Taking a snapshot of Android device

We can dump the framebuffer of a android device and save it as a file on our host in one single step:

  $ adb pull /dev/graphics/fb0 fb.dump

The dumped raw data is in RGB565 pixel format. So we'd like to convert it into a more common image format that could be recognized by common image viewers.
And FFMPEG could be handy for doing the conversion like this:

  $ ffmpeg -f rawvideo -pix_fmt rgb565 -s 320x480 -i fb.dump output%d.png

In this case, more than one images might be generated and named output1.png, output2.png, etc. This happens because the framebuffer on android is required to support page flipping, and the backed buffer can be dumped from the /dev/graphics/fb0 as well.

Alternatively, we can use the fbgrab utility to do the conversion:

  $ fbgrab -f fb.dump -w 320 -h 960 -b 16 output.png

Here we explicitly specify the width, height(x2 for the backed buffer), and color depth. In this case, we get a single image which is composed by two cascaded snapshots.

Of course we can also cross-compile the fbgrab utility and have it take the snapshot directly on the target device, and then pull it back with adb.

  $ fbgrab -d /dev/graphics/fb0 output.png

However, the only benefit with this, I think, is saving some time to type the dimension of the framebuffer, since the fbgrab would figure it out with IOCTL on the device node.

Estimating memory usage on Android and Linux

Earlier kernels export Rss and Vss of each single process to show how many page frames it occupies and how the address space it spans on a particular time. But this information is process-wide, and it doesn't help when you estimate system-wide memory usage. The problem is the lacking of information about how the processes share pages with each other. Simply summing up the Rss of all processes would over calculate the number of pages shared by multiple processes.