[libvirt-users] Quadratic function-like call delay

[Daniel P. Berrange]

On Mon, Sep 24, 2012 at 11:48:44AM +0400, Andrey Korolyov wrote:
> Hi,
> 
> There is a quite annoying thing, don`t sure if I can call it a bug.
> 
> How to reproduce:
> 
> - start a bunch of VMs,
> - stop libvirt,
> - start libvirt and immediately issue any call, say, 'virsh version',
> - delay until call completion may be fitted nearly as quadratic
> function from number of running VMs, qemu-kvm in my case (see link
> below). After this delay passed, any calls executed without slowdown,
> so problem is only a 'dead gap' after restarting service.

When you restart the libvirtd service, while KVM guests are running,
the first thing libvirtd needs todo is to connect to the QEMU monitor
for each guest and figure out its status. This can take some time
depending on how loaded the host is in general. It will definitely
slow libvirt API calls which need to query individual VMs, but I'm
rather surprised that you saw any slowdown of the 'virsh version'
command, since that should not touch VMs at all. So I think I *would*
class this as a bug. It could be something silly like the main I/O
event loop having some badly written bit of non-scalable code. Please
do file a bug about this, providing as much raw data as you have
managed to collect.


Daniel
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