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This page contains information on optimizing build performance for Android apps specifically. For general build performance optimization with Bazel, see Optimizing Performance. The flags are in the bazelrc configuration syntax, so they can be pasted directly into a bazelrc file and invoked with --config=<configuration_name> on the command line. Profiling performance Bazel writes a JSON trace profile by default to a file called command.profile.gz in Bazel’s output base. See the JSON Profile documentation for how to read and interact with the profile. Persistent workers for Android build actions. A subset of Android build actions has support for persistent workers. These actions’ mnemonics are:
  • DexBuilder
  • Javac
  • Desugar
  • AaptPackage
  • AndroidResourceParser
  • AndroidResourceValidator
  • AndroidResourceCompiler
  • RClassGenerator
  • AndroidResourceLink
  • AndroidAapt2
  • AndroidAssetMerger
  • AndroidResourceMerger
  • AndroidCompiledResourceMerger
Enabling workers can result in better build performance by saving on JVM startup costs from invoking each of these tools, but at the cost of increased memory usage on the system by persisting them. To enable workers for these actions, apply these flags with --config=android_workers on the command line: 
build:android_workers --strategy=DexBuilder=worker
build:android_workers --strategy=Javac=worker
build:android_workers --strategy=Desugar=worker

# A wrapper flag for these resource processing actions:
# - AndroidResourceParser
# - AndroidResourceValidator
# - AndroidResourceCompiler
# - RClassGenerator
# - AndroidResourceLink
# - AndroidAapt2
# - AndroidAssetMerger
# - AndroidResourceMerger
# - AndroidCompiledResourceMerger
build:android_workers --persistent_android_resource_processor
The default number of persistent workers created per action is 4. We have measured improved build performance by capping the number of instances for each action to 1 or 2, although this may vary depending on the system Bazel is running on, and the project being built. To cap the number of instances for an action, apply these flags: 
build:android_workers --worker_max_instances=DexBuilder=2
build:android_workers --worker_max_instances=Javac=2
build:android_workers --worker_max_instances=Desugar=2
build:android_workers --worker_max_instances=AaptPackage=2
# .. and so on for each action you're interested in.
Using AAPT2 aapt2 has improved performance over aapt and also creates smaller APKs. To use aapt2, use the --android_aapt=aapt2 flag or set aapt2 on the aapt_version on android_binary and android_local_test. SSD optimizations The --experimental_multi_threaded_digest flag is useful for optimizing digest computation on SSDs.