Page 109 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes Priority inversion is often solved by priority inheritance. With priority inheritance, a thread that
holds a lock that is wanted by a thread with a higher priority has its priority temporarily and
silently raised: its new priority becomes the same as the priority of the thread that it is causing
to block. When the thread releases the lock, its priority is lowered to its original value. The
goal of priority inheritance is to allow the high-priority thread to run as soon as possible. It is
a common feature of operating systems, and Java virtual machines running on those operating
systems are subject to it. However, it is not a requirement of the Java specification.

3.8.1 Complex Priorities
The second case involves the priority assigned to threads by the operating system. We
mentioned that Java has 11 priority levels (10 of which are available to developers), but this
is an abstraction of the Java language. Operating systems usually have many more priorities.
More important, though, is that the priority that the operating system assigns to a thread
is a complex formula that takes many pieces of information into account. A simple version
of this formula might be this:
Real Priority = Java Priority + SecondsWaiting For CPU

This type of formula accounts for the length of time that the thread has been waiting for a
CPU. After a sufficient amount of time has passed, a thread with a Java priority of 3 has a
real priority that is higher than a currently running Java thread with a priority of 5. This
gives the priority 3 thread an opportunity to run, even though it has a lower priority than
other unblocked threads.

Complex priorities are advantageous because they help to prevent thread starvation. Without
such a model, a low-priority thread would have to wait for all other high-priority threads to
block before it is given a chance to execute; it is likely that it might have to wait forever. With
complex priorities, it can still run much less often than it is higher-priority peers, but at least
it will run sometimes.
On the other hand, complex priorities mean that you cannot guarantee thread scheduling. In
particular, you cannot use thread priorities to try and prevent race conditions in data access: a lower-
priority thread can interrupt a higher-priority thread while it is in the process of updating shared
data. You also cannot use thread priorities to ensure a certain order of execution between tasks.

1. C Program For Round Robin Scheduling Method

2. C Program For Priority Scheduling Algorithm
3. C Program for Shortest Job First Scheduling Algorithm

Applying Process Design Principles

an Madison is a principal in Value Creation partners. He facilitates process
improvement using lean, six sigma, reengineering, and continuous improvement
Dtechniques. Dan is the author of Process Mapping, Process Improvement and Process
Management.

Dan Madison has studied what a business process should look like for fifteen years. He
studied what major corporations did in process improvement that made them successful
and distilled his findings into design principles that anyone can use. He has come up with
38 design principles that apply to all business processes.

Contd...

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