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| <title>Controlling the Embedded VM</title> |
| <link rel=stylesheet href="android.css"> |
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| <h1>Controlling the Embedded VM</h1> |
| |
| <ul> |
| <li><a href="#introduction">Introduction</a> (read this first!) |
| <li><a href="#checkjni">Extended JNI Checks</a> |
| <li><a href="#assertions">Assertions</a> |
| <li><a href="#verifier">Bytecode Verification and Optimization</a> |
| <li><a href="#execmode">Execution Mode</a> |
| <li><a href="#stackdump">Stack Dumps</a> |
| <li><a href="#dexcheck">DEX File Checksums</a> |
| <li><a href="#general">General Flags</a> |
| </ul> |
| |
| <h2><a name="introduction">Introduction (read this first!)</a></h2> |
| |
| <p>The Dalvik VM supports a variety of command-line arguments |
| (use <code>adb shell dalvikvm -help</code> to get a summary), but |
| it's not possible to pass arbitrary arguments through the |
| Android application runtime. It is, however, possible to affect the |
| VM behavior through certain system properties. |
| |
| <p>For all of the features described below, you would set the system property |
| with <code>setprop</code>, |
| issuing a shell command on the device like this: |
| <pre>adb shell setprop <name> <value></pre> |
| |
| <p><strong>The Android runtime must be restarted before the changes will take |
| effect</strong> (<code>adb shell stop; adb shell start</code>). This is because the |
| settings are processed in the "zygote" process, which starts early and stays |
| around "forever". |
| |
| <p>You may not be able to set <code>dalvik.*</code> properties or restart |
| the system as an unprivileged user. You can use |
| <code>adb root</code> or run the <code>su</code> command from the device |
| shell on "userdebug" builds to become root first. When in doubt, |
| <pre>adb shell getprop <name></pre> |
| will tell you if the <code>setprop</code> took. |
| |
| <p>If you don't want the property to evaporate when the device reboots, |
| add a line to <code>/data/local.prop</code> that looks like: |
| <pre><name> = <value></pre> |
| |
| <p>Such changes will survive reboots, but will be lost if the data |
| partition is wiped. (Hint: create a <code>local.prop</code> |
| on your workstation, then <code>adb push local.prop /data</code>. Or, |
| use one-liners like |
| <code>adb shell "echo name = value >> /data/local.prop"</code> -- note |
| the quotes are important.) |
| |
| |
| <h2><a name="checkjni">Extended JNI Checks</a></h2> |
| |
| <p>JNI, the Java Native Interface, provides a way for code written in the |
| Java programming language |
| interact with native (C/C++) code. The extended JNI checks will cause |
| the system to run more slowly, but they can spot a variety of nasty bugs |
| before they have a chance to cause problems. |
| |
| <p>There are two system properties that affect this feature, which is |
| enabled with the <code>-Xcheck:jni</code> command-line argument. The |
| first is <code>ro.kernel.android.checkjni</code>. This is set by the |
| Android build system for development builds. (It may also be set by |
| the Android emulator unless the <code>-nojni</code> flag is provided on the |
| emulator command line.) Because this is an "ro." property, the value cannot |
| be changed once the device has started. |
| |
| <p>To allow toggling of the CheckJNI flag, a second |
| property, <code>dalvik.vm.checkjni</code>, is also checked. The value |
| of this overrides the value from <code>ro.kernel.android.checkjni</code>. |
| |
| <p>If neither property is defined, or <code>dalvik.vm.checkjni</code> |
| is set to <code>false</code>, the <code>-Xcheck:jni</code> flag is |
| not passed in, and JNI checks will be disabled. |
| |
| <p>To enable JNI checking: |
| <pre>adb shell setprop dalvik.vm.checkjni true</pre> |
| |
| <p>You can also pass JNI-checking options into the VM through a system |
| property. The value set for <code>dalvik.vm.jniopts</code> will |
| be passed in as the <code>-Xjniopts</code> argument. For example: |
| <pre>adb shell setprop dalvik.vm.jniopts forcecopy</pre> |
| |
| |
| <h2><a name="assertions">Assertions</a></h2> |
| |
| <p>Dalvik VM supports the Java programming language "assert" statement. |
| By default they are off, but the <code>dalvik.vm.enableassertions</code> |
| property provides a way to set the value for a <code>-ea</code> argument. |
| |
| <p>The argument behaves the same as it does in other desktop VMs. You |
| can provide a class name, a package name (followed by "..."), or the |
| special value "all". |
| |
| <p>For example, this: |
| <pre>adb shell setprop dalvik.vm.enableassertions all</pre> |
| enables assertions in all non-system classes. |
| |
| <p>The system property is much more limited than the full command line. |
| It is not possible to specify more than one <code>-ea</code> entry, and there |
| is no way to specify a <code>-da</code> entry. There is presently no |
| equivalent for <code>-esa</code>/<code>-dsa</code>. |
| |
| |
| <h2><a name="verifier">Bytecode Verification and Optimization</a></h2> |
| |
| <p>The system tries to pre-verify all classes in a DEX file to reduce |
| class load overhead, and performs a series of optimizations to improve |
| runtime performance. Both of these are done by the <code>dexopt</code> |
| command, either in the build system or by the installer. On a development |
| device, <code>dexopt</code> may be run the first time a DEX file is used |
| and whenever it or one of its dependencies is updated ("just-in-time" |
| optimization and verification). |
| |
| <p>There are two command-line flags that control the just-in-time |
| verification and optimization, |
| <code>-Xverify</code> and <code>-Xdexopt</code>. The Android framework |
| configures these based on the <code>dalvik.vm.dexopt-flags</code> |
| property. |
| |
| <p>If you set: |
| <pre>adb shell setprop dalvik.vm.dexopt-flags v=a,o=v</pre> |
| then the framework will pass <code>-Xverify:all -Xdexopt:verified</code> |
| to the VM. This enables verification, and only optimizes classes that |
| successfully verified. This is the safest setting, and is the default. |
| <p>You could also set <code>dalvik.vm.dexopt-flags</code> to <code>v=n</code> |
| to have the framework pass <code>-Xverify:none -Xdexopt:verified</code> |
| to disable verification. (We could pass in <code>-Xdexopt:all</code> to |
| allow optimization, but that wouldn't necessarily optimize more of the |
| code, since classes that fail verification may well be skipped by the |
| optimizer for the same reasons.) Classes will not be verified by |
| <code>dexopt</code>, and unverified code will be loaded and executed. |
| |
| <p>Enabling verification will make the <code>dexopt</code> command |
| take significantly longer, because the verification process is fairly slow. |
| Once the verified and optimized DEX files have been prepared, verification |
| incurs no additional overhead except when loading classes that failed |
| to pre-verify. |
| |
| <p>If your DEX files are processed with verification disabled, and you |
| later turn the verifier on, application loading will be noticeably |
| slower (perhaps 40% or more) as classes are verified on first use. |
| |
| <p>For best results you should force a re-dexopt of all DEX files when |
| this property changes. You can do this with: |
| <pre>adb shell "rm /data/dalvik-cache/*"</pre> |
| This removes the cached versions of the DEX files. Remember to |
| stop and restart the runtime (<code>adb shell stop; adb shell start</code>). |
| |
| <p>(Previous version of the runtime supported the boolean |
| <code>dalvik.vm.verify-bytecode</code> property, but that has been |
| superceded by <code>dalvik.vm.dexopt-flags</code>.)</p> |
| |
| |
| <h2><a name="execmode">Execution Mode</a></h2> |
| |
| <p>The current implementation of the Dalvik VM includes three distinct |
| interpreter cores. These are referred to as "fast", "portable", and |
| "debug". The "fast" interpreter is optimized for the current |
| platform, and might consist of hand-optimized assembly routines. In |
| constrast, the "portable" interpreter is written in C and expected to |
| run on a broad range of platforms. The "debug" interpreter is a variant |
| of "portable" that includes support for profiling and single-stepping. |
| |
| <p>The VM may also support just-in-time compilation. While not strictly |
| a different interpreter, the JIT compiler may be enabled or disabled |
| with the same flag. (Check the output of <code>dalvikvm -help</code> to |
| see if JIT compilation is enabled in your VM.) |
| |
| <p>The VM allows you to choose between "fast", "portable", and "jit" with an |
| extended form of the <code>-Xint</code> argument. The value of this |
| argument can be set through the <code>dalvik.vm.execution-mode</code> |
| system property. |
| |
| <p>To select the "portable" interpreter, you would use: |
| <pre>adb shell setprop dalvik.vm.execution-mode int:portable</pre> |
| If the property is not specified, the most appropriate interpreter |
| will be selected automatically. At some point this mechanism may allow |
| selection of other modes, such as JIT compilation. |
| |
| <p>Not all platforms have an optimized implementation. In such cases, |
| the "fast" interpreter is generated as a series of C stubs, and the |
| result will be slower than the |
| "portable" version. (When we have optimized versions for all popular |
| architectures the naming convention will be more accurate.) |
| |
| <p>If profiling is enabled or a debugger is attached, the VM |
| switches to the "debug" interpreter. When profiling ends or the debugger |
| disconnects, the original interpreter is resumed. (The "debug" interpreter |
| is substantially slower, something to keep in mind when evaluating |
| profiling data.) |
| |
| <p>The JIT compiler can be disabled on a per-application basis by adding |
| <code>android:vmSafeMode="true"</code> in the <code>application</code> |
| tag in <code>AndroidManifest.xml</code>. This can be useful if you |
| suspect that JIT compilation is causing your application to behave |
| incorrectly. |
| |
| |
| <h2><a name="stackdump">Stack Dumps</a></h2> |
| |
| <p>Like other desktop VMs, when the Dalvik VM receives a SIGQUIT |
| (Ctrl-\ or <code>kill -3</code>), it dumps stack traces for all threads. |
| By default this goes to the Android log, but it can also be written to a file. |
| |
| <p>The <code>dalvik.vm.stack-trace-file</code> property allows you to |
| specify the name of the file where the thread stack traces will be written. |
| The file will be created (world writable) if it doesn't exist, and the |
| new information will be appended to the end of the file. The filename |
| is passed into the VM via the <code>-Xstacktracefile</code> argument. |
| |
| <p>For example: |
| <pre>adb shell setprop dalvik.vm.stack-trace-file /tmp/stack-traces.txt</pre> |
| |
| <p>If the property is not defined, the VM will write the stack traces to |
| the Android log when the signal arrives. |
| |
| |
| <h2><a name="dexcheck">DEX File Checksums</a></h2> |
| |
| <p>For performance reasons, the checksum on "optimized" DEX files is |
| ignored. This is usually safe, because the files are generated on the |
| device, and have access permissions that prevent modification. |
| |
| <p>If the storage on a device becomes unreliable, however, data corruption |
| can occur. This usually manifests itself as a repeatable virtual machine |
| crash. To speed diagnosis of such failures, the VM provides the |
| <code>-Xcheckdexsum</code> argument. When set, the checksums on all DEX |
| files are verified before the contents are used. |
| |
| <p>The application framework will provide this argument during VM |
| creation if the <code>dalvik.vm.check-dex-sum</code> property is enabled. |
| |
| <p>To enable extended DEX checksum verification: |
| <pre>adb shell setprop dalvik.vm.check-dex-sum true</pre> |
| |
| <p>Incorrect checksums will prevent the DEX data from being used, and will |
| cause errors to be written to the log file. If a device has a history of |
| problems it may be useful to add the property to |
| <code>/data/local.prop</code>. |
| |
| <p>Note also that the |
| <code>dexdump</code> tool always verifies DEX checksums, and can be used |
| to check for corruption in a large set of files. |
| |
| |
| <h2><a name="general">General Flags</a></h2> |
| |
| <p>In the "Gingerbread" release, a general mechanism for passing flags to |
| the VM was introduced: |
| |
| <pre>adb shell setprop dalvik.vm.extra-opts "flag1 flag2 ... flagN"</pre> |
| |
| <p>The flags are separated by spaces. You can specify as many as you want |
| so long as they all fit within the system property value length limit |
| (currently 92 characters). |
| |
| <p>The extra-opts flags will be added at the end of the command line, |
| which means they will override earlier settings. This can be used, for |
| example, to experiment with different values for <code>-Xmx</code> even |
| though the Android framework is setting it explicitly. |
| |
| <address>Copyright © 2008 The Android Open Source Project</address> |
| |
| </body></html> |