用cloudsim实现如下仿真:2个用户,2个数据中心,每个数据中心有7个host,执行7个云任务。

好的，让我们来一步步实现这个仿真：

1. 下载和安装CloudSim

首先，你需要从CloudSim的官方网站下载和安装CloudSim。安装完毕后，你可以使用Eclipse或者NetBeans等集成开发环境来编写和运行Java代码。

2. 创建Hosts和Datacenters

首先，我们需要创建两个数据中心，每个数据中心都包含7个主机。在CloudSim中，我们可以使用`DatacenterCharacteristics`类来设置数据中心的特征（如处理能力、存储容量等）。在本例中，我们假设每个主机的处理能力和存储容量都相同。以下是相应的Java代码：

int numHosts = 7;

// Create DatacenterCharacteristics
String arch = "x86"; // CPU architecture
String os = "Linux"; // Operating System
String vmm = "Xen"; // Virtual Machine Monitor
double time_zone = 10.0; // Time zone of the resource
double cost = 3.0; // The cost of using processing in this resource
double costPerMem = 0.05; // The cost of using memory in this resource
double costPerStorage = 0.1; // The cost of using storage in this resource
double costPerBw = 0.1; // The cost of using bw in this resource
LinkedList<Storage> storageList = new LinkedList<Storage>(); // We are not adding SAN devices by now

DatacenterCharacteristics characteristics = new DatacenterCharacteristics(arch, os, vmm, storageList, time_zone, cost, costPerMem, costPerStorage, costPerBw);

// Create Hosts
for (int i = 0; i < numHosts; i++) {
    int hostId = i;
    long storage = 1000000; // host storage
    int bw = 10000;

    // Create PEs
    ArrayList<Pe> peList = new ArrayList<Pe>();
    int mips = 1000;
    for (int j = 0; j < 2; j++) {
        peList.add(new Pe(j, new PeProvisionerSimple(mips))); // create and add PE to the list
    }

    // Create Host
    Host host = new Host(
            hostId,
            new RamProvisionerSimple(512), // RAM capacity of the host
            new BwProvisionerSimple(bw), // bandwidth capacity of the host
            storage,
            peList,
            new VmSchedulerSpaceShared(peList)
    );

    hostList.add(host);
}

// Create Datacenters
for (int i = 0; i < numDatacenters; i++) {
    String name = "Datacenter_" + i;
    Datacenter datacenter = null;
    try {
        datacenter = new Datacenter(name, characteristics, new VmAllocationPolicySimple(hostList), storageList, 0);
    } catch (Exception e) {
        e.printStackTrace();
    }
    datacenterList.add(datacenter);
}

3. 创建VM和Cloudlets

我们需要创建7个Cloudlet和7个VM，然后将它们分配给两个用户。以下是相应的Java代码：

int numCloudlets = 7;
int numVms = 7;

// Create VMs
for (int i = 0; i < numVms; i++) {
    int vmId = i;
    int mips = 250;
    long size = 10000;
    int ram = 512;
    long bw = 1000;
    int pesNumber = 2;
    String vmm = "Xen";

    // Create VM
    Vm vm = new Vm(vmId, userIdList.get(i % 2), mips, pesNumber, ram, bw, size, vmm, new CloudletSchedulerSpaceShared());

    // Add VM to the list
    vmList.add(vm);
}

// Create Cloudlets
for (int i = 0; i < numCloudlets; i++) {
    int cloudletId = i;
    int pesNumber = 2;
    long length = 40000;
    long fileSize = 300;
    long outputSize = 300;
    UtilizationModel utilizationModel = new UtilizationModelFull();

    // Create Cloudlet
    Cloudlet cloudlet = new Cloudlet(cloudletId, length, pesNumber, fileSize, outputSize, utilizationModel, utilizationModel, utilizationModel);

    // Add Cloudlet to the list
    cloudletList.add(cloudlet);
}

// Assign Cloudlets to VMs
for (int i = 0; i < numCloudlets; i++) {
    Cloudlet cloudlet = cloudletList.get(i);
    Vm vm = vmList.get(i % numVms);

    cloudlet.setUserId(vm.getUserId());
    cloudlet.setVmId(vm.getId());

    // Add Cloudlet to VM's list
    vm.getCloudletList().add(cloudlet);
}

4. 运行仿真

最后，我们需要将所有的主机、数据中心、VM和Cloudlet都添加到仿真中，并开始仿真。以下是相应的Java代码：

// Create Simulation
CloudSim simulation = new CloudSim();

// Add Hosts to Datacenters
for (int i = 0; i < hostList.size(); i++) {
    datacenterList.get(i % numDatacenters).getHostList().add(hostList.get(i));
}

// Add Datacenters to Simulation
for (Datacenter datacenter : datacenterList) {
    simulation.addEntity(datacenter);
}

// Add VMs to Simulation
for (Vm vm : vmList) {
    simulation.addEntity(vm);
}

// Add Cloudlets to Simulation
for (Cloudlet cloudlet : cloudletList) {
    simulation.addEntity(cloudlet);
}

// Start Simulation
simulation.start();

这样，我们就完成了这个仿真的实现。你可以根据自己的需求调整参数和代码。