HP-UX vPars and Integrity VM V6.1.5 Administrator Guide (5900-2295, April 2013)

on the size of the vPar or VM. Given below is a rough estimate of the memory overhead required
for the VSP as well as individual vPars and VMs.
The vPar/VM memory pool reserved is roughly about 92% of the system free memory available
at the time of V6.1.5 product startup. The remaining memory is left out as free memory available
for VSP use. This is in addition to the memory taken up by HP-UX to boot on the VSP. The memory
taken up by HP-UX to boot depends on the size of the system, including total memory, number of
cores and the I/O devices on the system. So the overall VSP memory overhead is the sum of the
memory HP-UX needs to boot up on the system plus the free memory left in the VSP for optimal
functioning of the VSP. In terms of overall system memory, the VSP memory overhead typically
equates roughly to 1500MB + 8.5% of the total physical memory.
Note that the calculation for how much memory is in VSP versus what is available for vPars and
VMS is done at product start time. You can see the hpvmhwmgmt –p memory –l output to see
how much memory is available for vPar/VM memory pool size.
In addition to the VSP memory overhead, individual vPars and VMs have a memory overhead
depending on their size. A rough estimation of the individual guest memory overhead can be done
using the following formula:
Guest memory overhead = cpu_count * (guest_mem * 0.4% + 64M)
Where:
guest_mem is the guest (vPar/VM) memory size
cpu_count is 1 for vPar and num_vcpus for VM. (Memory overhead in a vPar is independent
of number of cpus in a vPar) .
For example, a 16G 4vCPU VM, the overhead is roughly about 512M. For the same 16G VM
with 1 vCPU, it would be around 128M. For a 16G, 16 cpu vPar, as well as a 16G, 1 CPU vPar,
memory overhead would be 128M.
When you create a 16G 4vCPU VM, additional 512M is used up by the VM. This memory is
taken from the vPar/VM pool. Note that there may be some amount of memory taken up from the
VSP memory when a vPar or VM is started. However, that is in most cases negligible compared
to the vPar/VMoverhead memory taken up from the vPar/VM pool.
The hpvmstatus –s command output has additional information displayed to take into account
the guest memory overhead required. In the following example:
# hpvmstatus –s
…
Available memory for vPars and VMs = 24320 Mbytes
Available memory for 7 (max avail.) CPU VM = 23552 Mbytes
Available memory for 7 (max avail.) CPU vPar = 24192 Mbytes
…
Although available memory in the vPar/VM memory pool is 24320 MB, if you want to create a
7 vCPU VM with all the available memory, the VM can be created only with 23552 MB. The
difference is the overhead memory required for a 7vCPU 23552MB VM. Note that the numbers
shown in hpvmstatus display are approximate and the actual memory allowed to create a VM
might have slight variations.
NOTE: The VSP overhead is valid for VSPs with base_pagesize=64.
2.6 Hyperthreading setting on the VSP
By default, the VSP has the hyperthreading (firmware setting) ON in the npartition or server;
whereas lcpu_attr is OFF in the VSP. This setting provides optimal performance and
responsiveness for the VSP. HP strongly discourages changing the default hyperthreading settings
in the VSP, unless it is recommended in the documentation or there is some other compelling reason
to do so.
32 Installing and configuring VSP for HP-UX vPars and Integrity VM V6.1.5