Ramping VUs

Description

A variable number of VUs execute as many iterations as possible for a specified amount of time. This executor is equivalent to the global stages option.

Options

In addition to the common configuration options this executor also adds the following options:

When to use

This executor is a good fit if you need VUs to ramp up or down during specific periods of time.

Example

In this example, we'll run a two-stage test, ramping up from 0 to 10 VUs over 20 seconds, then down to 0 VUs over 10 seconds.

1import http from 'k6/http';
2import { sleep } from 'k6';
3
4export const options = {
5  discardResponseBodies: true,
6  scenarios: {
7    contacts: {
8      executor: 'ramping-vus',
9      startVUs: 0,
10      stages: [
11        { duration: '20s', target: 10 },
12        { duration: '10s', target: 0 },
13      ],
14      gracefulRampDown: '0s',
15    },
16  },
17};
18
19export default function () {
20  http.get('https://test.k6.io/contacts.php');
21  // We're injecting a processing pause for illustrative purposes only!
22  // Each iteration will be ~515ms, therefore ~2 iterations/second per VU maximum throughput.
23  sleep(0.5);
24}

Note the setting of gracefulRampDown to 0 seconds, which could cause some iterations to be interrupted during the ramp down stage.

Observations

The following graph depicts the performance of the example script:

Based upon our test scenario inputs and results:

• We've defined 2 stages for a total test duration of 30 seconds;
• stage 1 ramps up VUs linearly from the startVUs of 0 to the target of 10 over a 20 second duration;
• from the 10 VUs at the end of stage 1, stage 2 then ramps down VUs linearly to the target of 0 over a 10 second duration;
• each iteration of the default function is expected to be roughly 515ms, or ~2/s;
• as the number of VUs changes, the iteration rate directly correlates; each addition of a VU increases the rate by ~2 iters/s, whereas each subtraction of a VU reduces by ~2 iters/s;
• our example performed ~300 iterations over the course of the test.

Get the stage index

To get the current running stage index, use the getCurrentStageIndex helper function from the k6-jslib-utils library. It returns a zero-based number equal to the position in the shortcut stages array or in the executor's stages array.

import { getCurrentStageIndex } from 'https://jslib.k6.io/k6-utils/1.3.0/index.js';

export const options = {
  stages: [
    { target: 10, duration: '30s' },
    { target: 50, duration: '1m' },
    { target: 10, duration: '30s' },
  ],
};

export default function () {
  if (getCurrentStageIndex() === 1) {
    console.log('Running the second stage where the expected target is 50');
  }
}

Using this feature, it is possible to automatically tag using the current running stage. Check the Tagging stages section for more details.