3 <title>Controlling the Embedded VM</title>
4 <link rel=stylesheet href="android.css">
8 <h1>Controlling the Embedded VM</h1>
11 <li><a href="#overview">Overview</a>
12 <li><a href="#checkjni">Extended JNI Checks</a>
13 <li><a href="#assertions">Assertions</a>
14 <li><a href="#verifier">Bytecode Verification and Optimization</a>
15 <li><a href="#execmode">Execution Mode</a>
16 <li><a href="#dp">Deadlock Prediction</a>
17 <li><a href="#stackdump">Stack Dumps</a>
18 <li><a href="#dexcheck">DEX File Checksums</a>
21 <h2><a name="overview">Overview</a></h2>
23 <p>The Dalvik VM supports a variety of command-line arguments
24 (use <code>adb shell dalvikvm -help</code> to get a summary), but
25 it's not possible to pass arbitrary arguments through the
26 Android application runtime. It is, however, possible to affect the
27 VM behavior through certain system properties.
29 <p>For all of the features described below, you would set the system property
30 with <code>setprop</code>,
31 issuing a shell command on the device like this:
32 <pre>adb shell setprop <name> <value></pre>
34 <p>The Android runtime must be restarted before the changes will take
35 effect (<code>adb shell stop; adb shell start</code>). This is because the
36 settings are processed in the "zygote" process, which starts early and stays
39 <p>You may not be able to set this as an unprivileged user. You can use
40 <code>adb root</code> or run the <code>su</code> command from the device
41 shell on "userdebug" builds to become root first. When in doubt,
42 <pre>adb shell getprop <name></pre>
43 will tell you if the <code>setprop</code> took.
45 <p>If you don't want the property to evaporate when the device reboots,
46 add a line to <code>/data/local.prop</code> that looks like:
47 <pre><name> = <value></pre>
49 <p>Such changes will survive reboots, but will be lost if the data
50 partition is wiped. (Hint: create a <code>local.prop</code>
51 on your workstation, then <code>adb push local.prop /data</code> . Or,
53 <code>adb shell "echo name = value >> /data/local.prop"</code> -- note
54 the quotes are important.)
57 <h2><a name="checkjni">Extended JNI Checks</a></h2>
59 <p>JNI, the Java Native Interface, provides a way for code written in the
60 Java programming language
61 interact with native (C/C++) code. The extended JNI checks will cause
62 the system to run more slowly, but they can spot a variety of nasty bugs
63 before they have a chance to cause problems.
65 <p>There are two system properties that affect this feature, which is
66 enabled with the <code>-Xcheck:jni</code> command-line argument. The
67 first is <code>ro.kernel.android.checkjni</code>. This is set by the
68 Android build system for development builds. (It may also be set by
69 the Android emulator unless the <code>-nojni</code> flag is provided on the
70 emulator command line.) Because this is an "ro." property, the value cannot
71 be changed once the device has started.
73 <p>To allow toggling of the CheckJNI flag, a second
74 property, <code>dalvik.vm.checkjni</code>, is also checked. The value
75 of this overrides the value from <code>ro.kernel.android.checkjni</code>.
77 <p>If neither property is defined, or <code>dalvik.vm.checkjni</code>
78 is set to <code>false</code>, the <code>-Xcheck:jni</code> flag is
79 not passed in, and JNI checks will be disabled.
81 <p>To enable JNI checking:
82 <pre>adb shell setprop dalvik.vm.checkjni true</pre>
84 <p>You can also pass JNI-checking options into the VM through a system
85 property. The value set for <code>dalvik.vm.jniopts</code> will
86 be passed in as the <code>-Xjniopts</code> argument. For example:
87 <pre>adb shell setprop dalvik.vm.jniopts forcecopy</pre>
89 <p>For more information about JNI checks, see
90 <a href="jni-tips.html">JNI Tips</a>.
93 <h2><a name="assertions">Assertions</a></h2>
95 <p>Dalvik VM supports the Java programming language "assert" statement.
96 By default they are off, but the <code>dalvik.vm.enableassertions</code>
97 property provides a way to set the value for a <code>-ea</code> argument.
99 <p>The argument behaves the same as it does in other desktop VMs. You
100 can provide a class name, a package name (followed by "..."), or the
103 <p>For example, this:
104 <pre>adb shell setprop dalvik.vm.enableassertions all</pre>
105 enables assertions in all non-system classes.
107 <p>The system property is much more limited than the full command line.
108 It is not possible to specify more than one <code>-ea</code> entry, and there
109 is no way to specify a <code>-da</code> entry. There is presently no
110 equivalent for <code>-esa</code>/<code>-dsa</code>.
113 <h2><a name="verifier">Bytecode Verification and Optimization</a></h2>
115 <p>The system tries to pre-verify all classes in a DEX file to reduce
116 class load overhead, and performs a series of optimizations to improve
117 runtime performance. Both of these are done by the <code>dexopt</code>
118 command, either in the build system or by the installer. On a development
119 device, <code>dexopt</code> may be run the first time a DEX file is used
120 and whenever it or one of its dependencies is updated ("just-in-time"
121 optimization and verification).
123 <p>There are two command-line flags that control the just-in-time
124 verification and optimization,
125 <code>-Xverify</code> and <code>-Xdexopt</code>. The Android framework
126 configures these based on the <code>dalvik.vm.dexopt-flags</code>
130 <pre>adb shell setprop dalvik.vm.dexopt-flags v=a,o=v</pre>
131 then the framework will pass <code>-Xverify:all -Xdexopt:verified</code>
132 to the VM. This enables verification, and only optimizes classes that
133 successfully verified. This is the safest setting, and is the default.
134 <p>You could also set <code>dalvik.vm.dexopt-flags</code> to <code>v=n</code>
135 to have the framework pass <code>-Xverify:none -Xdexopt:verified</code>
136 to disable verification. (We could pass in <code>-Xdexopt:all</code> to
137 allow optimization, but that wouldn't necessarily optimize more of the
138 code, since classes that fail verification may well be skipped by the
139 optimizer for the same reasons.) Classes will not be verified by
140 <code>dexopt</code>, and unverified code will be loaded and executed.
142 <p>Enabling verification will make the <code>dexopt</code> command
143 take significantly longer, because the verification process is fairly slow.
144 Once the verified and optimized DEX files have been prepared, verification
145 incurs no additional overhead except when loading classes that failed
148 <p>If your DEX files are processed with verification disabled, and you
149 later turn the verifier on, application loading will be noticeably
150 slower (perhaps 40% or more) as classes are verified on first use.
152 <p>For best results you should force a re-dexopt of all DEX files when
153 this property changes. You can do this with:
154 <pre>adb shell "rm /data/dalvik-cache/*"</pre>
155 This removes the cached versions of the DEX files. Remember to
156 stop and restart the runtime (<code>adb shell stop; adb shell start</code>).
158 <p>(Previous version of the runtime supported the boolean
159 <code>dalvik.vm.verify-bytecode</code> property, but that has been
160 superceded by <code>dalvik.vm.dexopt-flags</code>.)</p>
163 <h2><a name="execmode">Execution Mode</a></h2>
165 <p>The current implementation of the Dalvik VM includes three distinct
166 interpreter cores. These are referred to as "fast", "portable", and
167 "debug". The "fast" interpreter is optimized for the current
168 platform, and might consist of hand-optimized assembly routines. In
169 constrast, the "portable" interpreter is written in C and expected to
170 run on a broad range of platforms. The "debug" interpreter is a variant
171 of "portable" that includes support for profiling and single-stepping.
173 <p>The VM may also support just-in-time compilation. While not strictly
174 a different interpreter, the JIT compiler may be enabled or disabled
175 with the same flag. (Check the output of <code>dalvikvm -help</code> to
176 see if JIT compilation is enabled in your VM.)
178 <p>The VM allows you to choose between "fast", "portable", and "jit" with an
179 extended form of the <code>-Xint</code> argument. The value of this
180 argument can be set through the <code>dalvik.vm.execution-mode</code>
183 <p>To select the "portable" interpreter, you would use:
184 <pre>adb shell setprop dalvik.vm.execution-mode int:portable</pre>
185 If the property is not specified, the most appropriate interpreter
186 will be selected automatically. At some point this mechanism may allow
187 selection of other modes, such as JIT compilation.
189 <p>Not all platforms have an optimized implementation. In such cases,
190 the "fast" interpreter is generated as a series of C stubs, and the
191 result will be slower than the
192 "portable" version. (When we have optimized versions for all popular
193 architectures the naming convention will be more accurate.)
195 <p>If profiling is enabled or a debugger is attached, the VM
196 switches to the "debug" interpreter. When profiling ends or the debugger
197 disconnects, the original interpreter is resumed. (The "debug" interpreter
198 is substantially slower, something to keep in mind when evaluating
202 <h2><a name="dp">Deadlock Prediction</a></h2>
204 <p>If the VM is built with <code>WITH_DEADLOCK_PREDICTION</code>, the deadlock
205 predictor can be enabled with the <code>-Xdeadlockpredict</code> argument.
206 (The output from <code>dalvikvm -help</code> will tell you if the VM was
207 built appropriately -- look for <code>deadlock_prediction</code> on the
208 <code>Configured with:</code> line.)
209 This feature tells the VM to keep track of the order in which object
210 monitor locks are acquired. If the program attempts to acquire a set
211 of locks in a different order from what was seen earlier, the VM logs
212 a warning and optionally throws an exception.
214 <p>The command-line argument is set based on the
215 <code>dalvik.vm.deadlock-predict</code> property. Valid values are
216 <code>off</code> to disable it (default), <code>warn</code> to log the
217 problem but continue executing, <code>err</code> to cause a
218 <code>dalvik.system.PotentialDeadlockError</code> to be thrown from the
219 <code>monitor-enter</code> instruction, and <code>abort</code> to have
222 <p>You will usually want to use:
223 <pre>adb shell setprop dalvik.vm.deadlock-predict err</pre>
224 unless you are keeping an eye on the logs as they scroll by.
226 <p>Please note that this feature is deadlock prediction, not deadlock
227 detection -- in the current implementation, the computations are performed
228 after the lock is acquired (this simplifies the code, reducing the
229 overhead added to every mutex operation). You can spot a deadlock in a
230 hung process by sending a <code>kill -3</code> and examining the stack
231 trace written to the log.
233 <p>This only takes monitors into account. Native mutexes and other resources
234 can also be the cause of deadlocks, but will not be detected by this.
237 <h2><a name="stackdump">Stack Dumps</a></h2>
239 <p>Like other desktop VMs, when the Dalvik VM receives a SIGQUIT
240 (Ctrl-\ or <code>kill -3</code>), it dumps stack traces for all threads.
241 By default this goes to the Android log, but it can also be written to a file.
243 <p>The <code>dalvik.vm.stack-trace-file</code> property allows you to
244 specify the name of the file where the thread stack traces will be written.
245 The file will be created (world writable) if it doesn't exist, and the
246 new information will be appended to the end of the file. The filename
247 is passed into the VM via the <code>-Xstacktracefile</code> argument.
250 <pre>adb shell setprop dalvik.vm.stack-trace-file /tmp/stack-traces.txt</pre>
252 <p>If the property is not defined, the VM will write the stack traces to
253 the Android log when the signal arrives.
256 <h2><a name="dexcheck">DEX File Checksums</a></h2>
258 <p>For performance reasons, the checksum on "optimized" DEX files is
259 ignored. This is usually safe, because the files are generated on the
260 device, and have access permissions that prevent modification.
262 <p>If the storage on a device becomes unreliable, however, data corruption
263 can occur. This usually manifests itself as a repeatable virtual machine
264 crash. To speed diagnosis of such failures, the VM provides the
265 <code>-Xcheckdexsum</code> argument. When set, the checksums on all DEX
266 files are verified before the contents are used.
268 <p>The application framework will provide this argument during VM
269 creation if the <code>dalvik.vm.check-dex-sum</code> property is enabled.
271 <p>To enable extended DEX checksum verification:
272 <pre>adb shell setprop dalvik.vm.check-dex-sum true</pre>
274 <p>Incorrect checksums will prevent the DEX data from being used, and will
275 cause errors to be written to the log file. If a device has a history of
276 problems it may be useful to add the property to
277 <code>/data/local.prop</code>.
279 <p>Note also that the
280 <code>dexdump</code> tool always verifies DEX checksums, and can be used
281 to check for corruption in a large set of files.
284 <address>Copyright © 2008 The Android Open Source Project</address>