Public bug reported:

HP DL380 G9 Openstack (Kilo 2015.1.4) hypervisor
755GB RAM/2x18 core E5-2699v3 2.30GHz
Ubuntu 14.04.3 with 4.4.0-28-generic kernel
Qemu version 2.0.0+dfsg-2ubuntu1.22
Libvirt 1.2.2-0ubuntu13.1.16

Up to about 100 instances residing on the hypervisor, no issues are
noticed. Once the server obtains about 100 instances, you see ksoftirqd
process jump to the top of processes consuming CPU. Load shoots into the
high 100's and low 200's and instances become inaccessible as the
hypervisor begins dropping packets (due to CPU).

Moved to Libvirt 1.2.2-0ubuntu13.1.17, qemu 2.0.0+dfsg-2ubuntu1.30, and
kernel 3.13.0-101-generic.

This appears to have resolved the issue and the server can once again
handle 150+ instances without skipping a beat.

I did find this error message repeated thousands of times in libvirtd.log up 
until kernel and package upgrade which fixed the issue:
error : qemuMonitorJSONCheckError:354 : internal error: unable to execute QEMU 
command 'qom-get': guest hasn't updated any stats yet

Let me know if I can provide extra detail


** Affects: qemu-kvm (Ubuntu)
     Importance: Undecided
         Status: New

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  KVM CPU slowdown on 4.4 kernel

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