Eager execution

    TensorFlow’s eager execution is an imperative programming environment that evaluates operations immediately, without building graphs: operations return concrete values instead of constructing a computational graph to run later. This makes it easy to get started with TensorFlow and debug models, and it reduces boilerplate as well. To follow along with this guide, run the code samples below in an interactive R interpreter.

    Eager execution is a flexible machine learning platform for research and experimentation, providing:

    • An intuitive interface—Structure your code naturally and use R data structures. Quickly iterate on small models and small data.
    • Easier debugging—Call ops directly to inspect running models and test changes. Use standard R debugging tools for immediate error reporting.
    • Natural control flow—Use R control flow instead of graph control flow, simplifying the specification of dynamic models.

    Eager execution supports most TensorFlow operations and GPU acceleration.

    Note: Some models may experience increased overhead with eager execution enabled. Performance improvements are ongoing, but please file a bug if you find a problem and share your benchmarks.

    Setup and basic usage

    library(tensorflow)
    library(tfautograph)
    library(keras)
    library(tfdatasets)

    In Tensorflow 2.0, eager execution is enabled by default.

    ## [1] TRUE

    Now you can run TensorFlow operations and the results will return immediately:

    x <- matrix(2, ncol = 1, nrow = 1)
    m <- tf$matmul(x, x)
    m
    ## tf.Tensor([[4.]], shape=(1, 1), dtype=float64)

    Enabling eager execution changes how TensorFlow operations behave—now they immediately evaluate and return their values to R tf$Tensor objects reference concrete values instead of symbolic handles to nodes in a computational graph. Since there isn’t a computational graph to build and run later in a session, it’s easy to inspect results using print() or a debugger. Evaluating, printing, and checking tensor values does not break the flow for computing gradients.

    Eager execution works nicely with R. TensorFlow math operations convert R objects and R arrays to tf$Tensor objects. The as.array method returns the object’s value as an R array.

    a <- tf$constant(matrix(c(1,2,3,4), ncol = 2))
    a
    ## tf.Tensor(
    ## [[1. 3.]
    ##  [2. 4.]], shape=(2, 2), dtype=float64)
    ## tf.Tensor(
    ## [[2. 4.]
    ##  [3. 5.]], shape=(2, 2), dtype=float64)
    ## tf.Tensor(
    ## [[ 2. 12.]
    ##  [ 6. 20.]], shape=(2, 2), dtype=float64)
    ##      [,1] [,2]
    ## [1,]    1    3
    ## [2,]    2    4

    Dynamic control flow

    A major benefit of eager execution is that all the functionality of the host language is available while your model is executing. So, for example, it is easy to write fizzbuzz:

    This has conditionals that depend on tensor values and it prints these values at runtime.

    Eager training

    Computing gradients

    Automatic differentiation is useful for implementing machine learning algorithms such as backpropagation for training neural networks. During eager execution, use tf$GradientTape to trace operations for computing gradients later.

    You can use tf$GradientTape to train and/or compute gradients in eager. It is especially useful for complicated training loops.

    Since different operations can occur during each call, all forward-pass operations get recorded to a “tape”. To compute the gradient, play the tape backwards and then discard. A particular tf$GradientTape can only compute one gradient; subsequent calls throw a runtime error.

    ## tf.Tensor(2.0, shape=(), dtype=float32)

    Train a model

    The following example creates a multi-layer model that classifies the standard MNIST handwritten digits. It demonstrates the optimizer and layer APIs to build trainable graphs in an eager execution environment.

    ## <BatchDataset shapes: ((None, 28, 28, 1), (None,)), types: (tf.float32, tf.int32)>
    mnist_model <- keras_model_sequential() %>% 
      layer_conv_2d(filters = 16, kernel_size = c(3,3), activation= "relu",
                    input_shape = shape(NULL, NULL, 1)) %>% 
      layer_conv_2d(filters = 16, kernel_size = c(3,3), activation = "relu") %>% 
      layer_global_average_pooling_2d() %>% 
      layer_dense(units = 10)

    Even without training, call the model and inspect the output in eager execution:

    ## tf.Tensor(
    ## [[-8.50985199e-03  9.76161857e-04 -2.50255484e-02 -5.79575971e-02
    ##   -3.91511843e-02 -2.02112067e-02  1.19331172e-02  2.99258605e-02
    ##    7.55230756e-03  3.86199094e-02]
    ##  [-5.54877939e-03  1.90716446e-03 -1.70769673e-02 -3.62131633e-02
    ##   -2.53974535e-02 -1.38209835e-02  7.40819378e-03  1.79758631e-02
    ##    5.72366873e-03  2.54252721e-02]
    ##  [-2.37655244e-03  1.36287510e-03 -1.33525934e-02 -3.33486199e-02
    ##   -2.12530848e-02 -1.39125455e-02  5.86056244e-03  1.53014306e-02
    ##    5.15997969e-03  2.24790853e-02]
    ##  [-8.92254990e-04  2.28004996e-03 -1.07957972e-02 -3.00190244e-02
    ##   -1.75903179e-02 -1.35528101e-02  4.88691870e-03  1.25359586e-02
    ##    5.23545966e-03  1.93263516e-02]
    ##  [-4.21859929e-03  3.05507542e-03 -1.49999214e-02 -3.11945472e-02
    ##   -2.06255876e-02 -1.27387317e-02  6.34148577e-03  1.41533548e-02
    ##    5.45461895e-03  2.25168187e-02]
    ##  [-3.46548553e-03  1.24341354e-03 -1.45013975e-02 -4.30306159e-02
    ##   -3.16537209e-02 -1.74248051e-02  6.47401158e-03  2.26319134e-02
    ##    5.64713310e-03  2.93269195e-02]
    ##  [-6.41194824e-03  1.38130516e-03 -1.84288751e-02 -4.14446481e-02
    ##   -3.08680199e-02 -1.57348588e-02  7.77957682e-03  2.17617080e-02
    ##    4.86520818e-03  2.92749219e-02]
    ##  [-8.39145947e-03 -2.43743139e-04 -2.18558982e-02 -5.35714217e-02
    ##   -3.83904204e-02 -1.82459299e-02  1.09588886e-02  3.00901663e-02
    ##    3.97703936e-03  3.47796679e-02]
    ##  [-6.90627703e-03 -2.02620332e-03 -1.62484460e-02 -4.23779786e-02
    ##   -3.48815881e-02 -1.46378912e-02  7.04134628e-03  2.60561779e-02
    ##    2.87065259e-03  3.00167799e-02]
    ##  [-3.46036465e-03  3.34005570e-03 -1.42491609e-02 -2.86302492e-02
    ##   -1.86991822e-02 -1.21941017e-02  5.76612214e-03  1.24817807e-02
    ##    5.39597031e-03  2.08636373e-02]
    ##  [-6.75824890e-03  1.80363667e-03 -1.81528479e-02 -3.73662151e-02
    ##   -2.79965084e-02 -1.33580044e-02  7.09015829e-03  1.83111280e-02
    ##    6.74578175e-03  2.72248089e-02]
    ##  [-3.44557199e-03  1.44218188e-03 -1.55201033e-02 -3.91926579e-02
    ##   -3.03197410e-02 -1.78610198e-02  6.71680411e-03  2.06990894e-02
    ##    5.46659622e-03  2.73653362e-02]
    ##  [-2.98760762e-03  6.68506909e-05 -1.03480723e-02 -2.65669450e-02
    ##   -2.34568883e-02 -1.09654916e-02  3.45098414e-03  1.51341530e-02
    ##    4.49841795e-03  2.06842236e-02]
    ##  [-6.59199711e-03  1.85408711e-03 -1.94277260e-02 -4.28726152e-02
    ##   -2.99611464e-02 -1.58806108e-02  9.21235979e-03  2.24604607e-02
    ##    5.33315912e-03  2.91829202e-02]
    ##  [-7.55199324e-03 -9.93973459e-04 -2.15730183e-02 -5.56724407e-02
    ##   -4.60459515e-02 -2.07579192e-02  9.57913976e-03  3.33841294e-02
    ##    4.62856423e-03  3.92136984e-02]
    ##  [-2.03214702e-03  4.08457185e-04 -1.21998340e-02 -3.37962173e-02
    ##   -2.65589673e-02 -1.54427039e-02  4.19362914e-03  1.70531943e-02
    ##    5.84620563e-03  2.43132822e-02]
    ##  [-3.81546142e-03  1.21751742e-04 -1.36933727e-02 -3.54161970e-02
    ##   -2.85060816e-02 -1.41160497e-02  5.15741529e-03  1.88995544e-02
    ##    5.81339980e-03  2.64615659e-02]
    ##  [-4.51849913e-03  1.79681068e-04 -1.25195542e-02 -2.85590179e-02
    ##   -2.36752722e-02 -1.03663774e-02  4.86267731e-03  1.64620187e-02
    ##    4.00224933e-03  2.16186680e-02]
    ##  [-6.88880868e-03  2.30632047e-03 -2.50062961e-02 -6.10050745e-02
    ##   -4.42578457e-02 -2.45542563e-02  1.10575147e-02  3.20139751e-02
    ##    7.40471063e-03  4.25111316e-02]
    ##  [-8.04840680e-03 -1.73170422e-03 -2.13432573e-02 -5.59643619e-02
    ##   -4.14501876e-02 -1.88157260e-02  1.08416816e-02  3.29777822e-02
    ##    3.58740776e-03  3.77420597e-02]
    ##  [-2.12463085e-03  1.40806718e-03 -1.62827484e-02 -4.21891250e-02
    ##   -2.92056706e-02 -1.80202033e-02  6.18648017e-03  1.89643912e-02
    ##    7.54634384e-03  2.85427365e-02]
    ##  [-6.24155253e-03  3.68376786e-04 -1.89247429e-02 -4.59269919e-02
    ##   -3.55105102e-02 -1.77306253e-02  7.98209663e-03  2.48527452e-02
    ##    5.78143680e-03  3.23706158e-02]
    ##  [-1.42555241e-03  4.77403111e-04 -1.20030018e-02 -3.56824584e-02
    ##   -2.53661703e-02 -1.50882667e-02  5.09238290e-03  1.77882873e-02
    ##    5.66911884e-03  2.43990738e-02]
    ##  [-1.21319480e-02 -3.38456419e-04 -3.04601416e-02 -6.95648193e-02
    ##   -5.14865555e-02 -2.32764110e-02  1.34642534e-02  3.88753153e-02
    ##    5.17100841e-03  4.83683124e-02]
    ##  [-7.99048692e-03  1.30610866e-03 -2.30237599e-02 -5.47382683e-02
    ##   -3.83893251e-02 -2.00371165e-02  1.12129143e-02  2.90373228e-02
    ##    5.98406652e-03  3.79212201e-02]
    ##  [-6.75235782e-03  9.91679379e-04 -1.80075001e-02 -3.80989239e-02
    ##   -2.85798106e-02 -1.37160970e-02  7.86706619e-03  2.10245345e-02
    ##    3.95417260e-03  2.71354374e-02]
    ##  [-7.98894465e-03  4.55419155e-04 -2.53729578e-02 -6.31851330e-02
    ##   -4.31225747e-02 -2.33430732e-02  1.32131195e-02  3.43508609e-02
    ##    5.29513042e-03  4.14416529e-02]
    ##  [-3.49725038e-03 -2.39763220e-04 -1.08997943e-02 -2.69409642e-02
    ##   -2.45306063e-02 -1.08287791e-02  3.64527735e-03  1.59635600e-02
    ##    3.91276646e-03  2.12638490e-02]
    ##  [-6.46782434e-03 -7.04026374e-04 -1.56770907e-02 -3.86993401e-02
    ##   -3.16020884e-02 -1.30451052e-02  6.17024768e-03  2.19560675e-02
    ##    4.18775994e-03  2.79887151e-02]
    ##  [-5.55107370e-03  2.06118939e-03 -1.58842616e-02 -3.25190350e-02
    ##   -2.33283471e-02 -1.21178171e-02  6.47215592e-03  1.56531073e-02
    ##    5.25392406e-03  2.41678189e-02]
    ##  [-8.96292087e-03 -5.41977119e-04 -2.13856287e-02 -4.84847501e-02
    ##   -3.47109959e-02 -1.52589623e-02  1.07035376e-02  2.74108090e-02
    ##    4.38953377e-03  3.33805010e-02]
    ##  [-5.13768382e-03 -8.29380937e-04 -1.55788297e-02 -4.19435799e-02
    ##   -3.52306105e-02 -1.68032013e-02  6.36776956e-03  2.41187550e-02
    ##    5.30361803e-03  3.00990045e-02]], shape=(32, 10), dtype=float32)

    While keras models have a builtin training loop (using the fit method), sometimes you need more customization. Here’s an example, of a training loop implemented with eager:

    Note: Use the assert functions in tf$debugging to check if a condition holds up. This works in eager and graph execution.

    history <- loss_history %>% 
      purrr::map(as.numeric) %>% 
      purrr::flatten_dbl()
    ggplot2::qplot(x = seq_along(history), y = history, geom = "line")

    Variables and optimizers

    tf$Variable objects store mutable tf$Tensor-like values accessed during training to make automatic differentiation easier.

    The collections of variables can be encapsulated into layers or models, along with methods that operate on them. See Custom Keras layers and models for details. The main difference between layers and models is that models add methods like fit, evaluate, and save.

    For example, the automatic differentiation example above can be rewritten:

    Next:

    1. Create the model.
    2. The Derivatives of a loss function with respect to model parameters.
    3. A strategy for updating the variables based on the derivatives.
    model <- Linear()
    optimizer <- optimizer_sgd(lr = 0.01)
    
    cat("Initial loss: ", as.numeric(loss(model, training_inputs, training_outputs), "\n"))
    ## Initial loss:  68.66985
    for (i in seq_len(300)) {
      grads <- grad(model, training_inputs, training_outputs)
      optimizer$apply_gradients(purrr::transpose(
        list(grads, list(model$w, model$b))
      ))
      if (i %% 20 == 0)
        cat("Loss at step ", i, ": ", as.numeric(loss(model, training_inputs, training_outputs)), "\n")
    }
    ## Loss at step  20 :  31.23723 
    ## Loss at step  40 :  14.52059 
    ## Loss at step  60 :  7.055109 
    ## Loss at step  80 :  3.721035 
    ## Loss at step  100 :  2.23201 
    ## Loss at step  120 :  1.566984 
    ## Loss at step  140 :  1.269965 
    ## Loss at step  160 :  1.137304 
    ## Loss at step  180 :  1.078052 
    ## Loss at step  200 :  1.051587 
    ## Loss at step  220 :  1.039765 
    ## Loss at step  240 :  1.034485 
    ## Loss at step  260 :  1.032126 
    ## Loss at step  280 :  1.031073 
    ## Loss at step  300 :  1.030602
    ## <tf.Variable 'weight:0' shape=() dtype=float32, numpy=3.0587368>
    ## <tf.Variable 'bias:0' shape=() dtype=float32, numpy=2.0177262>

    Note: Variables persist until the last reference to the object is removed, and is the variable is deleted.

    Object-based saving

    A Keras model includes a convinient save_weights method allowing you to easily create a checkpoint:

    save_model_weights_tf(model, "weights")
    load_model_weights_tf(model, filepath = "weights")

    Using tf$train$Checkpoint you can take full control over this process.

    This section is an abbreviated version of the guide to training checkpoints.

    x$assign(2) # Assign a new value to the variables and save.
    ## <tf.Variable 'UnreadVariable' shape=() dtype=float32, numpy=2.0>
    ## [1] "ckpt/-1"
    x$assign(11) # Change the variable after saving.
    ## <tf.Variable 'UnreadVariable' shape=() dtype=float32, numpy=11.0>
    ## <tensorflow.python.training.tracking.util.CheckpointLoadStatus>
    ## <tf.Variable 'Variable:0' shape=() dtype=float32, numpy=2.0>

    To save and load models, tf$train$Checkpoint stores the internal state of objects, without requiring hidden variables. To record the state of a model, an optimizer, and a global step, pass them to a tf$train$Checkpoint:

    model <- keras_model_sequential() %>% 
      layer_conv_2d(filters = 16, kernel_size = c(3,3), activation = "relu") %>% 
      layer_global_average_pooling_2d() %>% 
      layer_dense(units = 10)
    
    optimizer <- optimizer_adam(lr = 0.001)
    
    checkpoint_dir <- 'path/to/model_dir'
    if (!dir.exists(checkpoint_dir))
      dir.create(checkpoint_dir, recursive = TRUE)
    
    checkpoint_prefix <- file.path(checkpoint_dir, "ckpt")
    
    root <- tf$train$Checkpoint(optimizer = optimizer, model = model)
    
    root$save(checkpoint_prefix)
    ## [1] "path/to/model_dir/ckpt-1"
    ## <tensorflow.python.training.tracking.util.CheckpointLoadStatus>

    Note: In many training loops, variables are created after tf\(train\)Checkpoint.restore is called. These variables will be restored as soon as they are created, and assertions are available to ensure that a checkpoint has been fully loaded. See the guide to training checkpoints for details.

    Object-oriented metrics

    tf$keras$metrics are stored as objects. Update a metric by passing the new data to the callable, and retrieve the result using the tf$keras$metrics$result method, for example:

    ## tf.Tensor(0.0, shape=(), dtype=float32)
    ## tf.Tensor(2.5, shape=(), dtype=float32)
    ## tf.Tensor(2.5, shape=(), dtype=float32)
    m(c(8, 9))
    ## tf.Tensor(5.5, shape=(), dtype=float32)
    ## tf.Tensor(5.5, shape=(), dtype=float32)

    Summaries and TensorBoard

    TensorBoard is a visualization tool for understanding, debugging and optimizing the model training process. It uses summary events that are written while executing the program.

    You can use tf$summary to record summaries of variable in eager execution. For example, to record summaries of loss once every 100 training steps:

    Advanced automatic differentiation topics

    Dynamic models

    tf$GradientTape can also be used in dynamic models. This example for a backtracking line search algorithm looks like normal R code, except there are gradients and is differentiable, despite the complex control flow:

    Custom gradients

    Custom gradients are an easy way to override gradients. Within the forward function, define the gradient with respect to the inputs, outputs, or intermediate results. For example, here’s an easy way to clip the norm of the gradients in the backward pass:

    Custom gradients are commonly used to provide a numerically stable gradient for a sequence of operations:

    ## tf.Tensor(0.5, shape=(), dtype=float32)
    ## tf.Tensor(nan, shape=(), dtype=float32)

    Here, the log1pexp function can be analytically simplified with a custom gradient. The implementation below reuses the value for tf$exp(x) that is computed during the forward pass—making it more efficient by eliminating redundant calculations:

    ## tf.Tensor(0.5, shape=(), dtype=float32)
    ## tf.Tensor(1.0, shape=(), dtype=float32)

    Performance

    Computation is automatically offloaded to GPUs during eager execution. If you want control over where a computation runs you can enclose it in a tf$device('/gpu:0') block (or the CPU equivalent):

    A tf$Tensor object can be copied to a different device to execute its operations:

    Benchmarks

    For compute-heavy models, such as ResNet50 training on a GPU, eager execution performance is comparable to tf_function execution. But this gap grows larger for models with less computation and there is work to be done for optimizing hot code paths for models with lots of small operations.

    Work with functions

    While eager execution makes development and debugging more interactive, TensorFlow 1.x style graph execution has advantages for distributed training, performance optimizations, and production deployment. To bridge this gap, TensorFlow 2.0 introduces functions via the tf_function API. For more information, see the tf_function guide.