[Simh] VMware and SIMH/VAX

[Zane Healy]

I finally found the root cause of the problems I’ve had with running the SIMH VAX 3900 emulation on an Ubuntu VM on my ESXI server.  I’d previously been using the VM for trying out a PDP-11 emulator on VMware, and the CPU was running at 100%, so I’d throttled it to 500Mhz.  That’s all well and good, but I forgot about that, and months later decided to start running some VAX emulators.  The I/O was somewhat “jerky” and the system lost time really bad.

In the mean time I brought another VAX VM up on a RPi2, and it runs just fine, and was largely out performing the VMware VAX.  Plus keeping perfect time, and for a month or so it’s been clustered with my Compaq XP1000/667.  It’s no speed demon, but it works great.

Finally I was reminded that I’d throttled the VM, and once I removed the throttle it was lighting fast with smooth I/O (way better than my VAXstation 4000/60, which is my fastest VAX).  It’s also been running for over 24 hours, with no noticeable clock drift.  Next step will be to make it part of my VMS cluster.

Basically I’m writing this to get the info out there that if you’re running SIMH VAX emulation, that throttling the CPU of the host x86 VM is bad. :-)  At least if you want smooth emulation, and accurate time, 

For years I’ve preferred to run VMS and my PDP-11’s OS’s on the real HW, but I have to admit, the idea of not having to spend so much on power and cooling, or depending on ancient HW is appealing.  I’ve done PDP-11 emulation for about as long as I’ve had real PDP-11’s, but hadn’t fully gotten VMS up and running under emulation until about three months ago.

Zane