Increasing capacity with extra CPUs

Extra CPUs are redundant, fully operable CPUs of a Server Unit that can be attached by the VM2000 administrator for ongoing VM2000 operation in the event of short-term additional CPU demand being placed on the capacity of the server (Capacity on Demand).

Selected Server Units are equipped with extras CPUs when the customer requests this.

The use of extra CPUs is regulated by an agreement. Your sales representative will be able to give you details.

Extra CPUs temporarily raise the CPU capacity of the Server Unit (see "Planning distribution of the CPU capacity to the VMs"), the CPU capacity of the CPU pool to which they are assigned, and indirectly the performance of the guest systems as well.

(Real) extra CPUs are managed like real normal CPUs in the CPU pools. They are always assigned to precisely one CPU pool.

Extra CPUs in normal operation

The extra CPUs remain logically disconnected when VM2000 operation is initialized and are added to the standard CPU pool. On SU /390, the number of available extra CPUs is indicated in message VMS5053 when the monitor system is started up. The state of the extra CPUs is displayed if /SHOW-VM-RESOURCES is specified.
Guest systems have nothing to do with extra CPUs.

Use of extra CPUs to raise capacity

If additional CPU capacity is required of the server, the VM2000 administrator can attach extra CPUs by means of /ATTACH-VM-RESOURCES UNIT=*EXTRA-CPU(...).

Extra CPUs must be used in accordance with an agreement. The usage intervals are recorded in the monitor system in a separate log file (RESLOG subsystem). The attachment or detachment of extra CPUs is reported via Remote Service.

	In the event of the failure of extra CPUs on SU /390, available hot spare CPUs are attached automatically by VM2000.
	In the event of the failure of extra CPUs on SU x86, installed cold spare CPUs only become available when the server is restarted.

The VM2000 administrator can detach extra CPUs again during VM2000 operation by specifying /DETACH-VM-RESOURCES UNIT=*EXTRA-CPU(...).

Extra CPUs are detached at the latest at VM2000 termination. In a new VM2000 session, extra CPUs have to be attached again, if required.

Distribution of the additional CPU capacity to the VMs

Attaching extra CPUs increases the CPU capacity of the Server Unit as a whole. In absolute terms, the server capacity (absolute CPU share) at the disposal of the active VMs of the CPU pool to which the extra CPU is added increases in accordance with the settings for the CPU quotas. If the additional CPU capacity is only to be assigned to one VM or to individual VMs, the CPU quotas of all of the VMs have to be adapted. For details, please also refer to the section "Planning distribution of the CPU capacity to the VMs".

Simple example of a Server Unit with 2 CPUs (standard CPU pool only)

Initial situation	CPU-QUOTA	Effective CPU share of the VM	Absolute CPU share of the VM
VM1 (BI)	60	60%	1.2
VM2 (BI)	30	30 %	0.6
VM3 (MONO)	10	10 %	0.2
Total	100 1	100 %	2.0

¹   A total of 100 for the CPU quotas makes it is easy to calculate the CPU shares.

   

When an extra CPU is attached, this results in the following shares:

With an extra CPU	CPU-QUOTA	Effective CPU share of the VM	Absolute CPU share of the VM
VM1 (BI)	60	60%	1.8 1
VM2 (BI)	30	30 %	0.9
VM3 (MONO)	10	10 %	0.3
Total	100	100 %	3.0

¹   Changes in comparison to the initial situation appear bold.

 

VM2 is to receive all of the additional server capacity:

Adaptation of CPU quotas	CPU-QUOTA	Effective CPU share of the VM	Absolute CPU share of the VM
VM1 (BI)	40	40 %	1.2
VM2 (BI)	53 1	53 %	1.6
VM3 (MONO)	7	7 %	0.2
Total	100	100 %	3.0

1	The new CPU quotas and relative CPU shares are calculated in this example on the basis of the relationship between the absolute CPU share of the VM (1.6 for VM2) and the number of CPUs (3).

   

In complex cases, the formulas and constraints described in the section entitled "Calculating the effective CPU share of a VM" (Planning distribution of the CPU capacity to the VMs) must be adhered to.

When setting up the VM with /CREATE-VM, ensure that the VM can accept the additional CPU capacity intended for it. You can do this by setting a sufficiently high multiprocessor level for the PROCESSOR operand (1 <= PROCESSOR <= number of available real normal CPUs) so that the absolute CPU share of the VM is lower than the number specified for PROCESSOR even after the capacity is increased. In the example on "Increasing capacity with extra CPUs", VM1 and VM2 therefore cannot be set up as monoprocessor VMs (because this would result in absolute CPU shares of 1.2 (VM1) and 1.6 (VM2) after the capacity was increased).

When a VM is even expected to accept the largest possible CPU capacity (overdimensioned VM), i.e. including the CPU capacity of the available extra CPUs, the PROCESSOR=*EXTRA-AND-NORMAL operand must be specified when the VM is configured with /CREATE-VM, see "CREATE-VM (Initialize a VM)".

The limit on the CPU utilization of a VM or VM group imposed by MAX-CPU-UTILIZATION < 100 (see "Planning distribution of the CPU capacity to the VMs") continues to apply to the nominal CPU capacity of the Server Unit. This corresponds to the performance of the real normal CPUs. Attaching or detaching extra CPUs does not change the limit.

The setting MAX-CPU-UTILIZATION = 100 allows a VM or VM group in normal operation as well as after attaching extra CPUs to utilize the capacity of all attached CPUs.