Benchmarking LLM Performance on Diverse Hardware

Effectively applying hardware acceleration techniques, optimizing kernels, or implementing distributed inference strategies requires rigorous validation. Without systematic performance measurement, it's impossible to quantify the benefits of these optimizations, compare different hardware platforms accurately, or identify remaining bottlenecks. Benchmarking provides this essential feedback loop, grounding optimization efforts in empirical data. It measures performance under specific conditions.

Defining Relevant Performance Metrics

Choosing the right metrics is fundamental to meaningful benchmarking. For LLMs, several important indicators capture different aspects of performance:

• Latency: Measures the time taken for inference. It's often broken down further:
  • Time To First Token (TTFT): The duration from sending the request to receiving the first generated token. This is critical for interactive applications (e.g., chatbots) where perceived responsiveness matters most.
  • Inter-Token Latency (ITL) or Time Per Output Token (TPOT): The average time taken to generate each subsequent token after the first. Lower ITL means faster streaming of the response. It's calculated as $(Total Generation Time - TTFT) / (Number Of Generated Tokens - 1)$.
  • Total Latency: The full time from request to receiving the complete response. Relevant for summarizing long outputs or batch processing.
• Throughput: Measures the rate at which the system can process requests or generate tokens.
  • Tokens Per Second (TPS): The total number of output tokens generated by the system per second across all concurrent requests. This is a primary metric for evaluating overall system capacity and efficiency, especially under load. $TPS = \frac{\text{Total Output Tokens}}{\text{Total Time}}$
  • Requests Per Second (RPS): The number of independent inference requests the system can handle per second. This is influenced by both latency and the system's ability to handle concurrent connections.
• Resource Utilization:
  • Memory Usage: Peak GPU/CPU memory consumption during inference. This determines the feasibility of running a model on specific hardware.
  • Power Consumption: Energy efficiency is increasingly important, measured in Watts or Joules per token/request.
• Cost: Often expressed as cost per million tokens or cost per inference request, factoring in hardware TCO (Total Cost of Ownership) or cloud instance pricing.

The choice of primary metric depends heavily on the application's requirements. Interactive systems prioritize low TTFT, while offline batch processing often aims to maximize TPS.

Establishing Benchmarking Methodologies

Meaningful benchmarking requires careful experimental design:

1. Define the Workload: Specify the exact task (e.g., text generation, summarization), model being used, input prompt characteristics (length distribution), and output generation parameters (number of tokens to generate, temperature, top-k/p sampling). Use workloads that mirror production traffic patterns. Synthetic workloads (e.g., fixed prompt length, fixed output length) can be useful for isolating specific behaviors but may not reflect real-world performance.
2. Control the Environment: Ensure tests are run on identical hardware and software stacks (OS, drivers, libraries like CUDA/ROCm, inference frameworks). Minimize background processes that could interfere with measurements.
3. Warm-up Phase: Execute several inference requests before starting measurements. This allows caches to warm up, GPU kernels to compile (if using JIT compilation), and the system to reach a steady state, avoiding artificially high latency from initial "cold starts."
4. Measurement Points: Decide whether to measure latency client-side (including network time) or server-side (excluding network time). Server-side measurements isolate model/hardware performance but might miss system-level bottlenecks.
5. Batching Strategy: LLM inference performance, especially throughput, is highly sensitive to batch size. Benchmark across a range of relevant batch sizes (including batch size 1 for latency-sensitive cases). Dynamic batching strategies, where the server groups incoming requests, add another layer of complexity to measure accurately.
6. Concurrency Level: For throughput measurements (TPS, RPS), simulate multiple concurrent clients making requests to understand how the system scales under load.
7. Statistical Rigor: Run each benchmark configuration multiple times (e.g., 10-100 runs) and report statistical aggregates like mean, median (P50), P90, P95, and P99 latencies. This accounts for variability and provides a more complete picture than single-run results. Standard deviation or confidence intervals help quantify uncertainty.

Benchmarking Across Diverse Hardware

Performance characteristics vary significantly across hardware types and even generations within the same type:

• CPUs: Generally offer the lowest throughput and highest latency but are universally available. Benchmarks should focus on core count, instruction sets (e.g., AVX-512), and cache performance impacts. Memory bandwidth is often a limiting factor.
• GPUs (NVIDIA, AMD, Intel): The workhorses for LLM training and inference. Main factors include:
  • Memory Capacity (VRAM): Determines the maximum model size runnable without complex offloading.
  • Memory Bandwidth: Important for loading weights, impacting TTFT and overall performance, especially for large models where inference is memory-bound.
  • Compute Units (CUDA Cores, Stream Processors, Xe-cores): Affect raw computational throughput (matrix multiplication, attention). Specialized units like Tensor Cores (NVIDIA) or Matrix Cores (AMD/Intel) significantly accelerate mixed-precision operations.
  • Software Stack: Performance heavily depends on optimized libraries (cuBLAS, rocBLAS, oneMKL) and runtimes (TensorRT, ROCm, OpenVINO). Benchmarks must compare performance within the context of these ecosystems.
• TPUs (Google): Designed specifically for ML workloads, often excelling at large batch sizes and specific matrix multiplication formats (like BFloat16). Benchmarking requires using the TensorFlow or JAX ecosystems.
• Specialized AI Accelerators (NPUs, Custom ASICs): Hardware from various vendors (Groq, Cerebras, SambaNova, mobile NPUs) designed for specific ML operations. Performance can be exceptional for certain workloads but often requires dedicated SDKs and compilation toolchains. Benchmarking needs to adhere to vendor-specific guidelines and tools.

When comparing hardware, ensure an "apples-to-apples" comparison: use the same model, precision (e.g., FP16, INT8), batch size, sequence lengths, and software framework where possible. If frameworks differ, document this clearly as part of the results.

Common Challenges and Best Practices

• Ignoring Cold Starts: Failing to include a warm-up phase leads to reporting artificially high initial latencies.
• Using Non-Representative Data: Benchmarking with inputs/outputs significantly different from production traffic (e.g., very short prompts when production uses long ones) yields misleading results.
• Comparing Different Precisions/Optimizations Unfairly: Claiming speedup without specifying that one test used FP16 and the other INT8, or that one used FlashAttention while the other didn't, is incomplete.
• Neglecting Framework Overhead: Some inference frameworks or servers add their own latency (request queuing, pre/post-processing). Isolate model inference time from total end-to-end time if needed.
• Insufficient Runs: Reporting results from a single run is unreliable due to system jitter and variability. Use multiple runs and report statistics.
• Lack of Context: Report all relevant configuration details: hardware, software versions (drivers, libraries), model name, precision, batch size, sequence lengths, generation parameters, and the specific metrics measured.

Visualizing and Interpreting Results

Presenting benchmark results clearly is important. Bar charts are effective for comparing metrics like P90 latency or average TPS across different hardware platforms or optimization levels.

Comparison of Time-To-First-Token (TTFT) and Inter-Token Latency (ITL) P90 latencies for an LLM across different hardware platforms, illustrating performance differences.

Comparison of throughput (Tokens Per Second) for the same LLM across different hardware platforms at various batch sizes, showing how throughput scales differently.

Interpretation involves looking past single numbers. Analyze the trade-offs: Hardware A might have lower latency at batch size 1, while Hardware B offers much higher throughput at larger batches. Relate these findings back to the specific application requirements and cost constraints to make informed deployment decisions. Rigorous benchmarking is the foundation for optimizing LLM inference performance in real-world systems.