BSD vs. System V, one last thing...

[Doug Gwyn]

In article <553 at umbio.MIAMI.EDU> jherr at umbio.MIAMI.EDU (Jack Herrington) writes:
>I just found out ... that SV has no virtual memory built in, this makes
>absolutely no sense to me since unix had it all the way up to V7 as I
>remember, and they scrapped vm for SV.  This utterly confuses me ...

Sounds like you started out confused.  First of all, you should distinguish
between "demand paging" (in which portions of a process are loaded from disk
as they are needed, instead of all in advance) and "virtual memory" (which
just means that the addresses used by a process are mapped into physical
addresses).  Most versions of UNIX, for example 6th Edition on the PDP-11,
utilized virtual memory, and this remains especially true today (including
UNIX System V).  The first widely-used demand-paging UNIX was 3BSD on the
VAX.  AT&T's VAX releases of UNIX did not use demand paging until UNIX
System V Release 2 Version 2 for DEC VAX, and around the same time frame
AT&T's 3B2 releases of UNIX System V started to use demand paging (based on
a generally cleaner implementation than found in 4BSD).  There really is not
much advantage to demand paging over partial swapping if your system runs
mostly small processes and is not heavily loaded.  Under some circumstances
demand paging can cause a horrible performance loss as the system goes into
a "thrashing" mode.

Why do you care?