Paralleelsus süvaõppes 2025/26 kevad

Practical 3

Profiling & Identifying Performance Bottlenecks

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Objective

In this practical session, you will:

• Measure training performance of a large model
• Compute throughput (samples per second)
• Analyze speedup behavior
• Identify computation vs I/O bottlenecks
• Use PyTorch’s built-in profiler
• Connect experimental results to Amdahl’s Law

This practical prepares you to reason about performance before scaling to multi-GPU training.

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Background

In Lecture 3, we introduced:

• Speedup
• Scalability
• Throughput
• Efficiency
• Amdahl’s Law
• Bottlenecks (computation vs I/O vs communication)
• PyTorch profiler

In this session, you will experimentally observe these concepts.

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Task 1—Baseline Measurement (Single GPU)

1) Create a file named: practical3.py
2) Use the following script:

import torch
import torch.nn as nn
import time

DIM = 8192
DEPTH = 6
BATCH_SIZE = 64
STEPS = 20

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

class LargeLinearModel(nn.Module):
    def __init__(self):
        super().__init__()
        layers = []
        for _ in range(DEPTH):
            layers.append(nn.Linear(DIM, DIM))
            layers.append(nn.ReLU())
        self.net = nn.Sequential(*layers)

    def forward(self, x):
        return self.net(x)

model = LargeLinearModel().to(device)
optimizer = torch.optim.Adam(model.parameters())
loss_fn = nn.MSELoss()

print("Running baseline experiment...")

total_start = time.time()

for step in range(STEPS):

    x = torch.randn(BATCH_SIZE, DIM, device=device)
    y = torch.randn(BATCH_SIZE, DIM, device=device)

    torch.cuda.synchronize()
    step_start = time.time()

    output = model(x)
    loss = loss_fn(output, y)

    loss.backward()
    optimizer.step()
    optimizer.zero_grad()

    torch.cuda.synchronize()
    step_time = time.time() - step_start

    throughput = BATCH_SIZE / step_time

    print(f"Step {step:02d} | "
          f"Step time: {step_time:.4f}s | "
          f"Throughput: {throughput:.2f} samples/sec")

total_time = time.time() - total_start
print(f"\nTotal training time: {total_time:.2f}s") 

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Task 2—Compute Throughput & Efficiency

1) Run the script with:

• BATCH_SIZE = 32
• BATCH_SIZE = 64
• BATCH_SIZE = 128

2) Create a table:

Batch size	Step Time (sec)	Throughput (samples/sec)

3) Answer:

• Does throughput increase linearly?
• When does GPU utilization improve?
• What happens to efficiency?

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Task 3—Artificial I/O Bottleneck

1) Add the following line before the forward pass: time.sleep(0.01)
2) Run the experiment again.
3) Answer:

• What happens to throughput?
• Is the GPU fully utilized?
• Which part of training becomes the bottleneck?

This simulates an I/O bottleneck.

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Task 4—Apply Amdahl’s Law

1) Assume that:

• 30% of total time is I/O
• 70% is computation

2) Using Amdahl’s Law:

{$ S_{max} = \frac{1 }{1-P} $}

3) Compute the theoretical maximum speedup if computation is infinitely fast.
4) Does this match your intuition from the experiment?

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Task 5—Use PyTorch Profiler

1) Place the profiler around one training step (forward + backward), right after data is created:

from torch.profiler import profile, ProfilerActivity

with profile(activities=[ProfilerActivity.CPU, ProfilerActivity.CUDA]) as prof:
    output = model(x)
    loss = loss_fn(output, y)
    loss.backward()

print(prof.key_averages().table(sort_by="cuda_time_total"))

2) Identify:

• Which operation consumes most CUDA time?
• Is backward more expensive than forward?
• Which layer dominates runtime?

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Task 6 (optional) — Monitor GPU Usage

1) While running the script, execute watch -n 1 nvidia-smi
2) Observe:

• GPU utilization (%)
• Memory usage
• Power consumption