[libvirt] How can I improve the performance of libvirt when running many virtual machines on same host

[Daniel Veillard]

On Wed, Jan 19, 2011 at 04:45:04PM -0200, Diego Dias wrote:
> Hi,
> 
>    I am using Xen 4 as hypervisor and using libvirt 0.8.4 to
> communicate with it.
>    When I run more than 35 virtual machines on the host, Libvirt
> starts to run extremely slow and I cant even execute a /"virsh
> list/". The memory usage of libvirtd is around 468288K.
>    Is there any way to improve the performance of libvirt? Is there
> any maximum number of virtual machines libvirt can manage without
> having a poor performace?

  Well I have run up to 400 guests, but running KVM on a rather large
machine, and the responsiveness of libvirt wasn't a problem (well we had
a trouble with cgroups, but once that one solved that was okay).
In a xen environemnt libvirt doesn't talk directly to the hypervisor
(well except for only a few simple commands where we can use the
hypervisor traps directly) and mostly talks to the xen daemon. Trying to
find out where the slowdown comes from is the only way to improve performances
reliably, and well it's hard to tell from your mail. I would assume 400MB
libvirtd instance is actually realatively small compared to the volume of
memeory used by the guests, just make sure that there is enough memory
in Dom0 and that libvirtd/xend/etc... aren't swapping, that would make the
whole thing slow down considerably.

Daniel

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