Posted by Ben Weiss - Senior Developer Relations Engineer

In today's world of mobile applications, a seamless user experience is not just a feature—it's a necessity. Slow load times, unresponsive interfaces, and instability can be significant barriers to user engagement and retention. During their work with the Android Developer Relations team, the engineering team at Reddit used the App Performance Score to evaluate their app. After assessing their performance, they identified significant improvement potential and decided to take the steps to enable the full power of R8, the Android app optimizer. This focused initiative led to remarkable improvements in startup times, reductions in slow or frozen frames and ANRs, and an overall increase in Play Store ratings. This case study breaks down how Reddit achieved these impressive results.

How the R8 Optimizer helped Reddit

• Tree shaking is the most important step to reduce an app's size. Here, unused code from app dependencies and the app itself is removed.

• Method inlining replaces method calls with the actual code, making the app more performant.

• Class merging, and other strategies are applied to make the code more compact. At this point it's not about human readability of source code any more, but making compiled code work fast. So abstractions, such as interfaces or class hierarchies don't matter here and will be removed.

• Identifier minification changes the names of classes, fields, and methods to shorter, meaningless names. So instead of MyDataModel you might end up with a class called a. 

• Resource shrinking removes unused resources such as xml files and drawables to further reduce app size.

Caption: Main stages of R8 Optimization

From hard data to user satisfaction: Identifying success in production

Reddit saw improved performance results immediately after a new version of the app was rolled out to users. By using Android Vitals and Crashlytics, Reddit was able to capture performance metrics on real devices with actual users, allowing them to compare the new release against previous versions.

Caption: How R8 improved Reddit's app performance

The team observed a 40% faster cold startup, a 30% reduction in "Application Not Responding" (ANR) errors, a 25% improvement in frame rendering, and a 14% reduction in app size.

These enhancements are crucial for user satisfaction. A faster startup means less waiting and quicker access to content. Fewer ANRs lead to a more stable and reliable app, reducing user frustration. Smoother frame rendering removes UI jank, making scrolling and animations feel fluid and responsive. This positive technical impact was also clearly visible in user sentiment.

User satisfaction indicators of the optimization's success were directly visible on the Google Play Store. Following the rollout of the R8-optimized version, the team saw a dramatic and positive shift in user sentiment and engagement.

Drew Heavner: "Enabling R8's full potential tool less than 2 weeks"

Most impressively, this was accomplished with a focused effort. Drew Heavner, the Staff Software Engineer at Reddit who worked on this initiative, noted that implementing the changes to enable R8's full potential took less than two weeks.

Confirming the gains: A deep dive with macrobenchmarks

After observing the significant real-world improvements, Reddit's engineering team and the Android Developer Relations team at Google conducted detailed benchmarks to scientifically confirm the gains and experiment with further optimizations. For this analysis, Reddit engineering provided two versions of their app: one without optimizations and another that applied R8 and two more foundational performance optimization tools: Baseline Profiles, and Startup Profiles.

Baseline Profiles effectively move the Just in Time (JIT) compilation steps away from user devices and onto developer machines. The generated Ahead Of Time (AOT) compiled code has proven to reduce startup time and rendering issues alike.

When an app is packaged, the d8 dexer takes classes and methods and constructs your app's classes.dex files. When a user opens the app, these dex files are loaded, one after the other until the app can start. By providing a Startup Profile you let d8 know which classes and methods to pack in the first classes.dex files. This structure allows the app to load fewer files, which in turn improves startup speed.

Jetpack Macrobenchmark was the core tool for this phase, allowing for precise measurement of user interactions in a controlled environment. To simulate a typical user journey, they used the UIAutomator API to create a test that opened the app, scrolled down three times, and then scrolled back up.

In the end all that was needed to write the benchmark was this:

The benchmark data confirmed the field observations and provided deeper insights. The fully optimized app started 55% faster and users could begin to browse 18% sooner. The optimized app also showed a two-thirds reduction in Just in Time (JIT) compilation occurrences and a one-third decrease in JIT compilation time. Frame rendering improved, resulting in 19% more frames being rendered over the benchmarked user journey. Finally, the app's size was reduced by over a third.

Caption: Reddit's overall performance improvements

You can measure the JIT compilation time with a custom Macrobenchmark trace section metric like this:

Enabling the technology behind the transformation: R8

To enable R8 in full mode, you configure your app/build.gradle.kts file by setting minifyEnabled and shrinkResources to true in the release build type.

This step has to be followed by holistic end to end testing, as performance optimizations can lead to unwanted behavior, which you better catch before your users do.

As shown earlier in this article, R8 performs extensive optimizations in order to maximize your performance benefits. R8 makes substantial modifications to the code including renaming, moving, and removing classes, fields and methods. If you observe that these modifications cause errors, you need to specify which parts of the code R8 shouldn't modify by declaring those in keep rules.

Follow Reddit's example in your app

Reddit's success with R8 serves as a powerful case study for any development team looking to make a significant, low-effort impact on their app's performance. The direct correlation between the technical improvements and the subsequent rise in user satisfaction underscores the value of performance optimization.

By following the blueprint laid out in this case study—using tools like the App Performance Score to identify opportunities, enabling R8's full optimization potential, monitoring real-world data, and using benchmarks to confirm and deepen understanding—other developers can achieve similar gains.

To get started with R8 in your own app, refer to the freshly updated official documentation and guidance on enabling, configuring and troubleshooting the R8 optimizer.