User manual

Miscellaneous Notes

TIME_CONTROL

SYSGEN parameter TIME_CONTROL should be set to 6 when running VAX MP with

synchronization window disabled. With synchronization window enabled, it can be set either to

6 or 2. See section “Interprocessor synchronization window” in the present document for valid

sets of parameter values. VSMP will refuse to load if incoherent parameter set is used.

Use of dedicated host machines

If host machine is dedicated to run VAX MP with no competing host workload, it is possible to

set configuration parameter

host_dedicated that may increase performance. This parameter

disables VAX MP VCPU priority elevation while executing system-critical code and saves the cost

of frequent “set thread priority” system calls.

Note that this setting affects the priority of VAX MP VCPU threads on the host system, it does

not affect relative priority of OpenVMS processes and OpenVMS scheduler.

Since the calls to elevate and downgrade VCPU threads priority are executed frequently by VAX

MP when performing system-intensive workload, saving can be noticeable under workloads that

execute a lot of interlocked instructions, such as workloads that perform heavy IO, heavy paging

or other activities resulting in high rate of spinlock and mutex acquisitions, manipulating

interlocked queues or performing other interlocked operations. By the way of example,

performance of a workload similar to OpenVMS boot sequence can be boosted by about 10-15%

if VCPU thread priority control is disabled.

For workloads that are mostly computational in nature with limited dependency and interaction

between the processes that execute on different processors , i.e. workloads that do not produce

high rate of interlocked instructions, disabling VAX MP thread priority control will not result in

noticeable performance improvement.

Thread priority control should be disabled only if there is no competing host workload and the

machine is dedicated to executing VAX MP.

Furthermore, since some host workload is also created by host OS system processes such as

system services and daemons, terminal emulator and window manager as well as VAX MP

auxiliary non-VCPU threads (IO threads and clock strobe thread), we advise to reserve one host

LCPU for these purposes if disabling VAX MP thread priority control and configure VAX MP with

the number of processors at least one less than the number of LCPUs on the host system.

Since reserving host LCPU reduces configured VAX system throughput, disabling thread priority

control only makes sense if the increase in throughput due to performance gain achieved by

Performance of actual OpenVMS bootstrap cannot be boosted by using HOST_DEDICATED. When VAX MP

operates in uniprocessor mode, as would be the case during initial boot phases, it does not control thread priority

and avoids most other overhead associated with multiprocessor synchronization.