kvmclock parameter in agent.properties

[mredaelli02]

Hi,
Is there any way to hide CPU steal time to VMs?
Looking around I found that set kvmclock.disable to true could hide the steal time to VMs. Does it works? Are there any known issues?

[herrickaccuwebhosting]

Hey! kvmclock.disable=true in agent.properties does work for what you're describing, it stops CloudStack from injecting the KVM paravirtual clock device into the guest XML, which is one of the things that exposes steal time visibility to the VM. Worth knowing though that it's more of a side effect than a purpose built "hide steal time" switch.
A few things worth checking before you flip it:

It only affects newly started/created VMs, existing running VMs need a stop/start (not just reboot) to pick up the XML change.
Without kvmclock, the guest falls back to its default clocksource, which on Linux guests is usually fine (TSC or hpet), but it's worth testing on Windows guests since clock drift issues are more common there without the paravirt clock.
Steal time itself doesn't go away, the hypervisor still schedules CPU the same way, you're just removing the guest's visibility into it. If you're trying to hide steal time to satisfy an SLA or monitoring dashboard rather than actually fixing contention, that's worth being honest with yourself about.
If actual steal time is high, the real fix is CPU overcommit ratio on the host, not hiding the metric.

We've tested kvmclock.disable in a few KVM/CloudStack environments here at AccuWeb.Cloud, and it does what it says without breaking anything else, just make sure it's solving the right problem and not just the visibility of one.
Claude

[mredaelli02]

Thanks you so much for the clarification