Load Testing

Load Testing

"Works on my machine" is not a performance guarantee. Load testing answers: "will it work when 1000 users hit it at once?"

Types of Load Tests

| Type | Purpose | Pattern | |------|---------|---------| | Load test | Does it handle expected traffic? | Ramp to target RPS, hold for 10-30 min | | Stress test | Where does it break? | Ramp until failure, note the breaking point | | Soak test | Does it leak resources over time? | Normal load for 4-24 hours | | Spike test | Does it handle sudden bursts? | Sudden jump to 5-10x normal, then back down | | Breakpoint test | What's the absolute max? | Incrementally increase until system fails |

Step 1: Define Targets

Before writing tests, define what "good" looks like:

TARGET SCENARIO: [what you're simulating]
EXPECTED USERS: [concurrent users]
EXPECTED RPS: [requests per second]
ACCEPTABLE LATENCY: p50  r.status === 200,
    'response time  r.timings.duration  1% |
| CPU utilization | Server monitoring | > 80% sustained |
| Memory usage | Server monitoring | Growing without stabilizing |
| Database connections | DB monitoring | Pool exhausted |
| Queue depth | Queue monitoring | Growing (consumers can't keep up) |
| Disk I/O | Server monitoring | Saturated |
| Network throughput | Server monitoring | Bandwidth cap hit |

## Step 5: Analyze Results

### Finding Bottlenecks

Is latency increasing with load? → YES at low concurrency: Application bottleneck (slow code, N+1 queries) → YES at high concurrency: Resource saturation (CPU, memory, connections) → NO: System is handling the load well

Is error rate increasing with load? → 5xx errors: Server overwhelmed (timeouts, OOM, connection exhaustion) → 429 errors: Rate limiting (increase limits or optimize client) → Connection errors: Network or load balancer limits

### Common Bottlenecks

| Bottleneck | Symptom | Fix |
|-----------|---------|-----|
| Database queries | Latency spikes, connection pool exhausted | Add indexes, optimize queries, connection pooling |
| No connection pooling | New TCP connection per request | Use connection pool for DB, HTTP clients |
| Synchronous I/O | Threads blocked, low throughput | Async I/O or increase thread pool |
| Single instance | CPU maxed on one server | Horizontal scaling, load balancer |
| Missing cache | Same expensive computation repeated | Add caching layer |
| Large payloads | Network bandwidth saturated | Compress, paginate, reduce payload |
| Lock contention | Threads waiting on shared resources | Reduce critical section, use optimistic locking |

## Step 6: Report

Load Test Report

Test Configuration

• Target: [X] concurrent users / [X] RPS
• Duration: [X] minutes
• Environment: [staging, production-mirror]
• Date: [YYYY-MM-DD]

Results

| Metric | Target | Actual | Status | |--------|--------|--------|--------| | p50 latency | < 200ms | 180ms | PASS | | p95 latency | < 500ms | 620ms | FAIL | | p99 latency | < 1000ms | 2400ms | FAIL | | Error rate | < 1% | 0.3% | PASS | | Max RPS achieved | 500 | 420 | FAIL |

Bottlenecks Identified

1. [Bottleneck] - [evidence] - [recommended fix]

Recommendations

1. [Action] - [expected improvement]

Next Steps

• [ ] Fix identified bottlenecks
• [ ] Re-run test to verify improvements
• [ ] Schedule soak test for [date]

## When to Run Load Tests

| Trigger | Test Type |
|---------|-----------|
| Before launch | Full load + stress + soak |
| Before major feature release | Load test for affected endpoints |
| Before expected traffic spike | Load test at expected peak |
| After significant architecture change | Full load test |
| Quarterly (routine) | Soak test (catch resource leaks) |

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [pvnarp](https://github.com/pvnarp)
- **Source:** [pvnarp/agent-skills](https://github.com/pvnarp/agent-skills)
- **License:** MIT

Install and usage instructions live in the source repository linked above.