Article ID: 120554, created on Mar 14, 2014, last review on May 10, 2014

  • Applies to:
  • Virtuozzo 6.0

Symptoms

A virtual machine with the CentOS 6.0 guest OS consumes much CPU resources when idle.

Cause

The Parallels Cloud Server 6.0 hypervisor does not expose Intel PMU in the guest OS, so the kernel cannot set up an NMI watchdog using hardware events.
 
Recent Linux distributions, like CentOS 6.1 and newer, detect this and disable the NMI watchdog unless explicitly instructed otherwise. Log messages indicating this are:
...
Performance Events: no PMU driver, software events only.
NMI watchdog disabled (cpu0): hardware events not enabled
...
The CentOS 6.0 kernel does not have the logic described above. If such a kernel fails to set up an NMI watchdog based on hardware events, it configures one using software events, forcing it out of the tickless mode. As a result, the kernel gets spammed by useless interrupts, which translates to increased CPU resource consumption. Log messages indicating this are:
...
Performance Events: no PMU driver, software events only.
...
Cannot setup NMI watchdog on CPU 0
...
APIC timer registered as dummy, due to nmi_watchdog=1!
...
Testing NMI watchdog ... OK.
...

Resolution

Update the guest OS to CentOS 6.1 or newer, or append nmi_watchdog=0 to the guest kernel command line.

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