Applying pruning techniques, whether unstructured or structured, introduces sparsity into the model's weight matrices. However, simply setting weights to zero doesn't automatically translate into reduced latency or computational cost during inference. Achieving actual performance gains requires explicit support from the software stack, specifically the compiler and the inference runtime, to recognize and exploit this sparsity. Without such support, computations involving zeroed weights are still performed, yielding no speedup.
Recognizing and Representing Sparsity
The first step for the software stack is to identify and represent the sparsity introduced by pruning.
- Unstructured Sparsity: This involves arbitrary zero entries scattered throughout weight matrices. Common representations include formats like Compressed Sparse Row (CSR), Compressed Sparse Column (CSC), or Coordinate List (COO). These formats store only the non-zero values along with their indices, significantly reducing memory footprint compared to dense representations, especially at high sparsity levels. However, they introduce overhead for index lookups during computation.
- Structured Sparsity: This involves removing entire blocks, channels, filters, or attention heads, resulting in predictable, regular patterns of zeros (e.g., entire rows or columns). Structured sparsity is often represented implicitly by modifying the model architecture definition (e.g., reducing the number of channels or heads) or by using block-sparse formats that represent larger zero regions more compactly than element-wise sparse formats.
Compiler Optimizations for Sparsity
Optimization techniques include:
- Sparse Format Selection: The compiler might analyze the sparsity pattern and density to choose the most efficient sparse storage format (CSR, CSC, COO, Block Sparse, etc.) for the target hardware.
- Sparse Kernel Generation: Instead of using standard dense matrix multiplication routines, the compiler generates or selects specialized kernels designed for sparse matrix operations. These kernels aim to skip computations involving zeros. For structured sparsity, this might involve generating code that operates on smaller, dense sub-matrices. For unstructured sparsity, it often involves indirect memory accesses based on stored indices.
- Instruction Selection: If the target hardware has specialized instructions for sparse computations (e.g., certain accelerators or future CPU/GPU instruction sets), the compiler maps the sparse operations onto these instructions.
- Memory Access Optimization: Compilers attempt to optimize memory access patterns for sparse formats, which can be irregular and lead to cache misses. Techniques like data reordering or tiling can be applied, although their effectiveness depends heavily on the sparsity pattern and hardware architecture.
- Operator Fusion: Similar to dense model compilation, compilers can fuse operations involving sparse tensors to reduce memory traffic and kernel launch overhead. Fusing a sparse matrix multiplication with a subsequent activation function, for instance.
Runtime Execution of Sparse Operations
The inference runtime environment (like ONNX Runtime, TensorRT, TensorFlow Lite, PyTorch JIT) executes the compiled model graph. Its responsibilities regarding sparsity include:
- Library Integration: Runtimes often link against highly optimized libraries for numerical computation, such as Intel MKL (Math Kernel Library) for CPUs or NVIDIA cuSPARSE for GPUs. These libraries provide efficient implementations of common sparse linear algebra operations (SpGEMM - Sparse General Matrix Multiply).
- Kernel Dispatch: The runtime selects the appropriate computational kernel (dense or sparse) based on the tensor's representation and potentially its sparsity level. There's often a threshold below which using a dense kernel is faster due to the overhead associated with sparse formats and kernels.
- Memory Management: Efficiently managing the memory for both the non-zero values and the associated index metadata of sparse tensors is handled by the runtime.
- Format Conversion: Sometimes, the runtime might need to convert between different sparse formats or between sparse and dense formats if different operators in the execution graph require different representations. This conversion introduces overhead.
High-level overview of how sparsity is handled from the pruned model definition through compilation and runtime execution on hardware.
Challenges and Approaches
Exploiting sparsity effectively is not straightforward:
- Overhead: Sparse formats require extra storage for indices, and sparse kernels often involve indirect memory accesses and control flow overhead. This overhead can sometimes negate the benefits of skipping zero-value computations, especially at lower sparsity levels (< 50-70%).
- Hardware Support: Performance gains are highly dependent on the target hardware's ability to efficiently execute sparse operations. GPUs, particularly with specialized units like Sparse Tensor Cores (introduced in NVIDIA's Ampere architecture), generally show better speedups for sparse matrix multiplications compared to CPUs, especially for unstructured sparsity. Structured sparsity often maps more efficiently to existing dense hardware capabilities.
- Sparsity Threshold: There's typically a "break-even" point in terms of sparsity level. Below this threshold, using dense kernels might still be faster due to the overheads of sparse computation. This threshold varies based on the hardware, software stack, matrix dimensions, and sparsity pattern.
- Structured vs. Unstructured: Structured pruning often leads to more predictable and significant performance gains because the resulting sparsity patterns are regular and easier for compilers and hardware to optimize. Accelerating highly unstructured, fine-grained sparsity remains a challenge, often requiring very high sparsity levels (>90%) to become effective.
Understanding how compilers and runtimes interact with sparse representations is important for selecting pruning strategies. While unstructured pruning might achieve higher compression ratios on paper, structured pruning often delivers more reliable inference acceleration due to better alignment with existing hardware and software optimization capabilities. Evaluating the end-to-end performance, considering the specific compiler, runtime, and target hardware, is necessary to assess the true benefit of any pruning approach.
