Add load pattern configuration guide to benchmarks (#26886)

Signed-off-by: Matvei Pashkovskii <mpashkov@amd.com>
Signed-off-by: Matvei Pashkovskii <matvei.pashkovskii@amd.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/contributing/benchmarks.md

@@ -321,6 +321,73 @@ The following arguments can be used to control the ramp-up:
 - `--ramp-up-start-rps`: The request rate at the beginning of the benchmark.
 - `--ramp-up-end-rps`: The request rate at the end of the benchmark.
 
+##### Load Pattern Configuration
+
+vLLM's benchmark serving script provides sophisticated load pattern simulation capabilities through three key parameters that control request generation and concurrency behavior:
+
+###### Load Pattern Control Parameters
+
+- `--request-rate`: Controls the target request generation rate (requests per second). Set to `inf` for maximum throughput testing or finite values for controlled load simulation.
+- `--burstiness`: Controls traffic variability using a Gamma distribution (range: > 0). Lower values create bursty traffic, higher values create uniform traffic.
+- `--max-concurrency`: Limits concurrent outstanding requests. If this argument is not provided, concurrency is unlimited. Set a value to simulate backpressure.
+
+These parameters work together to create realistic load patterns with carefully chosen defaults. The `--request-rate` parameter defaults to `inf` (infinite), which sends all requests immediately for maximum throughput testing. When set to finite values, it uses either a Poisson process (default `--burstiness=1.0`) or Gamma distribution for realistic request timing. The `--burstiness` parameter only takes effect when `--request-rate` is not infinite - a value of 1.0 creates natural Poisson traffic, while lower values (0.1-0.5) create bursty patterns and higher values (2.0-5.0) create uniform spacing. The `--max-concurrency` parameter defaults to `None` (unlimited) but can be set to simulate real-world constraints where a load balancer or API gateway limits concurrent connections. When combined, these parameters allow you to simulate everything from unrestricted stress testing (`--request-rate=inf`) to production-like scenarios with realistic arrival patterns and resource constraints.
+
+The `--burstiness` parameter mathematically controls request arrival patterns using a Gamma distribution where:
+
+- Shape parameter: `burstiness` value
+- Coefficient of Variation (CV): $\frac{1}{\sqrt{burstiness}}$
+- Traffic characteristics:
+    - `burstiness = 0.1`: Highly bursty traffic (CV ≈ 3.16) - stress testing
+    - `burstiness = 1.0`: Natural Poisson traffic (CV = 1.0) - realistic simulation  
+    - `burstiness = 5.0`: Uniform traffic (CV ≈ 0.45) - controlled load testing
+
+![Load Pattern Examples](../assets/contributing/load-pattern-examples.png)
+
+*Figure: Load pattern examples for each use case. Top row: Request arrival timelines showing cumulative requests over time. Bottom row: Inter-arrival time distributions showing traffic variability patterns. Each column represents a different use case with its specific parameter settings and resulting traffic characteristics.*
+
+Load Pattern Recommendations by Use Case:
+
+| Use Case           | Burstiness   | Request Rate    | Max Concurrency | Description                                               |
+| ---                | ---          | ---             | ---             | ---                                                       |
+| Maximum Throughput | N/A          | Infinite        | Limited         | **Most common**: Simulates load balancer/gateway limits with unlimited user demand |
+| Realistic Testing  | 1.0          | Moderate (5-20) | Infinite        | Natural Poisson traffic patterns for baseline performance |
+| Stress Testing     | 0.1-0.5      | High (20-100)   | Infinite        | Challenging burst patterns to test resilience             |
+| Latency Profiling  | 2.0-5.0      | Low (1-10)      | Infinite        | Uniform load for consistent timing analysis               |
+| Capacity Planning  | 1.0          | Variable        | Limited         | Test resource limits with realistic constraints           |
+| SLA Validation     | 1.0          | Target rate     | SLA limit       | Production-like constraints for compliance testing        |
+
+These load patterns help evaluate different aspects of your vLLM deployment, from basic performance characteristics to resilience under challenging traffic conditions.
+
+The **Maximum Throughput** pattern (`--request-rate=inf --max-concurrency=<limit>`) is the most commonly used configuration for production benchmarking. This simulates real-world deployment architectures where:
+
+- Users send requests as fast as they can (infinite rate)
+- A load balancer or API gateway controls the maximum concurrent connections
+- The system operates at its concurrency limit, revealing true throughput capacity
+- `--burstiness` has no effect since request timing is not controlled when rate is infinite
+
+This pattern helps determine optimal concurrency settings for your production load balancer configuration.
+
+To effectively configure load patterns, especially for **Capacity Planning** and **SLA Validation** use cases, you need to understand your system's resource limits. During startup, vLLM reports KV cache configuration that directly impacts your load testing parameters:
+
+```text
+GPU KV cache size: 15,728,640 tokens
+Maximum concurrency for 8,192 tokens per request: 1920
+```
+
+Where:
+
+- GPU KV cache size: Total tokens that can be cached across all concurrent requests
+- Maximum concurrency: Theoretical maximum concurrent requests for the given `max_model_len`
+- Calculation: `max_concurrency = kv_cache_size / max_model_len`
+
+Using KV cache metrics for load pattern configuration:
+
+- For Capacity Planning: Set `--max-concurrency` to 80-90% of the reported maximum to test realistic resource constraints
+- For SLA Validation: Use the reported maximum as your SLA limit to ensure compliance testing matches production capacity
+- For Realistic Testing: Monitor memory usage when approaching theoretical limits to understand sustainable request rates
+- Request rate guidance: Use the KV cache size to estimate sustainable request rates for your specific workload and sequence lengths
+
 </details>
 
 #### 📈 Offline Throughput Benchmark