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<title>Dalvik Heap Profiling</title>
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<h1>Dalvik Heap Profiling</h1>
<p>
The Dalvik virtual machine can produce a complete dump of the contents
of the virtual heap. This is very useful for debugging memory usage
and looking for memory leaks. Getting at the information can be tricky,
but has become easier in recent releases.
</p><p>
In what follows, the version number refers to the software release
running on the phone. To take advantage of the DDMS integration, you will
also need a sufficiently recent version of DDMS.
<h2>Getting the data</h2>
<p>
The first step is to cause the VM to dump its status, and then pull the hprof
data off. The exact manner for doing so has changed over time.
</p><p>
There is a <code>runhat</code> shell function, added by
<code>build/envsetup.sh</code>, that partially automates these steps. The
function changes in each release to accommodate newer behavior, so you have
to be careful that you don't use the wrong version.
</p><p>
<h3>Early releases (1.0/1.1)</h3>
<p>
You can only generate heap data on the emulator or a device with root
access, because of the way the dump is initiated and where the output
files go.
</p><p>
Get a command shell on the device:
<blockquote><pre>
$ adb shell
</pre></blockquote>
</p><p>
You can verify that you're running as root with the <code>id</code> command.
The response should look like <code>uid=0(root) gid=0(root)</code>. If not,
type <code>su</code> and try again. If <code>su</code> fails, you're out
of luck.
</p><p>
Next, ensure the target directory exists:
<blockquote><pre>
# mkdir /data/misc
# chmod 777 /data/misc
</pre></blockquote>
</p><p>
Use <code>ps</code> or DDMS to determine the process ID of your application,
then send a <code>SIGUSR1</code> to the target process:
<blockquote><pre>
# kill -10 <pid>
</pre></blockquote>
</p><p>
The signal causes a GC, followed by the heap dump (to be completely
accurate, they actually happen concurrently, but the results in the heap
dump reflect the post-GC state). This can take a couple of seconds,
so you have to watch for the GC log message to know when it's complete.
</p><p>
Next:
<blockquote><pre>
# ls /data/misc/heap-dump*
# exit
</pre></blockquote>
</p><p>
Use <code>ls</code> to check the file names, then <code>exit</code> to quit
the device command shell.
</p><p>
You should see two output files, named
<code>/data/misc/heap-dump-BLAH-BLAH.hprof</code> and
<code>.hprof-head</code>, where BLAH is a runtime-generated value
that ensures the filename is unique. Pull them off of the device and
remove the device-side copy:
<blockquote><pre>
$ adb pull /data/misc/heap-dump-BLAH-BLAH.hprof tail.hprof
$ adb pull /data/misc/heap-dump-BLAH-BLAH.hprof-head head.hprof
$ adb shell rm /data/misc/heap-dump-BLAH-BLAH.hprof /data/misc/heap-dump-BLAH-BLAH.hprof-head
</pre></blockquote>
</p><p>
Merge them together and remove the intermediates:
<blockquote><pre>
$ cat head.hprof tail.hprof > dump.hprof
$ rm head.hprof tail.hprof
</pre></blockquote>
</p><p>
You now have the hprof dump in <code>dump.hprof</code>.
</p><p>
<h3>Android 1.5 ("Cupcake")</h3>
<p>
Some steps were taken to make this simpler. Notably, the two output
files are now combined for you, and a new API call was added that allows
a program to write the dump at will to a specific file. If you're not
using the API call, you still need to be on an emulator or running as root.
(For some builds, you can use <code>adb root</code> to restart the adb
daemon as root.)
</p><p>
The basic procedure is the same as for 1.0/1.1, but only one file will
appear in <code>/data/misc</code> (no <code>-head</code>), and upon
completion you will see a log message that says "hprof: heap dump completed".
It looks like this in the log:
<blockquote><pre>
I/dalvikvm( 289): threadid=7: reacting to signal 10
I/dalvikvm( 289): SIGUSR1 forcing GC and HPROF dump
I/dalvikvm( 289): hprof: dumping VM heap to "/data/misc/heap-dump-tm1240861355-pid289.hprof-hptemp".
I/dalvikvm( 289): hprof: dumping heap strings to "/data/misc/heap-dump-tm1240861355-pid289.hprof".
I/dalvikvm( 289): hprof: heap dump completed, temp file removed
</pre></blockquote>
</p><p>
Summary: as above, use <code>mkdir</code> and <code>chmod</code>
to ensure the directory exists and is writable by your application.
Send the <code>SIGUSR1</code> or use the API call to initiate a dump.
Use <code>adb pull <dump-file></code> and <code>adb shell rm
<dump-file></code> to retrieve the file and remove it from the
device. The concatenation step is not needed.
</p><p>
The new API is in the <code>android.os.Debug</code> class:
<blockquote><pre>
public static void dumpHprofData(String fileName) throws IOException
</pre></blockquote>
When called, the VM will go through the same series of steps (GC and
generate a .hprof file), but the output will be written to a file of
your choice, e.g. <code>/sdcard/myapp.hprof</code>. Because you're
initiating the action from within the app, and can write the file to
removable storage or the app's private data area, you can do this on a
device without root access.
<h3>Android 1.6 ("Donut")</h3>
<p>
No real change to the way profiling works.
However, 1.6 introduced the <code>WRITE_EXTERNAL_STORAGE</code>
permission, which is required to write data to the SD card. If you're
accustomed to writing profile data to <code>/sdcard</code>, you will
need to enable the permission in your application's manifest.
</p>
<h3>Android 2.0 ("Eclair")</h3>
<p>
In 2.0, features were added that allow DDMS to request a heap dump on
demand, and automatically pull the result across. Select your application
and click the "dump HPROF file" button in the top left. This always
writes files to the SD card, so
you must have a card inserted and the permission enabled in your application.
</p>
<h3>Android 2.2 ("Froyo")</h3>
<p>
DDMS heap dump requests are now streamed directly out of the VM, removing
the external storage requirement.
</p>
<h2>Examining the data</h2>
<p>
The data file format was augmented slightly from the common hprof format,
and due to licensing restrictions the modified <code>hat</code> tool cannot
be distributed. A conversion tool, <code>hprof-conv</code>, can be used
to strip the Android-specific portions from the output. This tool was
first included in 1.5, but will work with older versions of Android.
</p><p>
The converted output should work with any hprof data analyzer, including
<code>jhat</code>, which is available for free in the Sun JDK, and
Eclipse MAT.
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<address>Copyright © 2009 The Android Open Source Project</address>
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