clock-app is rather a busy app (analogue clock display)

hmm this happens because in the analog clock mode, we need to update the UI every 10 ms to show a super smooth seconds hand movement. In the old clock app (purple background) we circumvented this issue by disabling the seconds hand by default. I am a bit unsure how to proceed. But this is an issue which needs to be tackled.

Idle phone, display on, draws ~201 mA, running the clock app (analogue display) draws ~426mA.

And here is the digital clock analysis (see attached)

I can't measure the current being drawn, it's too small a difference really.

Changing the property to:
        rotation: visible ? 360 * analogTime.getSeconds() / 60 : 0

reduces the cpu at least on my laptop well... from a 3/4 to a 0/1

Only thing is, that it is not a smooth movement.

Added two branches; Both use less CPU

one branch tries to go for simplicity, just updating the position every second, but without any smooth animation that cpus love to eat
The other branch goes with a small linear animation every second, that is slightly more appealing to the eye. one caveat to this however is once it reaches 359, it loops back. This may be actually a neat feature for someone to have, because it lets the user know when a minute has keeled over.

"we need to update the UI every 10 ms"

I would image that the screen runs at 60 Hz, so if you use vsync, it will run every 16.7 ms and look 100% identical. That should drop the CPU usage by about 40%, which is a start.

I'm not sure why you would need such a large amount of CPU usage just for an animation though. Is there processing in there which could be split out and updated only once every few seconds, separate from the animation? I would assume that an animation would not need any of the calls mentioned in the description.

@Sam Bull

There is no qml animation in the current branch. What it does rather is just update the rotation property every millisecond, so it appears to be a smooth linear animation.

 lp:~akiva/ubuntu-clock-app/1363968-linear-animation-every-second
However has an animation if you would like to see the difference.

I think the hertz idea is very interesting. I would not know how to pipe that signal though.

I would expect some kind of support in the SDK for vsync, if not you should probably register a bug. It's a moderately important feature for 3D games and other visual applications, in order to stop screen tearing.

Every millisecond is definitely overkill. That means you're updating the animation about 16 times between each screen update.

@Sam Bull

Every millisecond is overkill, thus the bug. My two branches that I linked significantly reduce this though.

Fix committed into lp:ubuntu-clock-app at revision None, scheduled for release in ubuntu-clock-app, milestone rtm