PDP-11 architecture Was: internet blocking problem ?

[Warner Losh]

On Fri, Sep 11, 2015 at 9:03 AM, Paul Koning <paulkoning at comcast.net> wrote:

>
> > On Sep 11, 2015, at 10:49 AM, Warner Losh <imp at bsdimp.com> wrote:
> >
> > ...
> > I loved the PDP-11 architecture, until I wanted to run programs on it
> that
> > relied on the overlay manager and the overlays got to be 8 or 9 deep.
> Then
> > it was... painful.
>
> Perhaps the program was too large.  But it may just be that the overlay
> structure was not right.  Overlays involve significant overhead, and it's
> well known that the flexibility of TKB can cost a lot.  (TKB itself is an
> example of that, which is why there was a button with the text "TKB
> forever... and ever... and ever...".)
>
> For efficient overlays, RT-11 with Link tends to be better.  It's less
> flexible but that reduced flexibility enforces more care in overlay design,
> and the implementation is a whole lot faster.


True. We were running under RSTS-E and so had some more flexibility.
However, by the time we got to this overlay structure, we knew we'd done
something horribly wrong and spent several weeks slimming down the program,
reducing the number of parameters to some routines and redesigning some
code to have either simpler algorithms, or inlining some code. We slimmed
it by 30% but more importantly went from about 8 overlay depth to 3. Still
slow, but it was acceptably slow rather than insane.

Warner