Appendix:
I wrote a viewer tool of loads of memories, CPUs, and network speeds of virtual machines using the Netdata library, as shown in the following video. This video shows the loading and allocating of virtual memory for a certain virtual machine and the virtual memories retrieved from the other VMs for the VM's memory allocation, under the critical states almost used up physical memory. The behaviors of the virtual memory allocation and retrieval of 14 VMs in the video are beneficial for tuning up memory ballooning configuration for improving the memory overcommitment of the virtual machines.