Load testing your web server after you deployed your application is very very important to give you an idea about how your application will perform under heavy load, it also helps you in capacity planning and also configuring your web server for optimal performance.

In this article we will learn how to use locust to run load tests against our web server and also how to visualize the results, all of this using locust tests runner.

What will we do?

We will use locust tests runner to:

  • Run a load test against google and facebook servers.
  • Visualize the results and show them in an HTML page.

Locust Introduction

Locust is an Open Source load testing tool, you can use it to load test your systems regardless of their type, however it is primary used for web server testing.

The primary features of locust are:

  • Define user behaviour in code: No need for clunky UIs or bloated XML. Just plain code.
  • Distributed & scalable: Locust supports running load tests distributed over multiple machines, and can therefore be used to simulate millions of simultaneous users
  • Proven & battle tested: Locust has been used to simulate millions of simultaneous users. Battlelog, the web app for the Battlefield games, is load tested using Locust, so one can really say Locust is Battletested ;).

To use locust first you need to install it with this command

sudo -H pip3 install locust

To use locust we must first create a file called, this file describes the tests we want to run using python code, this is an example of the file:

from locust import HttpUser, between, task
class WebsiteUser(HttpUser):
    wait_time = between(5, 15)
    host = ""
    def index(self):

To run the tests use this command


This will run an embedded web server which can be accessed using this url as shown here.

Enter the number of user, hatch rate and the host you want to test and press Start swarming to start the load test, you can keep the test running as long as you want and press Stop to stop it.

Running locust tests without the web UI and save results

You can run tests without using the web UI and save results to a CSV file using this command

locust --headless -u 100 -r 10 -t1m --only-summary --csv=output

Here is a description of the command line arguments:

  • –headless: This switch disables the web UI and run tests immediately.
  • -u 100: Run 100 users for the test.
  • -r 10: This is the hatch rate, create 10 users every second until the number of users is reached.
  • -t1m: This is the test duration, run tests for 1 minute.
  • –only-summary: This switch disables showing stats continuously, and only show summary at the end.
  • –csv=output: Save the stats in a CSV file which is starts with output, it creates multiple files one of them is called output_stats.csv.

After one minute the testing stops and you can read the results.

Run multiple tests using locust tests runner

Now we learned how to run a single test using locust and save results to CSV files, next we will use a simple web page to display results in charts.

First clone the repository and install dependencies using these commands

git clone
cd locust-tests-runner
sudo -H pip3 install -r requirements.txt

The repository comes with two example files for and tests.json, copy them using these two commands

cp tests.example.json tests.json

We have already used, now we will show the structure of tests.json:

  "tests": [
      "users": 10,
      "rps": 5,
      "time": "1m"

This is a JSON file with a single key called tests and it has the value of an array, each element of the array is a JSON object with these keys.

  • users: The number of users to simulate in this test.
  • rps: The hatch rate for this test.
  • time: The duration for running the test.

We can define multiple tests here, and run them all.

Now make sure you have in the host field in file and also add these tests to tests.json file.

  "tests": [
      "users": 100,
      "rps": 10,
      "time": "2m"
      "users": 200,
      "rps": 20,
      "time": "4m"
      "users": 500,
      "rps": 30,
      "time": "6m"

This will run three tests, with the defined attributes and save results to CSV files in a directory called google, use this command to run them

python -n google

Once tests are done change to and run tests again using this command

python -n facebook

Once tests finish we can move to next step Visualize results.

Visualize results

After we ran the tests and saved results in folders with different names we can use the supplied script called to run a simple server used to display results.

Run the server with this command


Navigate to this url to display results as shown bellow.

The previous images show results for facebook and google, we can see that facebook had a consistent average response time of around 154ms, while google started to give errors after the first test, but if we return to the errors we can see it is “429” response code, which is used to rate limit the number of requests to some routes, this is used as a protection by google to defend against attacks.


In this article we learned about the use of locust for load testing, and also the use of locust tests runner to run tests and visualize results.

I will work to improve the runner, to be able to define tests using a web interface and display and compare results easily using the interface.

I hope you find the content useful for any comments or questions you can contact me on my email address

Stay tuned for more articles. :) :)