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planetwilson

High CPU when idling

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I have recently downsized my unRAID server to an i5 8400 6 core CPU. However I noticed that when running a Windows 10 VM, the CPU usage seems quite high. If I use "top" in an unRAID ssh session it shows qemu-system-x86 as the highest consumption. On the dashboard I can see one of the cores running at 45% and the others moving around 5-15% up and down. The actual VM is doing nothing and task manager in Windows 10 shows 99% idle. My previous 14 core Xeon never used to do this at all, if the WIndows VM was idle then so was the server largely.

 

Any ideas?

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Posted (edited)

I also get a fair amount of CPU usage when all I have is a single VM suspended? If I actually stop it properly then the CPU drops off. I thought suspended meant basically hibernate?

 

UPDATE: okay read the redhat docs on this and suspend just suspends in RAM aso I could see how some CPU is used if qemu is still running in the background. I need to set my scripts to do a managedsave instead. I still have the initial issue though. Given that CPU usage is still high when suspended I think I must have a lot of ambient CPU use by qemu itself for some reason.

Edited by planetwilson

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Posted (edited)

Okay did a bit of digging and found this thread:-

 

https://askubuntu.com/questions/1033985/kvm-high-host-cpu-load-after-upgrading-vm-to-windows-10-1803

 

<clock offset='localtime'>
  <timer name='rtc' tickpolicy='catchup'/>
  <timer name='pit' tickpolicy='delay'/>
  <timer name='hpet' present='yes'/>
  <timer name='hypervclock' present='yes'/>
</clock>

All much better now, apparently the hpet line is important

Edited by planetwilson

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One of the reasons for high CPU usage is that Windows seems to throw a lot of interrupts.  I've had mixed success with enabling HPET - sometimes it works, and sometimes it doesn't.

 

What I have found that works all the time is disabling the unused USB Root Hubs.  CPU usage will drop to nearly nothing at idle when I do that.

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