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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 191
pecking order may be the president first, the faculty deans next, then professors,
secretaries, janitors, and finally students. The need to take external factors into ac-
count leads to
priority scheduling
The basic idea is straightforward: each proc-
ess is assigned a priority, and the runnable process with the highest priority is al-
lowed to run.
Even on a PC with a single owner, there may be multiple processes, some of
them more important than others. For example, a daemon process sending elec-
tronic mail in the background should be assigned a lower priority than a process
displaying a video film on the screen in real time.
To prevent high-priority processes from running indefinitely, the scheduler
may decrease the priority of the currently running process at each clock tick (i.e.,
at each clock interrupt).
If this action causes its priority to drop below that of the
next highest process, a process switch occurs. Alternatively, each process may be
assigned a maximum time quantum that it is allowed to run. When this quantum is
used up, the next-highest-priority process is given a chance to run.
Priorities can be assigned to processes statically or dynamically.
On a military
computer, processes started by generals might begin at priority 100, processes
started by colonels at 90, majors at 80, captains at 70, lieutenants at 60, and so on
down the totem pole. Alternatively, at a commercial computer center, high-priority
jobs might cost $100 an hour, medium priority $75 an hour, and low priority $50
an hour. The UNIX system has a command,
, which allows a user to voluntar-
ily reduce the priority of his process, in order to be nice to the other users. Nobody
ever uses it.
Priorities can also be assigned dynamically by the system to achieve certain
system goals. For example, some processes are highly I/O bound and spend most
of their time waiting for I/O to complete. Whenever such a process wants the CPU,
it should be given the CPU immediately, to let it start its next I/O request, which
can then proceed in parallel with another process actually computing. Making the
I/O-bound process wait a long time for the CPU will just mean having it around
occupying memory for an unnecessarily long time.
A simple algorithm for giving
good service to I/O-bound processes is to set the priority to 1/
, where
is the frac-
tion of the last quantum that a process used.
A process that used only 1 msec of its
50-msec quantum would get priority 50, while a process that ran 25 msec before
blocking would get priority 2, and a process that used the whole quantum would
get priority 1.
It is often convenient to group processes into priority classes and use priority
scheduling among the classes but round-robin scheduling within each class.
2-43 shows a system with four priority classes. The scheduling algorithm is as fol-
lows: as long as there are runnable processes in priority class 4, just run each one
for one quantum, round-robin fashion, and never bother with lower-priority classes.
If priority class 4 is empty, then run the class 3 processes round robin.
If classes 4
and 3 are both empty, then run class 2 round robin, and so on.
If priorities are not
adjusted occasionally, lower-priority classes may all starve to death.

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