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Comparing HPU-Optimized safe_softmax with Native PyTorch safe_softmax

This article demonstrates how to benchmark and compare the performance of the Habana Processing Unit (HPU)-optimized `safe_softmax` operation against the native PyTorch implementation. The provided Python script guides you through the process step-by-step, with detailed explanations for each part. Additionally, we will provide some context about `safe_softmax`, its purpose, and its use cases.

##
Important Note: No Special Setup Required

The `safe_softmax` operation works out-of-the-box in PyTorch. When running your code on Habana hardware, the HPU-optimized implementation is automatically utilized without any additional configuration. This seamless integration allows you to benefit from performance improvements without modifying your existing code.

##
What is safe_softmax ?

The `softmax` function is a common operation in machine learning, particularly in classification tasks. It converts raw logits into probabilities by applying the exponential function and normalizing the results. However, the standard `softmax` can encounter numerical instability when dealing with very large or very small values in the input tensor, leading to overflow or underflow issues.

To address this, `safe_softmax` is implemented. It stabilizes the computation by subtracting the maximum value in each row (or along the specified dimension) from the logits before applying the exponential function. This ensures that the largest value in the exponent is zero, preventing overflow.

###
Why is safe_softmax important?

- Numerical Stability: Prevents overflow/underflow issues during computation.
- Widely Used: Commonly used in neural networks, especially in the final layer for classification tasks.
- Efficiency: Optimized implementations can significantly improve performance on specialized hardware like GPUs or HPUs.

##
Step-by-Step Explanation of the Code

###
1. Importing Required Libraries

    import torch
    import timeit
    import habana_frameworks.torch as ht
    from torch._decomp.decompositions import safe_softmax as native_safe_softmax

- `torch`: The core PyTorch library for tensor operations.
- `timeit`: A Python module for measuring execution time.
- `habana_frameworks.torch`: Provides support for Habana hardware (HPUs).
- `safe_softmax`: The native PyTorch implementation of `safe_softmax` is imported for comparison.

###
2. Defining the HPU-Optimized safe_softmax

    hpu_safe_softmax = torch.ops.aten._safe_softmax.default

- The HPU-optimized version of `safe_softmax` is accessed via the `torch.ops.aten` namespace. This implementation is specifically designed to leverage the Habana hardware for faster execution.

###
3. Preparing the Input Tensor

    input_tensor = torch.tensor([[1.0, 2.0, float("-inf")], [3.0, 4.0, 5.0]]).to("hpu")

- A 2D tensor is created with some typical values, including `-inf` to simulate edge cases.
- The tensor is moved to the HPU device using `.to("hpu")`.

###
4. Warmup for Fair Benchmarking

    hpu_safe_softmax(input_tensor, dim=1); ht.hpu.synchronize()
    native_safe_softmax(input_tensor, dim=1); ht.hpu.synchronize()

- Both the HPU-optimized and native implementations are executed once before benchmarking. This ensures that any initialization overhead is excluded from the timing measurements.
- `ht.hpu.synchronize()` ensures that all HPU operations are completed before proceeding.

###
5. Benchmarking the Implementations

    num_iterations = 10000
    hpu_time = timeit.timeit(
        "hpu_safe_softmax(input_tensor, dim=1); ht.hpu.synchronize()",
        globals=globals(),
        number=num_iterations
    )
    native_time = timeit.timeit(
        "native_safe_softmax(input_tensor, dim=1); ht.hpu.synchronize()",
        globals=globals(),
        number=num_iterations
    )

- The `timeit` module is used to measure the execution time of each implementation over 10,000 iterations.
- The `globals=globals()` argument allows the `timeit` module to access the defined variables and functions in the script.

###
6. Printing the Results

    print(f"Performance comparison over {num_iterations} iterations:")
    print(f"Native safe_softmax: {native_time:.6f} seconds")
    print(f"HPU safe_softmax: {hpu_time:.6f} seconds")

- The execution times for both implementations are printed, allowing for a direct comparison of their performance.

##
Example Output

After running the script, you might see output similar to the following (lower is better):

    Performance comparison over 10000 iterations:
    Native safe_softmax: 1.004057 seconds
    HPU safe_softmax: 0.104004 seconds

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