Lecture Outline

Aims and Objectives
General Issues
Allocating Resources
Allocation Mechanisms
Allocation Policies
Summary

• Some issues of resource allocation

• Allocation Mechanisms

• Allocation Policies

• In a multi-processing environment, usually processes request more resources than there are available.

• It would be too expensive to provide enough non-shareable resources to cope with maximum demand.

• OS must ensure that processes get the resources they request without compromising system integrity.

• E.g., non-shareable resources cannot be shared amongst processes, because otherwise input or output could be meaningless (Printer intermingling output from several processes).

Issues:

• Mutual Exclusion of processes from non-shareable resources.

• Deadlock should be handled sensibly.

• Ensure high level of resource utilisation.

• Processes should be allocated resources within a reasonable length of time.

• The last two points compromise each other. OS can ensure high level of resource utilisation, but sometimes compromising response times for process completion.

• A resource is a component of the computer system for which processes can compete. Typically these are:

• Central Processors
  • Usually 1, but in a multi-processor environment there will be more than one.
    
  • Processors may be several of the same type, or some may have specific characteristics, to be dedicated to certain tasks.
    
  • OS needs to know what each processor can do to help it allocate processor to a process - uses Processor Descriptor.
    
  • Each processor may have its own processor queue, to queue jobs.

• Memory
  • Processes and data need to be in primary memory to be operated on.
    
  • Memory is finite, and Memory Manager must create enough space in primary memory to accommodate a process for it to become runnable.

• Peripherals
  • Each device has a device descriptor
    
  • IORBs are added to device request queue.
    
  • Mutual exclusion to non-shareable peripherals is managed by setting semaphore's initial value to 1.

• Backing Store
  • Can be used for Virtual Memory - managed by memory manager
    
  • Can also be used for File Store - managed by filing system.
    
  • Requests for space granted, unless there is no space left, or user has exceeded quota.
    
  • Requests cannot be queued.

• Files
  • Collections of related information (Programs, Data, Etc.)
    
  • Access may be restricted - security
    
  • Access may be limited - non-shareable in write mode
    
  • Access may be unlimited - shareable in read mode
    
  • Requests may be queued until the expiration of some timeout.

• Allocation mechanisms are handled by the OS at the appropriate Layer, but policies for using the mechanisms should be standard throughout the OS.

Deadlock

• When Process A holds non-shareable resource X and requests non-shareable resource Y, and Process B holds resource Y and requests resource X.

• 4 Necessary and Sufficient conditions for deadlock
  • Mutual Exclusion - non-shareable resources

  • Hold and Wait

  • No Pre-emption

  • Circular Waiting

• Deadlock can be solved by
  • preventing any one of the conditions from holding

  • Detection and recovery

  • Avoidance by anticipation

• Issues - to identify some allocation problems

• Allocation Mechanisms - to identify how different resource types may be allocated

• Allocation Policies - to show that any policy that is adopted needs to handle deadlock

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