Code slows down massively when running multiple seeds at once

David_Ireland: <p>I have a script that takes about 20 mins to run on my machine when running a single seed at a time. However, when I run several (10) seeds at once, the code slows down massively, taking around 3h to complete. I would expect running several seeds at once to slow down a bit but not so drastically, especially given that each seed uses around 950MB of GPU memory and the machine I am running on has 4 GPU’s each with 40GB of memory, and I split the jobs evenly across the GPU’s (so a max of 3 seeds on one GPU). I’ve profiled the code and found that what starts to become a lot slower is the <code>to.(device)</code> method. Unfortunately I cannot store all of the data on the GPU since it is an RL problem and so there is no static dataset since we collect new data online. Is there a reason why this becomes so much slower with more seeds running? When profiling the code, the <code>.to(device)</code> method goes from using 12107ms to 657858ms.</p>
<p>The version is 1.12.1. Here is a reproducible script that does essentially what the full script does, just cleaned up a bit:</p>
<pre><code class="lang-auto">import math
import numpy as np
import torch
import torch.nn as nn


class MLPResidualLayer(nn.Module):
    def __init__(self, dim):
        super(MLPResidualLayer, self).__init__()

        self.fc1 = nn.Linear(dim, dim)
        self.fc2 = nn.Linear(dim, dim)

    def forward(self, x):
        residual = x
        x = torch.relu(self.fc1(x))
        x = torch.relu(self.fc2(x))
        return residual + x


class Network(nn.Module):
    def __init__(self, state_dim, hidden_dim, num_actions, num_heads):
        super(Network, self).__init__()
        self.input_layer = nn.Linear(state_dim, hidden_dim)
        self.resnet = MLPResidualLayer(hidden_dim)
        self.layer_norm = nn.LayerNorm(hidden_dim)
        self.output_heads = VectorizedLinear(hidden_dim, num_actions, num_heads)
        self.num_heads = num_heads

    def forward(self, x):
        x = self.input_layer.forward(x)
        x = self.layer_norm(self.resnet.forward(x))
        x = x.unsqueeze(dim=0).repeat(self.num_heads, 1, 1)
        vals = self.output_heads.forward(x).transpose(0, 1)
        return vals


class VectorizedLinear(nn.Module):
    def __init__(self, in_features, out_features, ensemble_size):
        super().__init__()
        self.in_features = in_features
        self.out_features = out_features
        self.ensemble_size = ensemble_size

        self.weight = nn.Parameter(torch.empty(ensemble_size, in_features, out_features))
        self.bias = nn.Parameter(torch.empty(ensemble_size, 1, out_features))

        self.reset_parameters()

    def reset_parameters(self):
        stdv = 1. / math.sqrt(self.weight.size(1))
        self.weight.data.uniform_(-stdv, stdv)
        if self.bias is not None:
            self.bias.data.uniform_(-stdv, stdv)

    def forward(self, x: torch.Tensor) -&gt; torch.Tensor:
        # input: [ensemble_size, batch_size, input_size]
        # weight: [ensemble_size, input_size, out_size]
        # out: [ensemble_size, batch_size, out_size]
        return x @ self.weight + self.bias


class ReplayBuffer:
    def __init__(self, capacity, state_dim, num_heads, action_dim, batch_size=128):
        self.capacity = capacity
        self.batch_size = batch_size
        self.states = torch.randn(size=(capacity, state_dim), dtype=torch.float)
        self.actions = torch.randint(low=0, high=action_dim, size=(capacity, num_heads), dtype=torch.long)
        self.rewards = torch.randn(size=(capacity, 1), dtype=torch.float)
        self.next_states = torch.randn(size=(capacity, state_dim), dtype=torch.float)
        self.dones = torch.randint(low=0, high=2, size=(capacity, 1), dtype=torch.long)
        self.state_dim = state_dim
        self.num_heads = num_heads

    def sample(self):
        idx = np.random.randint(low=0, high=self.capacity, size=self.batch_size)  # when buffer large the probability of sampling a transition more than once -&gt; 0
        return self.states[idx], self.actions[idx], self.rewards[idx], self.next_states[idx], self.dones[idx]

    def __len__(self):
        return self.capacity


device = 'cuda' if torch.cuda.is_available() else 'cpu'
state_dim = 10
action_dim = 3
num_heads = 6
batch_size = 256

net = Network(state_dim, 512, action_dim, num_heads).to(device)
optimiser = torch.optim.Adam(net.parameters())
loss_fn = nn.HuberLoss()
buffer = ReplayBuffer(100000, state_dim, num_heads, action_dim, batch_size)

for update in range(100000):
    if (update + 1) % 1000 == 0:
        print(f"Update {update + 1}")
    states, actions, rewards, next_states, dones = buffer.sample()
    states = states.to(device)
    actions = actions.to(device)
    rewards = rewards.to(device)
    next_states = next_states.to(device)
    dones = dones.to(device)

    vals = net.forward(states).gather(2, actions.unsqueeze(dim=-1)).squeeze(dim=-1).mean(dim=1)
    with torch.no_grad():
        targets = net.forward(next_states).max(dim=-1)[0].mean(dim=-1, keepdim=True)
        targets = rewards + 0.99 * (1 - dones) * targets
    loss = loss_fn(vals, targets.flatten())
    optimiser.zero_grad()
    loss.backward()
    optimiser.step()
</code></pre>

ptrblck: <p>I assume you are launching multiple processes when describing “seeds”?<br>
In this case you are sharing the memory and compute resources of your device. If a single process is already utilizing e.g. all compute resources you won’t be able to execute anything else in parallel.<br>
Also, the <code>to()</code> operation might just be accumulating execution times from already scheduled kernels, so I would recommend creating a full visual profile with e.g. Nsight Systems.</p>

David_Ireland: <p>Thanks for the reply <a class="mention" href="/u/ptrblck">@ptrblck</a></p>
<p>Yes, running the same file multiple times from the command line is what I mean when I say different seeds.</p>
<p>Sure, I would expect it to slow down, but how much slower it becomes does not seem right. The machine has a lot of resources, around 270GB of RAM (each seed uses about 2.4GB of RAM), 4 GPU’s with 40GB memory each (a single seed is only 900MB), and 128 cores. So, the total resources used should be quite low, relative to what the machine has. For instance, the MuJoCo environment used also slows down when running the 10 seeds at once but only by about twice as much.</p>
<p>I can look into Nsight Systems. Is anybody able to run the reproducible script I attached to see if the problem happens there too? I also tried it on my personal workstation and the same pattern occurs, though it would make more sense here because the GPU is smaller.</p>

ptrblck: <p>Cross-post from <a href="https://github.com/pytorch/pytorch/issues/109111#issuecomment-1716873219">here</a> with a detailed explanation and examples including Nsight Systems profiles.</p>