Configuring ART

This page discusses how to configure ART and its compilation options. Topics addressed here include configuration of pre-compilation of the system image, dex2oat compilation options, and how to trade off system partition space, data partition space, and performance.

See ART and Dalvik, the Dalvik Executable format, and the remaining pages on source.android.com to work with ART. See Verifying App Behavior on the Android Runtime (ART) to ensure your apps work properly.

How ART works

ART uses ahead-of-time (AOT) compilation, and starting in Android 7.0 (Nougat or N), it uses a hybrid combination of AOT, just-in-time (JIT) compilation, and profile-guided compilation. The combination of all these compilation modes is configurable and will be discussed in this section. As an example, Pixel devices are configured with the following compilation flow:

  1. An application is initially installed without any AOT compilation. The first few times the application runs, it will be interpreted, and methods frequently executed will be JIT compiled.
  2. When the device is idle and charging, a compilation daemon runs to AOT-compile frequently used code based on a profile generated during the first runs.
  3. The next restart of an application will use the profile-guided code and avoid doing JIT compilation at runtime for methods already compiled. Methods that get JIT-compiled during the new runs will be added to the profile, which will then be picked up by the compilation daemon.

ART comprises a compiler (the dex2oat tool) and a runtime (libart.so) that is loaded for starting the Zygote. The dex2oat tool takes an APK file and generates one or more compilation artifact files that the runtime loads. The number of files, their extensions, and names are subject to change across releases, but as of the Android O release, the files being generated are:

  • .vdex: contains the uncompressed DEX code of the APK, with some additional metadata to speed up verification.
  • .odex: contains AOT compiled code for methods in the APK.
  • .art (optional): contains ART internal representations of some strings and classes listed in the APK, used to speed application startup.

Compilation options

Compilation options for ART are of two categories:

  1. System ROM configuration: what code gets AOT-compiled when building a system image.
  2. Runtime configuration: how ART compiles and runs applications on a device.

One core ART option to configure these two categories is compiler filters. Compiler filters drive how ART compiles DEX code and is an option passed to the dex2oat tool. Starting in Android O, there are four officially supported filters:

  • verify: only run DEX code verification.
  • quicken: run DEX code verification and optimize some DEX instructions to get better interpreter performance.
  • speed: run DEX code verification and AOT-compile all methods.
  • speed-profile: run DEX code verification and AOT-compile methods listed in a profile file.

System ROM configuration

There are a number of ART build options available for configuring a system ROM. How to configure these options depends on the available storage space for /system and the number of pre-installed applications. The JARs/APKs that are compiled into a system ROM can be divided in four categories:

  • Boot classpath code: compiled with the speed compiler filter by default.
  • System server code: compiled with the speed compiler filter by default.
  • Product-specific core applications: compiled with the speed compiler filter by default.
  • All other applications: compiled with the quicken compiler filter by default.

Makefile options

  • WITH_DEXPREOPT
  • Whether dex2oat is invoked on DEX code installed on the system image. Enabled by default.

  • DONT_DEXPREOPT_PREBUILTS (since Android 5.0)
  • Enabling DONT_DEXPREOPT_PREBUILTS prevents the prebuilts from being pre-optimized. These are apps that have include $(BUILD_PREBUILT) specified in their Android.mk, such as Gmail. Skipping pre-optimization of prebuilt apps that are likely to be updated via Google Play saves /system space but does add to first boot time.

  • WITH_DEXPREOPT_BOOT_IMG_AND_SYSTEM_SERVER_ONLY (new in Android O MR1)
  • Enabling WITH_DEXPREOPT_BOOT_IMG_AND_SYSTEM_SERVER_ONLY pre-optimizes only the boot classpath and system server jars.

  • LOCAL_DEX_PREOPT
  • Pre-optimization can also be enabled or disabled on an individual app basis by specifying the LOCAL_DEX_PREOPT option in the module definition. This can be useful for disabling pre-optimization of apps that may immediately receive Google Play updates since the updates would render the pre-optimized code in the system image obsolete. This is also useful to save space on major version upgrade OTAs since users may already have newer versions of apps in the data partition.

    LOCAL_DEX_PREOPT supports the values ‘true’ or ‘false’ to enable or disable pre-optimization, respectively. In addition, ‘nostripping’ can be specified if pre-optimization should not strip the classes.dex file from the APK or JAR file. Normally this file is stripped since it’s no longer needed after pre-optimization, but this last option is necessary to allow third-party APK signatures to remain valid.

  • PRODUCT_DEX_PREOPT_BOOT_FLAGS
  • Passes options to dex2oat to control how the boot image is compiled. It can be used to specify customized image classes lists, compiled classes lists, and compiler filters.

  • PRODUCT_DEX_PREOPT_DEFAULT_FLAGS
  • Passes options to dex2oat to control how everything besides the boot image is compiled.

  • PRODUCT_DEX_PREOPT_MODULE_CONFIGS
  • Provides the ability to pass dex2oat options for a particular module and product configuration. It is set in a product’s device.mk file by $(call add-product-dex-preopt-module-config,<modules>,<option>) where <modules> is a list of LOCAL_MODULE and LOCAL_PACKAGE names for JAR and APK files, respectively.

  • PRODUCT_DEXPREOPT_SPEED_APPS (New in Android O)
  • List of applications that have been identified as core to the products and which are desirable to compile with the speed compiler filter. For example, persistent apps such as SystemUI get a chance to use profile-guided compilation only at the next reboot, so it may be better for the product to have these apps always AOT-compiled.

  • PRODUCT_SYSTEM_SERVER_APPS (New in Android O)
  • List of applications that are loaded by the system server. These applications will be compiled by default with the speed compiler filter.

  • PRODUCT_ART_TARGET_INCLUDE_DEBUG_BUILD(Post Android O)
  • Whether to include a debug version of ART on the device. By default, this is enabled for userdebug and eng builds. The behavior can be overridden by explicitly setting the option to true or false.

    By default, the device will use the non-debug version (libart.so). To switch, set the system property persist.sys.dalvik.vm.lib.2 to libartd.so.

  • WITH_DEXPREOPT_PIC (Removed in Android O)
  • In Android 5.1.0 through Android 6.0.1, WITH_DEXPREOPT_PIC can be specified to enable position-independent code (PIC). With this, compiled code from the image doesn’t have to be relocated from /system into /data/dalvik-cache, saving space in the data partition. However, there is a slight runtime impact because it disables an optimization that takes advantage of position-dependent code. Typically, devices wanting to save space in /data should enable PIC compilation.

    In Android 7.0, PIC compilation was enabled by default.

  • WITH_DEXPREOPT_BOOT_IMG_ONLY (removed in Android O MR1)
  • This option was replaced with WITH_DEXPREOPT_BOOT_IMG_AND_SYSTEM_SERVER_ONLY that also preopts the system server jars.

Boot classpath configuration

  • Preloaded Classes List
  • The preloaded classes list is a list of classes the zygote will initialize on startup. This saves each app from having to run these class initializers separately, allowing them to start up faster and share pages in memory. The preloaded classes list file is located at frameworks/base/preloaded-classes by default, and it contains a list that is tuned for typical phone use. This may be different for other devices, such as wearables, and should be tuned accordingly. Be careful when tuning this since adding too many classes wastes memory loading unused classes; meanwhile, adding too few forces each app to have to have its own copy, again wasting memory.

    Example usage (in product’s device.mk):

    PRODUCT_COPY_FILES += <filename>:system/etc/preloaded-classes
    

    Note: This line must be placed before inheriting any product configuration makefiles that get the default one from: build/target/product/base.mk

  • Image Classes List
  • The image classes list is a list of classes that dex2oat initializes ahead of time and stores in the boot.art file. This allows the zygote to load these results out of the boot.art file on startup instead of running the initializers for these classes itself during preloading. A key feature of this is that the pages loaded from the image and shared between processes can be clean, allowing them to be swapped out easily in low-memory situations. In L, by default the image classes list uses the same list as the preloaded classes list. Beginning post-L in AOSP, a custom image classes list can be specified using:

    PRODUCT_DEX_PREOPT_BOOT_FLAGS
    

    Example use (in product’s device.mk):

    PRODUCT_DEX_PREOPT_BOOT_FLAGS += --image-classes=<filename>
    
  • Compiled Classes List
  • In post-L AOSP, a subset of classes from the boot classpath can be specified to be compiled during pre-optimization using the compiled classes list. This can be a useful option for devices that are very tight on space and can’t fit the entire pre-optimized boot image. However, note classes not specified by this list will not be compiled - not even on the device - and must be interpreted, potentially affecting runtime performance. By default, dex2oat will look for a compiled classes list in $OUT/system/etc/compiled-classes, so a custom one can be copied to that location by the device.mk. A particular file location can also be specified using:

    PRODUCT_DEX_PREOPT_BOOT_FLAGS
    

    Example usage (in product’s device.mk):

    PRODUCT_COPY_FILES += <filename>:system/etc/compiled-classes
    

    Note: This line must be placed before inheriting any product configuration makefiles that get the default one from: build/target/product/base.mk

Runtime configuration

Jit options

The following options affect Android releases only where the ART JIT compiler is available.

  • dalvik.vm.usejit: whether or not the JIT is enabled.
  • dalvik.vm.jitinitialsize (default 64K): the initial capacity of the code cache. The code cache will regularly GC and increase if needed.
  • dalvik.vm.jitmaxsize (default 64M): the maximum capacity of the code cache.
  • dalvik.vm.jitthreshold: (default 10000) - This is the threshold that the "hotness" counter of a method needs to pass in order for the method to be JIT compiled. The "hotness" counter is a metric internal to the runtime. It includes the number of calls, backward branches, and other factors.
  • dalvik.vm.usejitprofiles: whether or not JIT profiles are enabled; this may be used even if dalvik.vm.usejit is false. Note that if this is false, the compiler filter speed-profile does not AOT-compile any method and is equivalent to quicken.
  • dalvik.vm.jitprithreadweight (default to dalvik.vm.jitthreshold / 20) - The weight of the JIT "samples" (see jitthreshold) for the application UI thread. Use to speed up compilation of methods that directly affect users experience when interacting with the app.
  • dalvik.vm.jittransitionweight: (default to dalvik.vm.jitthreshold / 10) the weight of the method invocation that transitions between compile code and interpreter. This helps make sure the methods involved are compiled to minimize transitions (which are expensive).

Package manager options

Since Android 7.0, there's a generic way to specify the level of compilation/verification that happened at various stages. The compilation levels can be configured via system properties with the defaults being:

  • pm.dexopt.install=quicken
  • This is the compilation filter used when installing applications through Google Play. For faster installs, try the quicken compiler filter.

  • pm.dexopt.bg-dexopt=speed-profile
  • This is the compilation filter used when the device is idle, charging and fully charged. Try the speed-profile compiler filter to take advantage of profile-guided compilation and save on storage.

  • pm.dexopt.boot=verify
  • The compilation filter used after an over-the-air update. We strongly recommend the verify compiler filter for this option to avoid very long boot times.

  • pm.dexopt.first-boot=quicken
  • The compilation filter for the first time the device ever boots. The filter used here will only affect the boot time after factory. We recommend the filter quicken for it to avoid long times before a user gets to use the phone for the very first time. Note that if all applications in /system are already compiled with the quicken compiler filter or are compiled with the speed or speed-profile compiler filter, the pm.dexopt.first-boot has no effect.

Dex2oat options

Note that these options affect dex2oat during on-device compilation as well as during pre-optimization, whereas most of the options discussed above affect only pre-optimization.

To control dex2oat while it’s compiling the boot image:

  • dalvik.vm.image-dex2oat-Xms: initial heap size
  • dalvik.vm.image-dex2oat-Xmx: maximum heap size
  • dalvik.vm.image-dex2oat-filter: compiler filter option
  • dalvik.vm.image-dex2oat-threads: number of threads to use

To control dex2oat while it’s compiling everything besides the boot image:

  • dalvik.vm.dex2oat-Xms: initial heap size
  • dalvik.vm.dex2oat-Xmx: maximum heap size
  • dalvik.vm.dex2oat-filter: compiler filter option

On releases through Android 6.0, one additional option is provided for compiling everything besides the boot image:

  • dalvik.vm.dex2oat-threads: number of threads to use

Starting with Android 6.1, this becomes two additional options for compiling everything besides the boot image:

  • dalvik.vm.boot-dex2oat-threads: number of threads to use during boot time
  • dalvik.vm.dex2oat-threads: number of threads to use after boot time

Starting with Android 7.1, two options are provided for controlling how memory is used when compiling everything besides the boot image:

  • dalvik.vm.dex2oat-very-large: minimum total dex file size in bytes to disable AOT compilation
  • dalvik.vm.dex2oat-swap: use dex2oat swap file (for low-memory devices)

The options that control initial and maximum heap size for dex2oat should not be reduced since they could limit what applications can be compiled.

A/B specific configuration

ROM configuration

Starting in Android 7.0, devices may use two system partitions to enable A/B system updates. To save on the system partition size, the preopted files can be installed in the unused second system partition. They are then copied to the data partition on first boot.

Example usage (in device-common.mk):

PRODUCT_PACKAGES += \
     cppreopts.sh
PRODUCT_PROPERTY_OVERRIDES += \
     ro.cp_system_other_odex=1

And in device's BoardConfig.mk:

BOARD_USES_SYSTEM_OTHER_ODEX := true

Note that boot classpath code, system server code, and product-specific core applications always compile to the system partition. By default, all other applications get compiled to the unused second system partition. This can be controlled with the SYSTEM_OTHER_ODEX_FILTER, which has a value by default of:

SYSTEM_OTHER_ODEX_FILTER ?= app/% priv-app/%

Background dexopt OTA

With A/B enabled devices, applications can be compiled in the background for updating to the new system image. See App compilation in background to optionally include the compilation script and binaries in the system image. The compilation filter used for this compilation is controlled with:

pm.dexopt.ab-ota=speed-profile

We recommend using speed-profile to take advantage of profile guided compilation and save on storage.