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Posted by Serban Constantinescu, Product Manager
Wouldn’t it be great if each update improved start-up times, execution speed, and memory usage of your apps? Google Play system updates for the Android Runtime (ART) do just that. These updates deliver performance improvements, the latest security fixes, and unify the core OpenJDK APIs across hundreds of millions of devices, including all Android 12+ devices and soon Android Go.
ART is the engine behind the Android operating system (OS). It provides the runtime and core APIs that all apps and most OS services rely on. Both Java and Kotlin are compiled down to bytecode executed by ART. Improvements in the runtime, compiler and core API benefit all developers making app execution faster and bytecode compilation more efficient.
While parts of Android are customizable by device manufacturers, ART is the same for all devices and Google Play system updates enable a path to modular updates.
Modularizing the OS
Android was originally designed for monolithic updates, which meant that OS components did not need to have clear API boundaries. This is because all dependent software would be built together. However, this made it difficult to update ART independently of the rest of the OS. Our first challenge was to untangle ART’s dependencies and create clear, well-defined, and tested API boundaries. This allowed us to modularize ART and make it independently updatable.
As a core part of the OS, ART had to blaze new trails and engineer new OS boundaries. These new boundaries were so extensive that manually adding and updating them would be too time-consuming. Therefore, we implemented automatic generation of those through introspection in the build system.
Another example is stack unwinding, which reports the functions last executed when an issue is detected. Before modularizing the OS, all stack unwinding code was built together and could change across Android versions. This made the transition even more challenging, since there is only one version of ART that is delivered to many versions of Android, we had to create a new API boundary as well as design it to be forward-compatible with newer versions of the ART APEX module on devices that are no longer getting full OS updates.
Recently, for Android 14, we refactored the interface between the Package Manager, the service that determines how to install and update apps, and ART. This moves the OS boundary from the ART dex2oat command line to a well-defined interface that enables future optimizations, such as finer-grained control over the compilation mode.
ART updatability also introduced new challenges. For example, the collection of Java libraries, referred to as the Boot Classpath, had to be securely recompiled to ensure good performance. This required introducing a new secure state for compilation during boot as well as a fallback JIT compilation mode.
On older devices, the secure compilation happens on the first reboot after an ART update. On newer devices that support the Android Virtualization Framework, the compilation happens while the device is idle, in an enclave called Isolated Compilation – saving up to 20 seconds of boot-time.
Testing the ART APEX module
The ART APEX module is a complex piece of software with an order of magnitude more APIs than any other APEX module. It also backs a quarter of the developer APIs available in the Android SDK. In addition, ART has a compiler that aims to make the most of the underlying hardware by generating chipset-specific instructions, such as Arm SVE. This, together with the multiple OS versions on which the ART APEX module has to run, makes testing challenging.
We first modularized the testing framework from per-platform release (e.g. Android CTS) to per module. We did this by introducing an ART-specific Mainline Test Suite (MTS), which tests both compiler and runtime, as well as core OpenJDK APIs, while collecting code coverage statistics.
Our target is 100% API coverage and high line coverage, especially for new APIs. Together with HWASan and fuzzing, all of the tests described above contribute to a massive test load that needs to be sharded across multiple devices to ensure that it completes in a reasonable amount of time.
We test the upcoming ART release every day by compiling over 18 million APKs and running app compatibility tests, and startup, performance, and memory benchmarks on a variety of Android devices that replicate the diversity of our ecosystem as closely as possible. Once tests pass with all possible compilation modes, all Garbage Collector algorithms, and supported OS versions, we begin gradually rolling out the next ART release.
Benefits of ART Google Play system updates
By updating ART independently of OS updates, users get the latest performance optimizations and security fixes as quickly as possible, while developers get OpenJDK improvements and compiler optimisations that benefit both Java and Kotlin.
As shown in the graph below, the runtime and compiler optimizations in the ART 13 update delivered real-world app start-up improvements of up to 30% on some devices.
ART updates allow us to frequently deploy fixes with little additional effort from our ecosystem partners. They include propagating upstream OpenJDK fixes to Android devices as quickly as possible, as well as runtime and compiler security fixes, such as CVE-2022-20502, which was detected by our automated fuzzing tests.
For developers, ART updates mean that you can now target the latest programming features. ART 13 delivered OpenJDK 11 core language features, which was the fastest-ever adoption of a new OpenJDK release on Android devices.
In the coming months, we’ll be releasing ART 14 to all compatible devices. ART 14 includes OpenJDK 17 support along with new compiler and runtime optimizations that improve performance while reducing code size. Stay tuned for more details on ART 14!
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