Saving memory in convolutions

There are different approaches to apply the Jacobian with respect to the kernel of a convolution. They exhibit a non-trivial trade-off between run time and memory consumption (see more details below). The default choice in BackPACK is a memory- intensive implementation. This can lead to out-of-memory errors.

Here, we show how to switch BackPACK’s vector-Jacobian product algorithm for the kernel (weight) of torch.nn.Conv2d modules to a memory-saving variant presented in [Rochette, 2019].

This can be helpful if you are experiencing memory overflows with CNNs.

Note

This feature is experimental and may change in future releases.

Note

Currently, the savings only affect BackPACK’s first-order extensions. This may change in future releases.

Let’s get the imports out of our way.

import time

import torch
from memory_profiler import memory_usage
from torch.nn import (
    Conv2d,
    CrossEntropyLoss,
    Flatten,
    Linear,
    MaxPool2d,
    ReLU,
    Sequential,
)

from backpack import backpack, extend, extensions
from backpack.core.derivatives.convnd import weight_jac_t_save_memory
from backpack.utils.examples import load_one_batch_mnist

We start with the utility function for setting up an extended CNN, loss function, and input data from MNIST.

def setup(device):
    """Load MNIST batch, create extended CNN and loss function. Load to device.

    Args:
        device (torch.device): Device that all objects are transferred to.

    Returns:
        inputs, labels, model, loss function
    """
    X, y = load_one_batch_mnist(batch_size=64)
    X, y = X.to(device), y.to(device)

    model = extend(
        Sequential(
            Conv2d(1, 128, 3, padding=1),
            ReLU(),
            MaxPool2d(3, stride=2),
            Conv2d(128, 256, 3, padding=1),
            ReLU(),
            MaxPool2d(3, padding=1, stride=2),
            Conv2d(256, 64, 3, padding=1),
            ReLU(),
            MaxPool2d(3, stride=2),
            Conv2d(64, 32, 3, padding=1),
            ReLU(),
            MaxPool2d(3, stride=2),
            Flatten(),
            Linear(32, 10),
        ).to(device)
    )

    lossfunc = extend(CrossEntropyLoss().to(device))

    return X, y, model, lossfunc

Let’s demonstrate the differences between the vector-Jacobian methods. we benchmark the following function that computes individual gradients on the specified setup using BackPACK’s BatchGrad extensions.

def compute_individual_gradients(device, seed=0):
    """Compute individual gradients for the seeded problem specified in ``setup``.

    Args:
        device (torch.device): Device that the computation should be performed on.
        seed (int): Random seed to set before setting up the problem.

    Returns:
        Dictionary with parameter name and individual gradients as key value pairs.
    """
    torch.manual_seed(seed)

    X, y, model, lossfunc = setup(device)

    loss = lossfunc(model(X), y)

    with backpack(extensions.BatchGrad()):
        loss.backward()

    return {name: param.grad_batch for name, param in model.named_parameters()}

The memory-saving strategy is enabled by wrapping the backward pass with BackPACK inside weight_jac_t_save_memory which accepts a boolean flag save_memory.

Peak memory comparison

Let’s see the differences between both vector-Jacobian methods in terms of peak memory consumption.

def compare_peakmem(device):
    """Print peak memory of different vector-Jacobian algorithms for convolution.

    Peak memory only makes sense when ``device`` is CPU as memory usage on GPU
    cannot be tracked by this implementation.

    Args:
        device (torch.device): Device that the computation should be performed on.
    """
    print(f"Device: {device}")

    for save_memory in True, False:

        with weight_jac_t_save_memory(save_memory=save_memory):

            def work():
                return compute_individual_gradients(device)

            interval = 1e-3
            peakmem = max(memory_usage(work, interval=interval))

        print(f"Save memory: {save_memory}\tPeak memory: {peakmem:.1f}")


compare_peakmem(torch.device("cpu"))

Out:

Device: cpu
/home/docs/checkouts/readthedocs.org/user_builds/backpack/envs/master/lib/python3.7/site-packages/torch/nn/modules/module.py:974: UserWarning: Using a non-full backward hook when the forward contains multiple autograd Nodes is deprecated and will be removed in future versions. This hook will be missing some grad_input. Please use register_full_backward_hook to get the documented behavior.
  warnings.warn("Using a non-full backward hook when the forward contains multiple autograd Nodes "
Save memory: True       Peak memory: 694.6
Save memory: False      Peak memory: 2039.2

As expected, the backpropagation with save_memory=True requires less RAM.

Run time comparison

Next, we inspect the run time of both strategies.

def compare_runtime(device):
    """Print run time of different vector-Jacobian algorithms for convolution.

    Args:
        device (torch.device): Device that the computation should be performed on.
    """
    print(f"Device: {device}")

    for save_memory in True, False:

        with weight_jac_t_save_memory(save_memory=save_memory):
            start = time.time()

            compute_individual_gradients(device)

            if str(device) == "cuda":
                torch.cuda.synchronize()

            run_time = time.time() - start

        print(f"Save memory: {save_memory}\tRun time: {run_time:.3f}")


compare_runtime(torch.device("cpu"))

Out:

Device: cpu
/home/docs/checkouts/readthedocs.org/user_builds/backpack/envs/master/lib/python3.7/site-packages/torch/nn/modules/module.py:974: UserWarning: Using a non-full backward hook when the forward contains multiple autograd Nodes is deprecated and will be removed in future versions. This hook will be missing some grad_input. Please use register_full_backward_hook to get the documented behavior.
  warnings.warn("Using a non-full backward hook when the forward contains multiple autograd Nodes "
Save memory: True       Run time: 0.642
Save memory: False      Run time: 1.613

In this case, saving memory comes at the cost of reduced run time performance.

If you have a GPU you will see a similar behavior, too:

if torch.cuda.is_available():
    compare_runtime(torch.device("cuda"))

Let’s quickly double-check that both algorithms computed the same result.

device = torch.device("cpu")

with weight_jac_t_save_memory(save_memory=True):
    individual_gradients = compute_individual_gradients(device)

with weight_jac_t_save_memory(save_memory=False):
    save_memory_individual_gradients = compute_individual_gradients(device)

print(f"{'Parameter':<10}| Same individual gradients?")
for param_name in individual_gradients.keys():
    same = torch.allclose(
        individual_gradients[param_name],
        save_memory_individual_gradients[param_name],
        atol=1e-7,
    )
    msg = f"{param_name:<10}| {same}"

    if same:
        print(msg)
    else:
        raise ValueError(msg)

Out:

/home/docs/checkouts/readthedocs.org/user_builds/backpack/envs/master/lib/python3.7/site-packages/torch/nn/modules/module.py:974: UserWarning: Using a non-full backward hook when the forward contains multiple autograd Nodes is deprecated and will be removed in future versions. This hook will be missing some grad_input. Please use register_full_backward_hook to get the documented behavior.
  warnings.warn("Using a non-full backward hook when the forward contains multiple autograd Nodes "
Parameter | Same individual gradients?
0.weight  | True
0.bias    | True
3.weight  | True
3.bias    | True
6.weight  | True
6.bias    | True
9.weight  | True
9.bias    | True
13.weight | True
13.bias   | True

When to enable save memory?

If your program crashes because BackPACK tries to allocate too much memory, you should give it a try. Other than that, it is difficult to identify tendencies. The trend observed in this example (saving memory means slower run time) does not hold true in general, and you may want to compare both approaches for your specific setting, like we did here.

You can also take a look at backpack-benchmark, where BackPACK’s run time and peak memory are continuously monitored for some neural nets from DeepOBS.

This benchmark can be inspected over the commit history. Commits between 567f079b and f72f666 were performed with save_memory=True. Compare them with any other commit benchmarked with save_memory=False to get an intuition how both algorithms differ.

Total running time of the script: ( 0 minutes 7.935 seconds)

Gallery generated by Sphinx-Gallery