Page 133 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes In this unit, we describe methods that an operating system can use to prevent or deal with
deadlocks. Most current operating systems do not provide deadlock-prevention facilities, but
such features will probably be added soon. Deadlock problems can only become more common,
given current trends, including larger numbers of processes, multi-threaded programs, many
more resources within a system, and the emphasis on long-lived file and database servers rather
than batch systems.

Keep away your system from the deadlock condition for the best processing
of the system.

4.6.1 System Model
A system consists of a finite number of resources to be distributed among a number of competing
processes. The resources are partitioned into several types, each of which consists of some number
of identical instances. Memory space, CPU cycles, files, and I/O devices (such as printers and
tape drives) are examples of resource types. If a system has two CPUs, then the resource type
CPU has two instances. Similarly, the resource type printer may have five instances. If a process
requests an instance of a resource type, the allocation of any instance of the type will satisfy the
request. If it will not, then the instances are not identical, and the resource type classes have not
been defined properly. For example, a system may have two printers. These two printers may
be defined to be in the same resource class if no one cares which printer prints which output.
However, if one printer is on the ninth floor and the other is in the basement, then people on
the ninth floor may not see both printers as equivalent, and separate resource classes may need
to be defined for each printer. A process must request a resource before using it, and must
release the resource after using it. A process may request as many resources as it requires to
carry out its designated task. Obviously, the number of resources requested may not exceed
the total number of resources available in the system. In other words, a process cannot request
three printers if the system has only two. Under the normal mode of operation, a process may
utilize a resource in only the following sequence:

1. Request: If the request cannot be granted immediately (for example, the resource is being
used by another process), then the requesting process must wait until it can acquire the
resource.
2. Use: The process can operate on the resource (for example, if the resource is a printer, the
process can print on the printer).
3. Release: The process releases the resource. The request and release of resources are system
calls. Examples are the request and release device, open and close file, and allocate and free
memory system calls. Request and release of other resources can be accomplished through
the waif and signal operations on semaphores. Therefore, for each use, the operating system
checks to make sure that the using process has requested and been allocated the resource.
A system table records whether each resource is free or allocated, and, if a resource is
allocated, to which process. If a process requests a resource that is currently allocated to
another process, it can be added to a queue of processes waiting for this resource.
A set of processes is in a deadlock state when every process in the set is waiting for an event
that can be caused only by another process in the set. The events with which we are mainly
concerned here are resource acquisition and release. The resources may be either physical
resources (for example, printers, tape drives, memory space, and CPU cycles) or logical resources
(for example, files, semaphores, and monitors). However, other types of events may result in

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