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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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 196
SEC. 2.4
SCHEDULING
165
m
i
=
1
Σ
C
i
P
i
≤
1
A real-time system that meets this criterion is said to be
schedulable
.
This means
it can actually be implemented.
A process that fails to meet this test cannot be
scheduled because the total amount of CPU time the processes want collectively is
more than the CPU can deliver.
As an example, consider a soft real-time system with three periodic events,
with periods of 100, 200, and 500 msec, respectively.
If these events require 50,
30, and 100 msec of CPU time per event, respectively, the system is schedulable
because 0. 5
+
0. 15
+
0. 2
< 1.
If a fourth event with a period of 1 sec is added, the
system will remain schedulable as long as this event does not need more than 150
msec of CPU time per event. Implicit in this calculation is the assumption that the
context-switching overhead is so small that it can be ignored.
Real-time scheduling algorithms can be static or dynamic. The former make
their scheduling decisions before the system starts running.
The latter make their
scheduling decisions at run time, after execution has started. Static scheduling
works only when there is perfect information available in advance about the work
to be done and the deadlines that have to be met. Dynamic scheduling algorithms
do not have these restrictions.
2.4.5 Policy Versus Mechanism
Up until now, we have tacitly assumed that all the processes in the system be-
long to different users and are thus competing for the CPU.
While this is often
true, sometimes it happens that one process has many children running under its
control. For example, a database-management-system process may have many
children. Each child might be working on a different request, or each might have
some specific function to perform (query parsing, disk access, etc.).
It is entirely
possible that the main process has an excellent idea of which of its children are the
most important (or time critical) and which the least. Unfortunately, none of the
schedulers discussed above accept any input from user processes about scheduling
decisions. As a result, the scheduler rarely makes the best choice.
The solution to this problem is to separate the
scheduling mechanism
from
the
scheduling policy
, a long-established principle (Levin et al., 1975).
What this
means is that the scheduling algorithm is parameterized in some way, but the pa-
rameters can be filled in by user processes. Let us consider the database example
once again. Suppose that the kernel uses a priority-scheduling algorithm but pro-
vides a system call by which a process can set (and change) the priorities of its
children. In this way, the parent can control how its children are scheduled, even
though it itself does not do the scheduling. Here the mechanism is in the kernel but
policy is set by a user process.
Policy-mechanism separation is a key idea.
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