# Dalvik Heap Profiling

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.

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.

## Getting the data

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.

There is a `runhat` shell function, added by `build/envsetup.sh`, 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.

### Early releases (1.0/1.1)

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.

Get a command shell on the device:

>     $ adb shell

You can verify that you're running as root with the `id` command. The response should look like `uid=0(root) gid=0(root)`. If not, type `su` and try again. If `su` fails, you're out of luck.

Next, ensure the target directory exists:

>     # mkdir /data/misc
>     # chmod 777 /data/misc

Use `ps` or DDMS to determine the process ID of your application, then send a `SIGUSR1` to the target process:

>     # kill -10 <pid>

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.

Next:

>     # ls /data/misc/heap-dump*
>     # exit

Use `ls` to check the file names, then `exit` to quit the device command shell.

You should see two output files, named `/data/misc/heap-dump-BLAH-BLAH.hprof` and `.hprof-head`, 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:

>     $ 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

Merge them together and remove the intermediates:

>     $ cat head.hprof tail.hprof > dump.hprof
>     $ rm head.hprof tail.hprof

You now have the hprof dump in `dump.hprof`.

### Android 1.5 ("Cupcake")

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 `adb root` to restart the adb daemon as root.)

The basic procedure is the same as for 1.0/1.1, but only one file will appear in `/data/misc` (no `-head`), and upon completion you will see a log message that says "hprof: heap dump completed". It looks like this in the log:

>     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

Summary: as above, use `mkdir` and `chmod` to ensure the directory exists and is writable by your application. Send the `SIGUSR1` or use the API call to initiate a dump. Use `adb pull <dump-file>` and `adb shell rm <dump-file>` to retrieve the file and remove it from the device. The concatenation step is not needed.

The new API is in the `android.os.Debug` class:

>     public static void dumpHprofData(String fileName) throws IOException

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. `/sdcard/myapp.hprof`. 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.

### Android 1.6 ("Donut")

No real change to the way profiling works. However, 1.6 introduced the `WRITE_EXTERNAL_STORAGE` permission, which is required to write data to the SD card. If you're accustomed to writing profile data to `/sdcard`, you will need to enable the permission in your application's manifest.

### Android 2.0 ("Eclair")

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.

### Android 2.2 ("Froyo")

DDMS heap dump requests are now streamed directly out of the VM, removing the external storage requirement.

## Examining the data

The data file format was augmented slightly from the common hprof format, and due to licensing restrictions the modified `hat` tool cannot be distributed. A conversion tool, `hprof-conv`, 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.

The converted output should work with any hprof data analyzer, including `jhat`, which is available for free in the Sun JDK, and Eclipse MAT.