UNIX performance on a VAX & 68020

[Guy Harris]

> Benchmarks show that a 68020 is at least as fast as a VAX-11.
> But if I start e.g. 6 C-compilers on a UNIX-system running on a 68020,
> the performance collapses completely (even when no swapping is necessary),
> where on a VAX the system-performance stays reasonable.
> I wonder why this is so, when the CPU-speeds are roughly the same.

I don't see that you can make a generic statement about "UNIX systems running
on a 68020" here.  (For that matter, you can't make generic statements about "a
VAX-11; I presume you have some particular VAX, such as a 780, in mind.)  68020
UNIX machines may share the same CPU, but may run at different clock speeds, or
have different memory management units, or....

Which 68020 machine did you try this on?  It is conceivable that if you start
lots of processes running on a Sun-3, for example, the performance may degrade
because the Sun-3 MMU has only 8 hardware contexts, and the context switching
overhead may increase substantially when you have more than 7 or 8 processes
running simultaneously because the kernel actually has to load and unload
translations from the MMU rather than just switching the context number.