Open Stack

Good day.

I can't find any options for nova scheduler to consolidate new instances 
on few hosts instead of 'spreading' them on all available hosts.

Simple example: let says we has 10 hosts, each host got 10Gb of memory 
for instances. We has flavors of 3Gb and 5Gb of RAM. If we run 20 new 
instances, they will consume about 6Gb per host and we will not able to 
run new instance with 6Gb of RAM (even we have 10*4=40 Gb of free memory 
on computes, none of the hosts has more than 4Gb of memory).

Is any nice way to say 'consolidate' to openstack? Thanks!