All of my computers run kde 4.x except for one which runs 5.x -- that is my "test" platform to see if it is ready to replace 4.x. A recent KDE Planet post indicated KDE 5.x was now ready for production usage at 5.5. I can't report the same for one single reason -- after a period of time at idle, the plasma desktop ramps up CPU usage for no apparent reason, from nominal to "quite a lot". Then the computer starts heating up. The only solution I have found for this is to log out and log back in.

I don't know how to properly bring this to plasma programmers' attention and I have kept hoping that it would be cured in successive releases.

*Why I don't file a bug report: My installs are not "standard" and so it's difficult to determine where the cause of this lies. I can't specifically blame Plasma5, though I believe it is the problem. My current install is openSUSE 13.2, but I use a number of additional repositories. So, my KDE version is 5.5.3 and my kernel version is 4.4.0-2. Also, I put "panels" on all 4 screen edges, run dual-monitor and use several widgets -- digital time, temp monitor, cpu monitor, network monitor. (I haven't found the widgets to make a difference -- and it's also the basis on which I can identify temp & cpu usage, other than from ramp up of the fan.

*Why I care. I run several computers on KDE/Plasma and it is the "best" I've used, at least for my purposes. I have volunteered at the KDE table at SCALE in Los Angeles several years -- and I think again this year. But, it's difficult to be a proponent when there is an obvious bug in the current desktop. (I skipped the last couple of years partly because I was busy and partly because I wasn't satisfied with 5.x.)

Since my computers are "always on", idle cpu usage is important. And, if plasma is for some reason using 50%+ of the cpu power, that takes way from available cpu for other uses.

My alternative on this same computer is to run xfce desktop -- I don't have the same cpu ramp up problem, but there are set up issues since xfce does not remember the dual monitor settings and does not have the same useful widgets. (For xfce, I use lxrandr to set up the monitors and Conky to provide monitoring and a clock ...)

how long in idle till the ramp up?

assume there is no issue using KDE4 ?

I assume you're using Wolfi's repo, I had found that running multiple sessons (4 and 5) concurrently caused cpu issues but can't remember if it was x11 or plasma.

can you try to isolate the cause:
- revert back to the std openSUSE kernel which is 3.16.x
- back up your plasma desktop and use a single panel
- test a single monitor
- instead of plasmoids try gKrellm or Conky for temp and cpu usage, plasmoids have in the past behaved badly, see https://forums.opensuse.org/showthread. ... ost2714107
- dual boot to Leap which is meant to run Plasma 5 and see it that fixes the issue and if so consider upgrading

band aid - instead of logging out restart plasma desktop with

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kquitapp plasma-desktop && plasma-desktop

To me this sounds like some background process going crazy.

What process exactly is hogging the CPU? Use "top" or KDE's system monitor (Ctrl+ESC) to check.
Is it plasmashell? That would be the "desktop", but you don't really state that explicitly.

If it's kded5, try to disable all background services (in Configure Desktop/systemsettings5->Startup and Shutdown->Background Services) and enable them one by one to find the culprit.

Also I'd suggest to try a fresh user account as a test.

@google01103: "plasma-desktop" it would be in KDE4, Plasma5's process is called "plasmashell"...
So the command to restart it would rather be:

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kquitapp plasmashell && plasmashell