PROBLEM:
We have N physical machines(PMs) each with ram R_i, cpu C_i and a set of currently scheduled VMs each with ram requirement r_i and c_i respectively
Moving(Migrating) any VM from one PM to other has a cost associated which depends on its ram r_i. A PM with no VMs is shut down to save power.
Our target is to minimize the weighted sum of (N,migration cost) by migrating some VMs i.e. minimize the number of working PMs as well as not to degrade the service level due to excessive migrations.

My Approach:
Brute Force approach is choosing the minimum loaded PM and try to fit its VMs to other PMs by First Fit Decreasing algorithm or we can select the victim PMs and target PMs based on their loading level and shut down victims if possible by moving their VMs to targets.
I tried this Greedy approach on the Data of Baadal(IIT-D cloud) but It isn't giving promising results.

I have also tried to study the Ant colony optimization for dynamic VM consolidating but was unable to understand very much. I used the links.
http://dumas.ccsd.cnrs.fr/docs/00/72/52/15/PDF/Esnault.pdf http://hal.archives-ouvertes.fr/docs/00/72/38/56/PDF/RR-8032.pdf

Would anyone please clarify the solution or suggest any new approach/resources for better performance.
I am basically searching for the algorithms not the physical optimizations and I also know that many commercial organizations have provided these solution but I just wanted to know more the underlying algorithms.

Thanks in advance.