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Load testing assesses how your system performs under expected normal and peak load conditions. It helps you understand capacity, identify bottlenecks, and validate that your system meets performance requirements.

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Purpose

Load tests help you:
  • Measure response times under typical load
  • Identify performance bottlenecks
  • Validate system capacity and scalability
  • Ensure SLAs are met under normal conditions

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Configuration Pattern

Load tests use ramping stages to gradually increase load: 
import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  stages: [
    { duration: '5m', target: 100 }, // Ramp up to 100 users
    { duration: '10m', target: 100 }, // Stay at 100 users
    { duration: '5m', target: 0 },   // Ramp down to 0 users
  ],
  thresholds: {
    http_req_duration: ['p(95)<500', 'p(99)<1000'],
    http_req_failed: ['rate<0.01'],
  },
};

export default function() {
  const res = http.get('https://quickpizza.grafana.com');
  check(res, {
    'status is 200': (r) => r.status === 200,
  });
  sleep(1);
}
The stages configuration above creates a classic load test profile: gradual ramp-up, steady state, and controlled ramp-down.

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Using the Ramping VUs Executor

The ramping-vus executor provides fine-grained control over VU ramping: 
export const options = {
  scenarios: {
    load_test: {
      executor: 'ramping-vus',
      startVUs: 0,
      stages: [
        { duration: '2m', target: 50 },   // Ramp up
        { duration: '5m', target: 50 },   // Steady state
        { duration: '2m', target: 100 },  // Step up
        { duration: '5m', target: 100 },  // Higher steady state
        { duration: '2m', target: 0 },    // Ramp down
      ],
      gracefulRampDown: '30s',
    },
  },
};

Ramp Up

2-5 minutes to target load

Steady State

5-15 minutes at peak load

Ramp Down

2-5 minutes to zero

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Load Test Stages Explained

1

Ramp Up

Gradually increase VUs to give the system time to scale and warm up caches. This prevents overwhelming the system immediately.
{ duration: '5m', target: 100 }
2

Steady State

Maintain peak load to measure sustained performance. This is where you gather meaningful metrics.
{ duration: '10m', target: 100 }
3

Ramp Down

Gracefully reduce load to allow the system to recover and finish processing requests.
{ duration: '5m', target: 0 }

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Advanced Load Testing Pattern

Multi-stage load test with multiple peaks: 
import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  stages: [
    // Morning traffic
    { duration: '2m', target: 50 },
    { duration: '5m', target: 50 },
    
    // Lunch peak
    { duration: '2m', target: 100 },
    { duration: '5m', target: 100 },
    
    // Afternoon dip
    { duration: '2m', target: 50 },
    { duration: '5m', target: 50 },
    
    // Evening peak
    { duration: '2m', target: 150 },
    { duration: '5m', target: 150 },
    
    // Ramp down
    { duration: '5m', target: 0 },
  ],
};

export default function() {
  const res = http.get('https://quickpizza.grafana.com/api/pizza');
  check(res, { 'status is 200': (r) => r.status === 200 });
  sleep(1);
}

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Using Constant Arrival Rate

For more realistic load patterns, use arrival rate instead of VUs: 
export const options = {
  scenarios: {
    load_test: {
      executor: 'constant-arrival-rate',
      rate: 100,              // 100 iterations per second
      timeUnit: '1s',
      duration: '10m',
      preAllocatedVUs: 50,    // Initially allocated VUs
      maxVUs: 200,            // Max VUs if needed
    },
  },
};
Arrival rate executors maintain a constant request rate regardless of response times, providing more realistic load simulation.

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Metrics to Monitor

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Key Performance Indicators

  • Response time percentiles: p(95), p(99)
  • Request rate: Requests per second
  • Error rate: Failed requests percentage
  • Throughput: Data transferred

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Setting Thresholds

export const options = {
  thresholds: {
    // 95% of requests should be below 500ms
    http_req_duration: ['p(95)<500'],
    
    // 99% of requests should be below 1s
    'http_req_duration{name:http://quickpizza.grafana.com/api/pizza}': ['p(99)<1000'],
    
    // Error rate should be below 1%
    http_req_failed: ['rate<0.01'],
    
    // 95% of checks should pass
    checks: ['rate>0.95'],
  },
};

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Best Practices

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Load Test Duration

Minimum Duration

Run for at least 15-20 minutes to observe steady-state behavior

Recommended Duration

30-60 minutes for comprehensive testing

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Realistic User Behavior

import { sleep } from 'k6';
import http from 'k6/http';

export default function() {
  // Browse homepage
  http.get('https://quickpizza.grafana.com');
  sleep(2); // Think time
  
  // View products
  http.get('https://quickpizza.grafana.com/api/pizza');
  sleep(3);
  
  // Add to cart
  http.post('https://quickpizza.grafana.com/api/cart', {
    pizzaId: 1,
    quantity: 2,
  });
  sleep(1);
}
Add sleep() calls to simulate realistic user think time between requests. Without sleep, k6 will execute requests as fast as possible.

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When to Use

  • Pre-production: Validate performance before releases
  • Capacity planning: Determine maximum sustainable load
  • SLA validation: Ensure performance requirements are met
  • Baseline establishment: Create performance benchmarks

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Common Patterns

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Gradual Load Increase

Test multiple load levels in one test: 
export const options = {
  stages: [
    { duration: '2m', target: 25 },   // 25% load
    { duration: '5m', target: 25 },
    { duration: '2m', target: 50 },   // 50% load
    { duration: '5m', target: 50 },
    { duration: '2m', target: 100 },  // 100% load
    { duration: '5m', target: 100 },
    { duration: '2m', target: 0 },
  ],
};

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