Working with the Unix OS

Process Scheduling
algorithm swapper /* swap in swapped out processes,
* swap out other processes to make room */
input: none
output: none
{
loop:
for (all swapped out processes that are ready to run)
pick process swapped out longest;
if (no such process)
{
sleep (event must swap in);
goto loop;
}
if (enough room in main memory for process)
{
swap process in;
goto loop;
}
for (all processes loaded in main memory, not zombie and not locked in memory)
{
if (there is a sleeping process)
choose process such that priority + residence time
is numerically highest;
else /* no sleeping processes */
choose process such that residence time + nice
is numerically highest;
}
if (chosen process not sleeping or residency requirements not satisfied)
sleep (event must swap process in);
else
swap out process;
goto loop;
}
Figure 60. Algorithm
for Swapper
No "ready-to-run" processes exist on swap device: swapper goes to sleep
Swapper finds an eligible process to swap in but system does not contain enough memory: swapper attempts to
swap another process out.
Zombie processes do not get swapped out, they do not take up any physical memory.
The kernel swaps out sleeping processes rather than II ready-to-run II processes, they have a greater chance of
being scheduled soon.
A "ready-to-run" process must be core resident for at least 2 seconds before being swapped out, and a process to be
swapped in must have been swapped out for at least seconds.
The swapper awakens
- once a second by the clock
- if another process goes to sleep
The swapper swaps out a process based on its
- priority
- memory residence time
- nice value
Swap out groups of processes only if they provide enough memory for the incoming process.
If the swapper sleeps because it could not find enough memory to swap in a process, searches again for a process to
swap in although it had previously chosen one. Other swapped processes have awakened in the meantime.
If the swapper attempts to swap out a process but cannot find space on the swap device, a system deadlock could
arise if:
- all processes in main memory are asleep
- all "ready-to-run" processes are swapped out
- there is no room on the swap device for new processes
- there is no room in main memory for incoming processes.
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