kwin (4.11.12) idle CPU usage

[Gerald Hofer]

On 27/01/16 03:40, René J.V. Bertin wrote:
> On Tuesday January 26 2016 17:20:38 Duncan wrote:
>
>> more flashing you see and the bigger area that's flashing, the more
>> repainting is being done and the higher both kwin_x11 and X's usage
>> should be.
> Maybe, but *why*? Kwin isn't supposed to do anything with window contents in my configuration, when I'm not interacting with the system.
>
>> Given that he said quitting Chrome dramatically reduced kwin's CPU usage,
>> I'd guess that with this effect enabled, Chrome will be flashing like
>> crazy.
> It isn't. The whole window rect is indeed flashing whenever a tab is active, but nothing happens when that is not the case. According to chrome's own task manager only the central browser process is active when the browser really ought to be idle (around 10% of whatever) plus around 2% for the GPU task. I can kill the GPU task (only effect is it'll restart and stay at 0%) but not the central process.
> I recall having seen what appeared to be tiny offscreen windows looking at an xwininfo dump recently, maybe those are the culprit. I'd hope kwin would be clever enough not to attempt to composite anything with invisible windows though.
>
>
I think I have seen what you are describing on my system. When I turned 
on paint, the whole chrome tab was refreshing constantly (indicated by 
the cycling colours across the whole of chrome, including the menu), 
with significant  CPU time for kwin_x11. The refreshing happened 
independent of it being in focus or not. Then I started 
google-chrome-stable to see if the stable version has the same problem 
and without changing anything in the running google-chrome-unstable, I 
don't see that problem any more. I did open some menus in 
google-chrome-unstable and that seem to have "fixed" the issue temporarily.

So it almost looks like that chrome can get into a state where it 
constantly refreshes the whole window and then is causing load through 
the kwin_x11. I can confirm that chrome itself was fairly idle. At least 
in my case it was temporary. But I certainly saw a similar effect. I 
will keep an eye out how often that happens...

Gerald

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