How R8 made Kotlin Coroutines on Android 2x faster

Posted by Andrei Shikov, Senior Software Engineer, Android Toolkit and Jonathan Starup, Software Engineer, R8 Team

Starting from AGP 9.2.0, R8 optimizes most Atomic*FieldUpdater calls into Unsafe variants that perform 2x to 4x better on common operations. This has a particularly large impact on the kotlinx.atomicfu library that implements atomics for kotlinx.coroutines, making launching and cancelling coroutines up to 2x faster. In order to get the benefits, update your AGP to 9.2.0 or above.

With the majority of Android apps adopting Kotlin as their main language of choice, kotlinx.coroutines has become a de-facto standard for asynchronous programming. The library offers a well-designed and structured way of managing concurrent flows that is native to Kotlin. Jetpack Compose was no exception, adopting coroutines for managing pointer events, animations and other interactions. At the time of writing, most concurrent APIs in Compose call suspend functions under the hood and are launching and/or cancelling coroutines to handle updates.

As the Compose team started to investigate performance, coroutines were discovered to be a bottleneck for many operations that happen outside of composition. As an example, 80% of the time spent on creating and updating Modifier.clickable was consumed by launching and cancelling internal coroutines that handled InteractionSource updates. Based on those observations, much of early performance work was focused on removing coroutines from the default path and delaying initialization until necessary.

The cost of a coroutine

The easiest way to analyze a function's internal behavior on Android is to capture an Android Runtime (ART) method trace. An ART method trace is a tool that records the execution flow of an app, showing exactly which methods are called, their order, and how much time is spent in each, allowing developers to identify performance bottlenecks. For an empty LaunchedEffect { } call, it would look something like this:

Launch…