Each guest system has its own exclusive share of the server’s main memory. Access to the main memory is carried out as efficiently as in native mode. The access times are the same.
The main memory distribution should be carefully planned. The main memory size required for a VM is initially estimated or derived from the existing system environment. After the paging rate has been measured using SM2 it may be necessary to make corrections. The paging rate should be low, in accordance with the load profile (TP- or dialog-oriented). The current main memory size of a VM (and on /390 servers also the location) can be determined using /SHOW-VM-RESOURCES INFORMATION=*MEMORY.
Because of the code expansion and the (resident) memory requirements for JIT buffers, the main memory requirement for a VM on an x86 server is greater than for one on a /390 server. On an x86 server, 40% of the main memory is by default reserved for JIT buffers.
The main memory size for the VM2000 hypervisor (/390 server) is calculated by VM2000 in accordance with the size of the peripherals.
The minimum main memory size (MIN-MEMORY-SIZE) for a VM should only be defined when the functions for main memory configuration (/EXPAND- /REDUCE-VM-MEMORY) are to be used. The size selected must then be enough to satisfy at least the resident memory requirements in the guest system.
The resident memory requirement depends on whether the software product DAB is used.
The JIT memory of the x86 servers (approx. 40% of the main memory) also resides in the resident memory. The MIN-MEMORY-SIZE of a VM may nevertheless be defined with a lower value because a corresponding amount of the resident JIT memory is released dynamically with /REDUCE-VM-MEMORY in order to comply with the 40% rule for JIT. The process is reversed with /EXTEND-VM-MEMORY.
On x86 servers the maximum main memory size (MAX-MEMORY-SIZE) determines the upper limit to which the VM's main memory can be extended (/EXTEND-VM-MEMORY). The default value is double the VM's MEMORY-SIZE. If the main memory of a VM is not to be extended, the value MEMORY-SIZE should be selected for the maximum main memory size.
The current utilization of resident memory can be determined by SM2 from the values of the MEMORY report group: Resident Memory = TOTAL - Pageable Frames.