User manual

Miscellaneous Notes
63
These issues do not apply to VAX MP executing under a hypervisor running directly on top of
hardware, rather than on top of host operating system.
Nevertheless it is possible to run VAX MP under a hypervisor executed on top of host operating
system.
Some hypervisors may report incorrect host processor topology to guest operating system and
misrepresent hyperthreaded/SMT units as separate cores. For example VMware 7.x executed
under Windows misrepresents hyperthreaded units to Linux guest as separate cores. If left
uncorrected this could cause OpenVMS to undercalibrate its timing loops. VAX MP allows to
check for this situation and correct it. When running VAX MP under host OS based hypervisor,
before starting VSMP (i.e. before VSMP LOAD command or executing VSMP$LOAD.COM)
execute command VSMP QUERY and check for reported value of “Advised host SMT slow-down
factor”:
$ VSMP QUERY
[…]
Advised host SMT slow-down factor: 1.8
[…]
On processor with non-hyperthreaded cores, reported value should be 1.0. On 2-way
hyperthreaded processor reported value should be 1.8, consistent with expectation that
hyperthreaded core normally operates not at 2x speed of an independent core but only at 1.2-
1.3x its speed and thus effective slow-down of hyperthreaded core compared to two
independent cores is 1/1.2-1.3 or approximately 1.8.
If you are executing VAX MP under a hypervisor on a 2-way hyperthreaded host processor and
see slow-down factor reported as 1.0, this means that hypervisor misrepresents processor
topology to a guest operating system executed inside the hypervisor and on top of which VAX
MP is being executed.
To correct this situation add the following command to your VAX MP startup script that
overrides the value implied by incorrect processor topology exposed by the hypervisor:
CPU SMT 1.8
Console ROM prompt
MicroVAX 3900 console ROM is not SMP-aware and can wreak havoc on the system if executed
while multiprocessing is active. For this reason VAX MP disables access to console ROM (>>>
prompt) when more than one virtual processor is currently active. It is possible to access console
ROM when only one processor is active, even after SMP had been enabled; however if multiple
processors are currently active, BREAK action (such as telnet BREAK command or console WRU
character such as Ctrl/P) will be ignored. Access to console ROM prompt is reenabled when
primary VCPU is the only CPU in the active set, such as before START /CPU command, after STOP
/CPU/ALL command, after system shutdown or during and after system bugcheck.