VM2000 permits simultaneous execution of multiple, self-contained BS2000 guest systems on a real BS2000 server. The individual guest systems run on virtual machines (VMs) and, as far as their functions are concerned, operate in the same way as in native mode.
The resources CPU, main memory and devices are assigned to the virtual machines and thus to the guest systems by VM2000.
A hypervisor controls the execution of the guest systems on the VMs. In particular it virtualizes the global resources CPU and main memory and starts execution of the operational guest system’s CPUs on the real CPUs (scheduling).
On /390 servers the VM2000 hypervisor is a separate load module of VM2000 which is loaded (automatically) when VM2000 operation is initialized.
On x86 servers the Xen hypervisor performs this role. Some of the hypervisor tasks are performed by the carrier system X2000.
VM2000 on /390 servers distinguishes two processor statuses:
Hypervisor mode (the VM2000 hypervisor runs in this status)
VM mode (the guest systems run in this status)
A context (a set of registers containing the states of the CPU and information specific to VM) is associated with each of these statuses. This context is loaded when the status is activated.
In contrast to native mode VM2000 overhead arises:
Hypervisor overhead (program execution in hypervisor mode)
Indirect overhead (reduced server performance through changing VM contexts)
With careful configuration the VM2000 overhead is between 5% and 15% of the server performance. How high the VM2000 overhead on a server will actually be depends on the VM2000 configuration. Refer to the information in the section "Recommendations for an optimal configuration" to keep the VM2000 overhead to a minimum.