Using GCC/GAS Xenix on AT&T Unix V/386.3.2

[Jack F. Vogel]

In article <2496 at crash.cts.com> jca at pnet01.cts.com (John C. Archambeau) writes:
>Maybe I'd better refine the question, what's the difference between ordinary
>swapping and demand paging?  I've heard of systems having both and the demand
>pager kicking in when the swapper is heavily loaded.  Or am I listening to
>misinformation?
  
Yes, you are listening to misinformation. There is no such thing as the
"demand pager". Demand paging means that a process only gets pages of its
process as it references them, when it references a page the first time it
gets a page fault and the page is brought in. Whether it keeps the page
depends on how frequently it references it. The kernel process known as
the "pager" or usually seen as "vhand" when you do a ps on V3.2 is really
what one should call the page stealer. It wakes up every so many system
ticks and checks process pages and if they have not been referenced 
recently it "steals" them returning them to the free list for other
processes to use. "Swapping" happens only when the pager is not able to
free enough pages to satisfy the need of the scheduler to bring in some
process into core to run. When the pager cannot free enough pages via its
normal stealing operation it will choose some process and "swap" it, that
is takes it out of core altogether and put its image on the swap device.
It is always desireable to not swap, you will not do so as long as the
scheduler can keep the context of all runnable processes in core.

Hope this sheds some light.

Disclaimer: These are my views, not necessarily my employer's.


-- 
Jack F. Vogel			jackv at locus.com
AIX370 Technical Support	       - or -
Locus Computing Corp.		jackv at turnkey.TCC.COM