User manual

Introduction

VAX MP (VSMP) is compatible with these VMS service patches and can successfully execute OpenVMS

7.3 system with those service patches applied.

VAX MP will also execute in uniprocessor mode any operating system that supports MicroVAX 3900 or

VAXserver 3900 machines, including all versions of VMS (VAX/VMS, OpenVMS), Ultrix, NetBSD or

OpenBSD that support MicroVAX 3900 or VAXserver 3900. These operating systems will recognize VAX

MP as regular uniprocessor MicroVAX 3900 or VAXserver 3900 and execute in uniprocessor mode. The

only operating system that will execute on VAX MP in multiprocessor mode at this time is OpenVMS 7.3

and possibly other versions of OpenVMS that VSMP may be able to cope with.

Host hardware requirements

Current release of VAX MP runs on Intel x86 and x64 processors. Total number of VAX virtual processors

that can be configured in VAX MP installation cannot exceed the number of active logical processors on

the host system.

Host logical processor is defined as available host execution unit, counting multiple cores and

hyperthreaded/SMT execution units. For example, quad-core hyperthreaded processor has total of 8

host logical CPUs (LCPUs). Six-core processor with hyperthreading disabled will have 6 LCPUs, and so on.

Note that performance of hyperthreaded x86/x64 processors is not 2x but typically only around 1.2-1.3x

the performance of non-hyperthreaded core. For example, combined maximum performance of quad-

core hyperthreaded i7 processor is not 8x of single-thread performance but rather closer to 5x. Real-world

application and system performance will be further affected by workload-specific scalability

characteristics.

VAX MP implements VAX virtual processors as host system threads executing in parallel.

Never configure VAX MP with number of virtual processors exceeding the number of LCPUs available on

the host system. Doing so can (and likely will) cause host system lock-up. OpenVMS kernel contains

sections of code that sometimes demand simultaneous execution of all virtual processors on the system.

If they cannot execute simultaneously, OpenVMS will hang with VCPU threads spinning waiting for each

other with all host CPUs 100% busy but OpenVMS unable to make any forward progress. Even if it

eventually clears past this stall, slow-down will be drastic. Furthermore, threads will spin at elevated

thread priority. If the number of configured VCPUs exceeds the number of host LCPUs, this will bring

host system to effective lock-up, with host GUI and shell interfaces becoming unresponsive, so it may be

impossible to recover from this state other than by restarting host computer via hardware reset.

Similarly, if you configure multiple instances of VAX MP running simultaneously on the same host system

in multiprocessor mode, make sure that combined number of configured VCPUs in all concurrently

executing instances does not exceed the number of host LCPUs available – otherwise multiple instances

The fact that VSMP built for baseline OpenVMS 7.3 is able to cope successfully with modified VMS executive

images provided by patch VAXSYS01_073 validates general approach of VSMP dynamic patching facility.