An implementation of Round-Robin VmAllocationPolicy of CloudSim framework

CircularHostList.java

package org.cloudbus.cloudsim.examples;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

import org.cloudbus.cloudsim.Host;

public final class CircularHostList implements Iterable<Host> {

	private final List<Host> host_list = new LinkedList<Host>();

	private int ini;

	public CircularHostList(List<? extends Host> hosts) {
		this.host_list.addAll(hosts);
	}

	public boolean add(Host host) {
		return this.host_list.add(host);
	}

	public boolean remove(Host host2Remove) {
		return this.host_list.remove(host2Remove);
	}

	public Host next() {
		Host host = null;

		if (!host_list.isEmpty()) {
			int index = (this.ini++ % this.host_list.size());
			host = this.host_list.get(index);
		}

		return host;
	}

	@Override
	public Iterator<Host> iterator() {
		return get().iterator();
	}

	public List<Host> get() {
		return Collections.unmodifiableList(this.host_list);
	}

	public Host getWithMinimumNumberOfPesEquals(int numberOfPes) {
		List<Host> hosts = this.orderedAscByAvailablePes().get();

		for (int i = 0; i < hosts.size(); i++) {
			if (hosts.get(i).getNumberOfFreePes() >= numberOfPes) {
				return hosts.get(i);
			}
		}
		return null;
	}

	public int size() {
		return this.host_list.size();
	}

	public CircularHostList orderedAscByAvailablePes() {
		List<Host> list = new ArrayList<Host>(this.host_list);

		Collections.sort(list, new Comparator<Host>() {
			@Override
			public int compare(Host o1, Host o2) {
				return Integer.valueOf(o1.getNumberOfFreePes()).compareTo(
						o2.getNumberOfFreePes());
			}
		});
		return new CircularHostList(list);
	}
}

RoudRobinVmAllocationPolicy.java

package org.cloudbus.cloudsim.examples;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.cloudbus.cloudsim.Host;
import org.cloudbus.cloudsim.Log;
import org.cloudbus.cloudsim.Vm;
import org.cloudbus.cloudsim.core.CloudSim;

/**
 * Round-Robin {@link Vm} allocation policy.
 * 
 * @author alessandro
 */
public class RoudRobinVmAllocationPolicy extends org.cloudbus.cloudsim.VmAllocationPolicy {
	
	private final Map<String, Host> vm_table = new HashMap<String, Host>();
	
	private final CircularHostList hosts;
	
	public RoudRobinVmAllocationPolicy(List<? extends Host> list) {
		super(list);
		this.hosts = new CircularHostList(list);
	}

	@Override
	public boolean allocateHostForVm(Vm vm) {
		if (this.vm_table.containsKey(vm.getUid())) {
			return true;
		}

		boolean vm_allocated = false;

		Host host = this.hosts.next();
		if (host != null) {
			vm_allocated = this.allocateHostForVm(vm, host);
		}

		return vm_allocated;
	}

	@Override
	public boolean allocateHostForVm(Vm vm, Host host) 
	{
		if (host != null && host.vmCreate(vm)) 
		{
			vm_table.put(vm.getUid(), host);
			Log.formatLine("%.4f: VM #" + vm.getId() + " has been allocated to the host#" + host.getId() + 
					" datacenter #" + host.getDatacenter().getId() + "(" + host.getDatacenter().getName() + ") #", 
					CloudSim.clock());
			return true;
		}
		return false;
	}

	@Override
	public List<Map<String, Object>> optimizeAllocation(List<? extends Vm> vmList) {
		return null;
	}

	@Override
	public void deallocateHostForVm(Vm vm) {
		Host host = this.vm_table.remove(vm.getUid());

		if (host != null) {
			host.vmDestroy(vm);
		}
	}

	@Override
	public Host getHost(Vm vm) {
		return this.vm_table.get(vm.getUid());
	}

	@Override
	public Host getHost(int vmId, int userId) {
		return this.vm_table.get(Vm.getUid(userId, vmId));
	}
}

where i have to put this code?

You can put this code in your CloudSim project, in the constructor of the creation datacenter exist a parameter that receive a VmAllocationPolicy class, and you can put this class as parameter of this constructor.
Although I not tested, but it is the right thing to do....