Virtualization is a technique to share the hardware resources among multiple systems or users to achieve optimal usage of resources and reducing costs.
Although virtualization is a generic one conceptually, lets talk on server virtualization. This means a bunch of HW resources like CPUs, Memory, Disks, ports etc., are shared among multiple OSs either of same type or multiple type. So, to achieve this we need someone or something to manage the underlying HW and above running guests (OSs). This 'manager' is what is called a 'Hypervisor'.
There are couple of types of Hypervisors
Native or bare metal hypervisor - this is the software which directly runs on host's hardware to control the hardware and monitor the guests OS. so imagine this something that mediates between guest OS and underlying hardware. Example of
such implementation are Oracle VM, VMware EXXi Xen, Microsoft Hyper-V
The other hypervisor is made to run within a traditional operating system and then guest OSs can run on top of it. Example Oracle VirtualBox (which can be installed on an PC where windows is the base OS but virtual box can then host another guest OS like linux..
Oracle VM Server:
This can be installed on X86 instruction set based platforms with Xen hypervisor (GPU licensed) or on SPARC platforms (which will have its own hypervisor).
In general, the above implementation has their own firmware/hardware, a hypervisor and then a super domain/vm which controls the resource allocation to other guest VMs (also called domains or simply guests)
So, simply Oracle VM server is a collection of hardware (CPU, Mem, Network, IO etc.,), hypervisor (for managing underlying baremetal i.e. the hardware), domains (the VMs with thier own set of OS except Dom0 which a complete linux kernel and manages all the other Domains).
Lets explore some interesting things related to Oracle VM
CPU capacity:
how to determine the cpu capacity on a vm server
xm info is the command to use. for example, as shown below, the number of cpus are 72 which are ideally the threads. There are 2 nodes, 18 cores per socket and 2 threads per core
i.e. 2 * 2 * 18 = 72 threads (0 to 71 total, 0-35 on sock1, 36-71 on sock2)
nr_cpus : 72
nr_nodes : 2
cores_per_socket : 18
threads_per_core : 2
The cpu topology can be viewed by using the commnad xenpm get-cpu-topology
CPU core socket node
CPU0 0 0 0
CPU1 0 0 0
CPU2 1 0 0
CPU3 1 0 0
..
xm info also gives the high level vm details like what bit it supports, what instruction set (like intel x86), number of real cpus, number of nodes, number of sockets, number of threads per core, cpu frequency,
memory, pagesize etc.,
In an hyperthreaded model, each core will run 2 threads instead of one. and this would have counter effects but could improve efficiency..
vCPUs
virtual cpus are the cpus that are assigned to a guest/domu i.e. a virtual machine which runs on a domu can be assigned 10 CPUs which are considered as virtual cpus and the actual bindings to real cpu depends on how they are configured. For example below, vm1 is a virtual machine with the id=1, has 3 vCPUs which are in bind state and mapped to CPUs 3,6 7. This vm1 is configured to have the cpu affinity as 2-35 which is first socket on a 2 socket 72 core machine. so, since there is no absolute binding, the mapping can change in runtime and depends on the availability,
the vcpus can be mapped to any of the real cpus in the range 2-35.
xm vcpu-list
Name ID VCPU CPU State Time(s) CPU Affinity
vm1 1 0 3 -b- 5354.1 2-35
vm1 1 1 6 -b- 2312.4 2-35
vm1 1 2 7 -b- 2337.8 2-35
you can pin the CPUs for guest vms runtime but to change any affinity to dom0 requires a reboot.. and dom0 always takes the top priority.
And, it is always good to monitor the real cpu usage from the vm server to check how in an oversubscribed case, the busy vms on the same socket could impact each other..
