User manual

Quick VAX MP Installation Guide
30
If asynchronous IO is not enabled, VAX MP will be executing host IO requests in the context of VCPU
threads, possibly causing a spinlock or mutex held by VCPU for a long time while synchronous host IO
operation is in progress and VCPU thread is blocked waiting for this operation to complete. This may bring
other VCPUs to a stall, should they need the resource protected by this spinlock or mutex.
Synchronous IO on VCPU thread (as opposed to relegating IO operations to IO threads and performing
them asynchronously of VCPUs execution) also distorts VCPU calibration.
For these reasons we highly recommend using asynchronous IO and enabling it with
set asynch.
Note that this configuration script disables RL disks, as had been advised before, and uses RQ (MSCP)
disks instead.
It also disables DZ multiplexor and uses VH multiplexor instead. DZ can be used with VAX MP, however
use of DZ is not recommended if you want to move your configuration back and forth between VAX MP
and older versions of SIMH (3.7 and earlier, and early builds of 3.8) because of the bug in these older
SIMH versions. See details in the notes later in this document.
The script also uses TQ instead of TS for tape drive (i.e. MSCP tape drive instead of TS11), as advised in
the notes below.
Value of
host_turbo should be set to the ratio of maximum host CPU frequency to minimum host CPU
frequency. Modern CPUs can dynamically change their operating frequency depending on thermal
conditions and workload. For example, when only one or two cores are loaded on a 4-core chip, the chip
with assistance of operating system can shut down unused cores and boost frequency on active cores.
OpenVMS timeout and busy-wait loops need to be adjusted for this possible change in frequency. The
adjustment is performed via
host_tubro setting. Use BIOS or host OS system information utilities to
determine exact model of CPU chip in your host computer, then look up chip manufacturer’s site for
CPU data. For example, suppose your computer is equipped with Intel i7-3770T processor. Intel online
data sheet for this processor indicates:
Clock Speed: 2.5 GHz
Max Turbo Frequency: 3.7 GHz
Accordingly, turbo factor for this processor will be 3.7 / 2.5 = 148%. This is host_turbo value setting
you should enter for the given host processor:
deposit host_turbo 148 ; enter as decimal
By default host_turbo is set to 120%, which is sufficient for many modern processors, but not for all of
them. You should enter the value that is correct for your particular computer. Current value of
host_turbo setting can be displayed with SIMH console command examine host_turbo (it displays
the value as decimal) or from inside OpenVMS using
VSMP QUERY command.
Value of
ws_lock is set to 1 to cause pre-paging of VAX memory and simulator’s own code and data
into VAX MP working set.