CMSC 421 Operating Systems Lecture Notes
(c) 1993 Howard E. Motteler


7.5.3  Unix Swapping
---------------------

Older Unix had a

     - swapper              (`high level' scheduler)
     - processes scheduler  (`low level' scheduler)

Swapping starts when kernel ran out of memory due to:

     - fork
     - brk  (process asked for more memory)
     - growing stack

Swapping can also start when some other process had been
`swapped out' on disk for too long

Swapper selects a `victim' to be swapped out:

     - a process that has been in mem. for at least 2
       seconds (to prevent thrashing), and is either
     - a blocked process (e.g., waiting on i/o) with 
       highest priority + residence time, or
     - a ready process with highest pri + res time

(high number -> low priority in Unix)

(sometimes more than one victim is needed, if a lot of memory
 must be freed up)

Swapped-out processes are checked every few sec's to see if they
are ready to run:

  = Swapper choses oldest ready process and does

     - an `easy swap' if there is enough free memory
     - a `hard swap' of some other process has to be
       swapped out first

  = This is repeated until
     - no other swapped out processes are ready, or
     - there is no more room for `new' processes
       (just brought in or less than 2 sec old)

Free memory and swap space is kept on linked lists, 
using first fit