Why would a load test report higher network throughput than my server?
When you run a load test with Loadster, you might notice that the reported download bytes or network throughput is higher than what your web hosting provider or server monitoring tools show. This discrepancy is fairly common and can happen for a few different reasons.
How Loadster measures network traffic
For Browser Bots, Loadster measures network throughput using an internal SOCKS proxy for each bot, which captures all network traffic generated by that bot’s browser. This means Loadster sees every single byte transferred during the test, regardless of where that traffic is going.
This is different from server-side monitoring tools, which typically only measure traffic hitting your specific server or domain, and don’t know anything about traffic to third parties.
External content and CDN usage
Most modern web applications and websites load content from external sources that aren’t hosted on your primary server. This commonly includes:
Content Delivery Networks (CDNs): Images, CSS files, JavaScript libraries, and other static assets are often served from CDNs like Cloudflare, AWS CloudFront, or Google Cloud CDN.
Third-party services: Analytics scripts (like Google Analytics or Mixpanel), ad networks, social media widgets, chat widgets, etc.
External APIs: Your application might make requests to external APIs for payment processing, embedded maps, or other functionality.
Fonts and web assets: Custom fonts from services like Google Fonts or Adobe Fonts, icon libraries, and other design assets often come from external servers.
Since Loadster’s bots capture all network traffic, they include these external requests in their bandwidth measurements, while your server monitoring probably only sees traffic directly hitting your infrastructure.
Browser background data transfers
Another source of additional traffic might come from the browsers themselves. When using Browser Bots powered by Google Chrome, the browser has been known perform background activities when it first starts up. These can contribute to bandwidth totals in your load test, even though they aren’t hitting your site or loading any of the on-page resources.
Chrome has been known to download dependencies like updates, fonts, video codecs, and other resources from Google’s
servers during initialization. Common culprits include requests to domains like optimizationguide-pa.googleapis.com,
gvt1.com, safebrowsingohttpgateway.googleapis.com, and other Google services. These downloads can sometimes even add
several megabytes per bot to your total bandwidth usage.
This happens because Chrome is designed to keep itself updated and optimized, even when running in automated testing environments where we don’t necessary need those updates. Only some of them can be turned off, as far as we know. While this behavior is typical for a real browser, it can contribute to your bandwidth measurements in load testing scenarios and is one reason why it might be higher than expected.
Interpreting the network traffic results
When you see higher bandwidth numbers in your Loadster test report than on your server, it’s usually because Loadster is measuring the total network impact of your application from the user’s perspective. This includes all the external content and browser activity that real users would also trigger.
If you need to isolate and count the traffic specifically hitting your servers, you can:
- Compare Loadster’s measurements with your server logs and hosting provider metrics
- Use network monitoring tools on your server side to measure only direct traffic
- Review the difference to understand your application’s external dependencies
The “extra” bandwidth isn’t necessarily a problem; it’s just something to take into account to better understand what’s really happening when users visit your site.
Understanding the bandwidth and data transfer requirements for users of your site can help you identify dependencies on external services that might become bottlenecks or opportunities to optimize your use of CDNs and third-party services.