Using sub-batches to avoid busting the memory?

nicolas-gervais: <p>I am facing an error, because my batch size of 128 does not fit in my GPU. However, I do not want to reduce the batch size too much, because the network will be too unstable. Is it possible to sample from the batch of size 128 in pieces of 8, calculate the loss, and calculate gradients after passing 16x8 = 128 samples? This way, it would avoid fitting all 128 instances, but it would still update every 128 instances.</p>
<p>Here’s a state of the art reproducible example:</p>
<pre><code class="lang-auto">import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.optim as optim
from torch.utils.data import DataLoader, Dataset
from torch.nn import Module

x = torch.rand((1000, 3, 30, 30))
y = torch.randint(0, 2, size=(1000,)).type('torch.LongTensor')


class ConvNet(Module):

    def __init__(self):
        super().__init__()
        a = 32
        b = 64
        c = 128
        self.conv1 = nn.Conv2d(3, a, 3)
        self.conv2 = nn.Conv2d(a, b, 3)
        self.conv3 = nn.Conv2d(b, c, 3)

        self.fc1 = nn.Linear(2 * 2 * c, 1024)
        self.fc2 = nn.Linear(1024, 2048)
        self.fc3 = nn.Linear(2048, 2)

    def forward(self, x):
        x = F.max_pool2d(F.relu(self.conv1(x)), (2, 2))
        x = F.max_pool2d(F.relu(self.conv2(x)), (2, 2))
        x = F.max_pool2d(F.relu(self.conv3(x)), (2, 2))

        x = x.view(x.size(0), -1)
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = F.dropout(x, 0.5)
        x = self.fc3(x)
        return x


net = ConvNet()

optimizer = optim.Adam(net.parameters(), lr=0.0001)

loss_function = nn.CrossEntropyLoss()


class DataSet():

    def __init__(self):
        self.len = x.shape[0]
        self.x_train = x
        self.y_train = y

    def __getitem__(self, index):
        return x[index], y[index]

    def __len__(self):
        return self.len


data = DataSet()

train_loader = DataLoader(dataset=data, batch_size=64, shuffle=True)

epochs = 2
steps = 0
train_losses, test_losses = [], []
for e in range(epochs):
    running_loss = 0
    net.train()
    for images, labels in train_loader:
        if torch.cuda.is_available():
            images, labels = images.cuda(), labels.cuda()
        optimizer.zero_grad()
        log_ps = net(images)
        loss = loss_function(log_ps, labels)
        loss.backward()
        optimizer.step()
        running_loss += loss.item()

        print("[Epoch: {}/{}] ".format(e + 1, epochs),
              "[Training Loss: {:.3f}] ".format(running_loss / len(train_loader)))
</code></pre>

albanD: <p>Hi,</p>
<p>I think this post <a href="https://discuss.pytorch.org/t/why-do-we-need-to-set-the-gradients-manually-to-zero-in-pytorch/4903/20" class="inline-onebox">Why do we need to set the gradients manually to zero in pytorch?</a> will give you all the details you need for how to do this in a memory efficient way (you want to use 2).</p>

nicolas-gervais: <p>I’ll look into it in detail later but it seems to be exactly what I’m looking for. So a big thanks from me.</p>

nicolas-gervais: <p>IIUC, this updates the weights every 100, but takes only 10 in memory at a time?</p>

albanD: <p>Option 2 does that yes.<br>
The multiplication of the backward is a small slowdown. But if you don’t have enough memory, you don’t have a choice <img src="https://discuss.pytorch.org/images/emoji/apple/slight_smile.png?v=9" title=":slight_smile:" class="emoji" alt=":slight_smile:"></p>