Make sure to indicate the
batch_size= to normalize the results (e.g. 100
iter/sec -> 3200 ex/sec).
ds = tfds.load('mnist', split='train').batch(32).prefetch() # Display some benchmark statistics tfds.core.benchmark(ds, batch_size=32) # Second iteration is much faster, due to auto-caching tfds.core.benchmark(ds, batch_size=32)
Small datasets (< GB)
All TFDS datasets store the data on disk in the
For small datasets (e.g. Mnist, Cifar,...), reading from
.tfrecord can add
As those datasets fit in memory, it is possible to significantly improve the performance by caching or pre-loading the dataset. Note that TFDS automatically caches small datasets (see next section for details).
Caching the dataset
Here is an example of a data pipeline which explicitly caches the dataset after normalizing the images.
def normalize_img(image, label): """Normalizes images: `uint8` -> `float32`.""" return tf.cast(image, tf.float32) / 255., label ds, ds_info = tfds.load( 'mnist', split='train', as_supervised=True, # returns `(img, label)` instead of dict(image=, ...) with_info=True, ) # Applying normalization before `ds.cache()` to re-use it. # Note: Random transformations (e.g. images augmentations) should be applied # after both `ds.cache()` (to avoid caching randomness) and `ds.batch()` (for # vectorization ). ds = ds.map(normalize_img, num_parallel_calls=tf.data.experimental.AUTOTUNE) ds = ds.cache() # For true randomness, we set the shuffle buffer to the full dataset size. ds = ds.shuffle(ds_info.splits['train'].num_examples) # Batch after shuffling to get unique batches at each epoch. ds = ds.batch(128) ds = ds.prefetch(tf.data.experimental.AUTOTUNE)
When iterating over this dataset, the second iteration will be much faster than the first one thanks to the caching.
By default, TFDS auto-caches datasets which satisfy the following constraints:
- Total dataset size (all splits) is defined and < 250 MiB
shuffle_filesis disabled, or only a single shard is read
It is possible to opt out of auto-caching by passing
tfds.load. Have a look at the dataset catalog
documentation to see if a specific dataset will use auto-cache.
Loading the full data as a single Tensor
If your dataset fits into memory, you can also load the full dataset as a single
Tensor or NumPy array. It is possible to do so by setting
batch all examples in a single
tf.Tensor. Then use
tfds.as_numpy for the
(img_train, label_train), (img_test, label_test) = tfds.as_numpy(tfds.load( 'mnist', split=['train', 'test'], batch_size=-1, as_supervised=True, ))
Large datasets are sharded (split in multiple files), and typically do not fit in memory so they should not be cached.
Shuffle and training
During training, it's important to shuffle the data well; poorly shuffled data can result in lower training accuracy.
In addition to using
ds.shuffle to shuffle records, you should also set
shuffle_files=True to get good shuffling behavior for larger datasets that are
sharded into multiple files. Otherwise, epochs will read the shards in the same
order, and so data won't be truly randomized.
ds = tfds.load('imagenet2012', split='train', shuffle_files=True)
shuffle_files=True, TFDS disables
which may give a slight performance boost. To get deterministic shuffling, it is
possible to opt-out of this feature with
tfds.ReadConfig: either by setting
read_config.shuffle_seed or overwriting
Auto-shard your data across workers
When training on multiple workers, you can use the
input_context argument of
tfds.ReadConfig, so each worker will read a subset of the data.
input_context = tf.distribute.InputContext( input_pipeline_id=1, # Worker id num_input_pipelines=4, # Total number of workers ) read_config = tfds.ReadConfig( input_context=input_context, ) ds = tfds.load('dataset', split='train', read_config=read_config)
This is complementary to the subsplit API. First the subplit API is applied (
train[:50%] is converted into a list of files to read), then a
is applied on those files. Example: when using
num_input_pipelines=2, each of the 2 worker will read 1/4 of the data.
shuffle_files=True, files are shuffled within one worker, but not across
workers. Each worker will read the same subset of files between epochs.
Faster image decoding
By default TFDS automatically decodes images. However, there are cases where it
can be more performant to skip the image decoding with
tfds.decode.SkipDecoding and manually apply the
- When filtering examples (with
ds.filter), to decode images after examples have been filtered.
- When cropping images, to use the fused
The code for both examples is available in the decode guide.