camera app is polling the file system every second when in foreground

[John McAleely]

from OEM bug #1471310

running fnotifystat, I can see that every second the camera-app is polling away as follows:

Total Open Close Read Write PID Process Pathname
  8.0 1.0 1.0 6.0 0.0 3175 camera-app /proc/3175/mounts
  2.0 1.0 1.0 0.0 0.0 3175 camera-app /dev/disk/by-label

Total Open Close Read Write PID Process Pathname
  8.0 1.0 1.0 6.0 0.0 3175 camera-app /proc/3175/mounts
  2.0 1.0 1.0 0.0 0.0 3175 camera-app /dev/disk/by-label

Total Open Close Read Write PID Process Pathname
  8.0 1.0 1.0 6.0 0.0 3175 camera-app /proc/3175/mounts
  2.0 1.0 1.0 0.0 0.0 3175 camera-app /dev/disk/by-label

so it is consistently polling away. That's kinda wasteful on resources (e.g. battery drain).

[John McAleely (john.mcaleely)]

Duplicate bug notes this as happening on recent vivid / mako

[Sturm Flut (sturmflut)]

As far as I can see this is intentional, the app continuously checks the remaining space and disables itself when disk space turns critically low. Also the polling only happens when the app is being actively used.

[Colin Ian King (colin-king)]

I'm not sure what mechanism is being used, but it certainly does not look like it is using a more efficient mechanism such as statfs(2) or fstatfs(2) both of which return struct statfs containing f_blocks and f_bfree that allow one to determine how much of a percentage of free space is available.

These system calls interrogate the kernel's current view of file system state rather than having to poll and read /proc entries. It is more efficient and better still returns correct data.

[Florian Boucault (fboucault)]

The app uses the Qt API QStorageInfo::bytesAvailable() method [1] every 1 second.
For lack of a better API, one that would not require polling, we decided to go ahead with this one.
As Simon said, the polling only happens when the app is being actively used which means that even though it is indeed wasteful the amount of CPU used is not much in comparison to what the viewfinder uses in CPU and GPU when the camera app is up.

[1] https://doc-snapshots.qt.io/qt5-5.4/qstorageinfo.html

[Colin Ian King (colin-king)]

OK, I guess that is the cost of a high level API that doesn't use statfs().