How GPUs Organize Work: Or What are GPU Warps

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GPUs are built for speed, handling thousands of tasks at once. But how do they organize all that work? This post dives into warps, a key concept in GPU performance, explained step-by-step from a beginner’s perspective. We’ll follow the journey of discovery, building from basic questions to practical insights, with examples to make it concrete.

What’s a Warp and Why Does It Matter?

A warp is a small group of threads on a GPU that work together, running the same code at the same time on different pieces of data. Think of it like a team of workers on an assembly line, all doing the same task but on separate items. Typically, a warp has 32 threads, though this can vary by GPU.

This grouping is crucial because GPUs are designed for parallel processing. By organizing threads into warps, the GPU can execute many tasks efficiently, making it perfect for jobs like graphics rendering or data crunching.

Understanding Kernels: The GPU’s Job Description

A kernel is the function you write for the GPU to execute. It’s the core of your program, run across thousands of threads simultaneously. Unlike a CPU, which processes tasks one-by-one, a GPU runs the kernel on many data points at once.

For example, imagine adding two large lists of numbers. A CPU would loop through each pair. A GPU kernel, however, assigns each pair to a thread, adding them all in parallel. Here’s a simple pseudocode example:

# Kernel to add two lists
kernel add_lists(a, b, result, n):
    index = get_thread_id()
    if index < n:
        result[index] = a[index] + b[index]

# Main program
list_a = [1, 2, 3, ...]
list_b = [4, 5, 6, ...]
result = new list of size n
launch add_lists on GPU with n threads (list_a, list_b, result, n)
# Output: result = [5, 7, 9, ...]

Each thread computes one addition, writing to its own spot in the result.

How Warps Organize Threads for Teamwork

Threads don’t work alone; they’re grouped into warps to streamline execution. All threads in a warp run the same kernel, but each processes a different piece of data. This is called SIMD (Single Instruction, Multiple Data).

In our addition example, if you have 1024 pairs to add, that’s 32 warps (1024 ÷ 32). Each warp handles 32 pairs at once, with all threads executing the same addition instruction.

Streaming Multiprocessors: The GPU’s Task Managers

GPUs are split into Streaming Multiprocessors (SMs), which are like mini-processors inside the GPU. Each SM manages multiple warps, scheduling them to keep the GPU busy. This lets the GPU juggle different tasks—like one SM handling a kernel for image processing while another tackles matrix math.

A single SM can manage many warps, switching between them to hide delays, like waiting for data from memory.

Why Shared Instructions Make Warps Fast

The biggest win for warps is shared instruction fetching. Since all threads in a warp run the same kernel, the GPU fetches the kernel’s instructions once for the whole group, not for each thread. This saves time and energy compared to a CPU, where each task needs its own instruction fetch.

In the list addition kernel, the SM loads the “add” instruction once per warp, applying it to 32 data pairs simultaneously. This efficiency is why GPUs excel at parallel tasks.

Feature	CPU	GPU Warp
Instruction Fetch	Per task	Per warp
Processing Style	Sequential/Multi-core	Parallel SIMD
Speed Advantage	Limited by cores	Scales with threads

The Pitfall of Warp Divergence

Warps are fast when all threads follow the same code path. But what happens if some threads need to take a different route, like in an if-else statement? This is called warp divergence, and it slows things down.

Suppose a kernel checks if a number is greater than 5:

kernel process_data(data, n):
    index = get_thread_id()
    if index < n:
        if data[index] > 5:
            data[index] = data[index] * 2  # Path A
        else:
            data[index] = data[index] + 1  # Path B

# Example input: data = [3, 7, 4, 6, 8]
# Run with 5 threads (1 warp)
# Output: data = [4, 14, 5, 7, 16]
# Threads for 3, 4, 6 take Path B; 7, 8 take Path A

The GPU runs both paths sequentially: first Path A (masking threads where false), then Path B (masking threads where true). This halves efficiency if threads split evenly.

Writing Divergence-Free Code for Speed

To avoid divergence, write data-parallel code with minimal conditionals. One trick is using math or ternary operators instead of if-else. For the above example, rewrite it as:

kernel process_data(data, n):
    index = get_thread_id()
    if index < n:
        data[index] = (data[index] > 5) ? data[index] * 2 : data[index] + 1

# Same input: data = [3, 7, 4, 6, 8]
# Output: data = [4, 14, 5, 7, 16]
# All threads execute the same instruction

This often compiles to a single instruction, keeping the warp unified. For complex logic, consider moving conditionals to the CPU or restructuring data to align threads.

Designing for GPU Performance

The key to GPU programming is a data-parallel mindset. Avoid nested conditionals, as they multiply divergent paths, which even smart compilers can’t always fix. Instead, design algorithms where all threads perform uniform operations, like matrix math or image filters.

For example, sorting data before a kernel can group similar conditions, reducing divergence. Test your code with profiling tools to spot and fix divergence.

Warps are the backbone of GPU performance, enabling massive parallelism through shared instructions. By understanding warps and writing divergence-free code, you can harness the GPU’s full power.