
Create a dataframe object from a parquet file
dataframe = spark.read.parquet(dataset_url)
Show a schema
dataframe.printSchema()
Count all
dataframe.count()
Show a single column
dataframe.select('id').show()
SQL can be used to query a Petastorm dataset:
.. code-block:: python
spark.sql(
'SELECT count(id) '
'from parquet.file:///tmp/hello_world_dataset
').collect()
You can find a full code sample here: pyspark_hello_world.py <https://github.com/uber/petastorm/blob/master/examples/hello_world/petastorm_dataset/pyspark_hello_world.py>
_,
Non Petastorm Parquet Stores
Petastorm can also be used to read data directly from Apache Parquet stores. To achieve that, use
make_batch_reader
(and not make_reader
). The following table summarizes the differences
make_batch_reader
and make_reader
functions.
================================================================== =====================================================
make_reader
make_batch_reader
================================================================== =====================================================
Only Petastorm datasets (created using materializes_dataset) Any Parquet store (some native Parquet column types
are not supported yet.
The reader returns one record at a time. The reader returns batches of records. The size of the
batch is not fixed and defined by Parquet row-group
size.
Predicates passed to make_reader
are evaluated per single row. Predicates passed to make_batch_reader
are evaluated per batch.
================================================================== =====================================================
Troubleshooting
See the Troubleshooting_ page and please submit a ticket_ if you can't find an
answer.
Publications
- Gruener, R., Cheng, O., and Litvin, Y. (2018) Introducing Petastorm: Uber ATG's Data Access Library for Deep Learning. URL: https://eng.uber.com/petastorm/
.. _Troubleshooting: docs/troubleshoot.rst
.. _ticket: https://github.com/uber/petastorm/issues/new
.. _Development: docs/development.rst
How to Contribute
We prefer to receive contributions in the form of GitHub pull requests. Please send pull requests against the github.com/uber/petastorm
repository.
- If you are looking for some ideas on what to contribute, check out
github issues <https://github.com/uber/petastorm/issues>
_ and comment on the issue. - If you have an idea for an improvement, or you'd like to report a bug but don't have time to fix it please a
create a github issue <https://github.com/uber/petastorm/issues/new>
_.
To contribute a patch:
- Break your work into small, single-purpose patches if possible. It's much harder to merge in a large change with a lot of disjoint features.
- Submit the patch as a GitHub pull request against the master branch. For a tutorial, see the GitHub guides on forking a repo and sending a pull request.
- Include a detailed describtion of the proposed change in the pull request.
- Make sure that your code passes the unit tests. You can find instructions how to run the unit tests
here <https://github.com/uber/petastorm/blob/master/docs/development.rst>
_. - Add new unit tests for your code.
Thank you in advance for your contributions!
See the Development_ for development related information.
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