[Simh] VAX Emulation Speed Ramblings...

[Zane H. Healy]

> According to 
> http://www.comwaretech.com/products/legacy_solutions/microvax.shtml,
> "The REVIVER-VAX performance scales linearly with the PC's CPU speed. 
> For integer operations, REVIVER-VAX provides a performance of about one 
> VAX Unit of Processing (VUP) per 100 MHz of CPU clock. Disk I/O is 5 to 
> 20 times faster than on the original VAX."

Interesting site, am I alone in thinking they're simply a CHARON-VAX AR that
have renamed the product?  This is the first time I've ever heard of them.


> How are they doing this?  Are they using some sort of JIT?  I get about 
> 5 VUPs from a 1.2ghz using simh.  According to their calculations, 12 
> VUPs should be possible.

They're able to do platform specific optimization that SIMH can't, since one
of things about SIMH is it is designed to be portable.

> What methods can be used to increase performance of simh/vax on any 
> given pc?

Replace it with a newer faster computer :^)  For disk, do a nice striped
volume over a few U320 15k SCSI drives, or have enough RAM you can fit your
VAX disk images on a RAM disk. Try the Intel C Compilier.  

All these may or may not increase your performance.  I've tried various
things in the past with the TS10 PDP-10 emulator (this was before SIMH added
PDP-10 support), and the SIMH PDP-11 emulator, I found whatever I tried 
offered tiny performance boosts at best, at worst, they actually slowed me 
down.

If performance is truely an issue for you, really only have the following
options:
1. Get faster hardware (this might mean using a real HW, instead of
   emulation)
2. Get an emulator that is optimized for your platform.

For me, I run OpenVMS, and I use it fairly heavily, emulation isn't an
option, I run it on a DEC PWS433au with 1GB RAM and 6 18GB 10,000RPM drives,
and I still need better performance :^( 

		Zane