Performance issue with virtualized DB

Hello PostgreSQL users,

I'm running PostgreSQL 9 in a VM, sing KVM as hypervisor. The whole
database is loaded in a RAM file system.
As other hypervisors, there are some parameters to limit CPU that are
very important to me. I usually need to change the values of these
parameters.

However I'm having a bit of unexpected performance problems. Shouldn't
the query execution times vary linearly with the allocated CPU?
I ran some tests, and I found out that the costs vary "kind of"
linearly until some point. However if I give my VM less than about 20%
of the available CPU, the costs "explode".
The problem is that I need to run multiple VMs on the same host, and I
can't afford that much deterioration. I never heard about a "safe
limit" for CPU allocation. I always thought that CPU would affect the
query linearly.

Regards,

Antonio Carlos Furtado