Load Testing Wordpress Amazon Lightsail Instances

Lightsail offers a range of virtual private servers with Wordpress installed. A frequent question is which size instance would be best for my application. Wordpress is used in many different ways ranging from relatively static sites, personal blogs, photography, and e-commerce. The flexibility of Wordpress complicates the choice of which Lightsail Wordpress instance to use. One key website metric is responsiveness. The amount of time a person has to wait before a site renders a page greatly affects whether they stay on the site or if they move on (bounce) to another site. According to studies, if page load time increases from 1 second to 3 seconds, the probability of bounce increase by 32%. If the page time load increases from 1 second to 5 seconds, the likelihood of bounce increases by 90%. Although there are many factors that affect page speed load, we’re interested in how the volume of requests affects page load speed. In this article, we will setup a Lightsail Wordpress site and test the site's responsiveness under multiple users making multiple requests using a load testing tool.

For a realistic test, we’ll take statistics from State of the Web Report by webarchive.org. The report includes data from 2010 to the present and it segments the data by type of site including word press. We’ll statistics based on data from January 2020 to the August 2023.

Here are the benchmarks of a typical Wordpress site from 2020 to 2023 based on a sample of 4,184,009 URLs.

For comparison, we can use the Wikimedia Foundation’s statistics as a high end benchmark. Note that Wikimedia has over 1,000 servers spread across five data centers.

We will configure a Wordpress to return a combined total of 3700 kb comprised of an image and text. Lightsail Wordpress instances use t3 EC2 instances and the instance Plan corresponds to a t3 instance.

Here is a list of Lightsail instance types and the corresponding EC2 types.

We will use the $10.00/month instance for the test Wordpress server. This is a frequently used instance for sites with a moderate amount of traffic.

Let’s create an example Wordpress instance. From the AWS home console, open the Lightsail console. You can use the search bar to find the Lightsail console.

Choose Create instance.

Next, choose the Linux/Unix instance with the Wordpress blueprint.

For this example, we will use a the $10/month instances (t3.small).

Name the instance, then choose Create instance.

The next step is to create a test page that can emulate a typical page weight. For convenience, we will use the default administrator account. Open a terminal with the browser-based SSH client.

In the terminal, get the default account password.

Using the public IP address, sign into Wordpress. You can find the public IP in the Wordpress instance console.

Next, sign in to Wordpress. Open a browser window to http://<your-ip-address>/wp-login.php.

Next, we create a new page in the user account. In the dashboard menu on the left, choose Pages, then Add New.

In the new page, add an image and text with the Wordpress editor and publish it.

In this example, the page has an image and text with a total page weight of 3.7 mb.

When the page is published, the URL of the page will be the title of page, e.g., http://<your-ip-address>/load-test-page.

Locust is a performance testing tool. It uses Python to programmatically create load tests for requesting Wordpress pages. Locust is capable of using a single process to generate requests for thousands of concurrent users with a minimum of hardware. Another nice feature of Locust is a web-based UI in addition to a command-line client.

Let’s start by creating an OS only Lightsail $3.5/month (t3.nano) instance.

Sign into the instance using the browser-based SSH client to install Locust.

In the terminal, check if Python is installed.

Next, install locust. First, we’ll create a directory to hold the configuration files. Locust is distributed as a Python package, and it’s good practice to create a virtual environment for packages with venv and activate it. Use the Python package manager, pip, to install locust. Check the installation by displaying the version.

We can write a simple test for our Wordpress instance that requests the home page and the test page we created earlier. Save the test as locustfile.py.

The locust web-ui uses port 8089. Before we can use it, we will need to open the port. In the instance page, choose Networking. Then under IPv4 Firewall, choose Add Rule. Select Custom Rule from the drop down, enter 8089 for the Port or range field. Choose Create to add the rule and open the port.

Now we’re ready to run the test plan with the web-ui. In the terminal, we want to run locust in the background so it will continue to run if the terminal closes.

Open a browser window to http://<your-ip-address>:8089/. We'll set the load to 12 users and 83 requests/second to match the typical Wordpress load. The test will run for 30 minutes. Choose Start swarming to begin the load test.

We’ll let the load test run for 30 minutes and we’ll review the results.

Locust provides both statistics and charts from the load test. Let’s take a look at the chart.

We’re interested in the aggregate statistics. During the 30 minute load testing run, Locust sent 43,413 requests or 24 requests per second (RPS) split between the two tasks specified in the script.

This is much lower than the 83 requests/second and is a function of the locust server’s resources. To achieve a higher request rate, we would need more locust workers in a distributed mode.

Although the request rate is lower than specified, the response time produces interesting results. At 24 requests/second the Wordpress server’s response time is within 1 second with zero failures 100% of the time. The chart shows that the aggregate response time is approximately 700 millisecond 95% of the time.

These results look promising, but that’s only part of the story. Wordpress sites typically do not run at full capacity and CPU utilization occurs in spikes. This Lightsail Wordpress instance is deployed on a t3 instance. T3 instances have a baseline CPU performance and are capable of bursting above the baseline. The baseline is designed to meet the requirements of general purpose workloads such as web servers, small and medium databases, and microservices.

Lightsail instances provide instance metrics for monitoring the performance. Lightsail divides CPU utilization into a sustainable zone where there is no impact to performance and a burstable zone to handle larger workloads for a limited period of time. In the graph below of Average CPU Utilization per 5 Minutes, the sustainable zone is at 20% utilization because we are using a $10/month instance. During the 30 minute load test, the Wordpress server used 100% CPU capacity.

However, the server can run in the burstable zone for a limited amount of time before it will drop back to the sustainable zone. In the second graph, Remaining CPU Burst Capacity (Percentage), the load test consumed roughly 50% of available burst capacity. Burst capacity consumption is relatively linear, and it is possible that running the load test longer or increasing the number of requests could consume all the burst capacity. In that case, the server would drop to the sustainable zone and would increase the response time or even fail to respond under the increased workload.

To see how many minutes of burst capacity available to the server, we can look at the Remaining CPU Burst Capacity metric. In this example, the server can run in the burstable zone for quite some time. A realistic Wordpress test plan would access a variety of pages including creating new pages, and may even vary the the number of requests over a period of time.

These metrics can assist in determining which Lightsail instance to use based on the expected workload. Lightsail provides additional metrics for instances and other Lightsail services, which are described in this document. For a deep dive into instance burst capacity and how it is calculated and consumed, checkout Viewing instance burst capacity in Amazon Lightsail.

We’ve looked Wordpress from both the output performance side and from the server instance side. We created a example Wordpress site and ran a simple load test with locust. The metrics from locust shows server performance, i,e,. what user experiences, and the Lightsail metrics shows the resources that the Wordpress server consumes and whether you have sufficient resources for your expected workload. Explore these resources to build a load testing plan for your Wordpress deployments: