The CPU quotas for the VMs should be set in several stages:
1. Rough setting of the CPU quotas
In order to make a first approximation, it is recommended that you set the CPU quotas in the same proportions as the expected CPU utilization of the individual guest systems.
Example
Two guest systems (VM1 and VM2) are planned for a biprocessor system. These are estimated to require 50% and 20% of the computer capacity respectively.
This results in the following CPU quotas:
VM1: CPU-Q=50 (standardized EFF-Q=71,43)
VM2: CPU-Q=20 (standardized EFF-Q=28,57)
Since guest system VM2 will only place a load of around 20% on the server, it should be run as a monoprocessor guest system. Guest system VM2 will certainly require both CPUs.
2. Performance limit
The limit for the CPU performance for a VM (see "VM2000 scheduling (/390 servers)") must be selected so that the utilization is around 75% of the performance limit.
3. Measurement
The VM and the guest system are observed using /SHOW-VM-STATUS and SM2 (see "Measurement tools for VM2000"). The measurement values must be used to check whether the actual CPU time consumption and the performance behavior are in line with expectations.
If the guest systems consume approximately the expected capacity, the settings can be left as they are.
4. Checking the performance requirement
If the guest systems do not consume approximately the expected capacity, you must check the actual CPU requirements of the individual guest systems. To do this the guest systems should only be started individually, i.e. without competition from another VM. The guest systems should at least be prioritized one after the other with a very high effective CPU quota.
It can be assumed that the measured VM-ACTIVE percentage quotation corresponds to the actual CPU requirement of the guest system. The CPU quotas can now be recalculated on the basis of these values.
5. Fine-tuning the CPU quotas
Even after the approximate CPU quotas have been set correctly (i.e. the CPU quotas correspond closely to the capacity requirements of the guest systems), it is possible that the capacity requirements of individual guest systems will deviate sharply from the expected values. Guest systems with a high proportion of I/O operations generally fall below the expected values, whereas guest systems with high CPU requirements generally produce values above those expected.
In addition, the response times of important applications sometimes fail to meet requirements. In cases like this, the CPU quotas must be tuned, i.e. individual CPU quotas must be set higher (“overplanning”) or lower than the recommended values from the approximation.
It is not unusual for the CPU quotas to be corrected by a factor of 2. The performance requirements of the guest systems determine whether corrections are necessary and the extent of such corrections. This phase requires close cooperation with the administrators of the guest systems.
6. Forming VM groups (/390 servers)
VMs can then be combined to form VM groups.
When VM groups are used, fine-tuning of the MEMBER-CPU-QUOTA is also recommended for the group members.
Overplanning of the CPU-QUOTA for the VM group is not recommended. If overplanning is necessary, individual VMs should be planned instead of VM groups.