Deadlock Prevention

Deadlock is the situation which arrives to an process which is in waiting state for using the resource, which is being used by another  process for long time.

Dead lock   occurs   when  all the following conditions  hold:

1)mutual exclusion

2)Hold and Wait

3)No preemption

4)Circular wait

So, the dead lock can be prevented by ensuring  at least  one of the four above conditions  does not  hold.

 

Ways   to  prevent  the  deadlock are:

A)No  Mutual Exclusion :

  • In Mutual Exclusion ,if one process uses an non-shareable resource, another process   willing to use the same resource ,it  must  wait  until the first process releases the resource.    
  • Consider  mutual exclusion in the case of ATM, people(process) who  stand in the queue for withdrawing the money from ATM machine must wait for their turn in order to use  the resource(ATM machine)
  • To eliminate mutual exclusion, all the resources in the system must  be  sharable.
  • It is impossible to dis-satisfy the mutual exclusion because there are some resources which are non-shareable.

Some examples of  the non-shareable resources  of  the system are  tap drive and printer.

B)No Hold  and  Wait Condition:

In the hold  and wait condition, process holds all the resources  it requires before its  execution.

To  Make this condition  false, there are two  approaches  :

1)One approach  is   process requests and  gets  allocated  to   all the  resources that  are  useful  before  the  starting  of  the  execution.

Disadvantage : It  is  time Consuming. Consider  copying   of  file to drive  and  printing it .The process requests  all  the  resources(DVD drive,printer) that  are  useful   for  execution  and  gets  allocated. Printer   is  used  at  last   for  the  printing  the file  after  the copying   file  in  the  DVD drive   to  disk. Another   processes   which  are   wants   to  use  the  printer  must  wait  until  the whole  execution  of  the  first  process   completes .   

2)Another approach  is  process  requests  some  of   the  resources  and  uses them .Before  requesting  another  resources  process  must  release  all  the resources  that  are previously requested.

Disadvantage : It  is  also  time  consuming. Consider  the  copying  of  the  file  to  drive   and  printing  it. The  process  requests  the  resources  that  it  wants  one  by  one .If   the   process  wants  the  resource   which  is  used  by  the  another  process  it  has  to   wait  each  and  every   time  to  use  the  resource. This  leads   to  the  starvation  of   the   process   for  the  resource.

 

C) Preemption  Condition : There  are    approaches   for   the  elimination ,

 

1)If  the  process  requesting  the  resource  which  is  held  by  the  another  waiting  process  then  the  resource  is   preempted  that  is   given  to   the  requested  process.

 

2)If   the  process1  requested  for  an  resource  which  is   using  by  the   another  process2,then   resources   held  by  the  process1   is  preempted   to  the  other  processes   which   are  requested   for  the  respective   resources  and  the   list  of   the  resouces  which  are  preempted   are  added  to  the  list  of  the  required  resources   with  the   new  resource  that  is  to  be allocated. When  all  the   resources   in  the  list  are  available  then   the process1  restart   its  execution.

 

3)Resources   are  allocated  to  the  processes   with   the    highest  priority  value.

 

4)Another  approach, by   using  TIME QUANTUM METHOD, each   and  every  process   is  given  some  certain  amount  of  time  for  execution after the  completion   of  the  time  the  process  must  release  the  resource  and  other  process  utilizes  the  resource  within  its   given  time.

D)No  Circular  Wait : First    all  the  resources  must  be  given  an  value  in  the  increasing  order. If  the   process  is  requested  for  an  resource  with  a  value  then  after  the  process   must  request  the  resources  with  value  higher  than  the  previous value.

consider   the  values  of  the resources  as  printer=2,scanner=5,camera=6.If   the  process   requested  for  the  scanner(it has value of 5) after its execution  if it has to request  another resource then  process   must  request  the  resource  higher  than 

 

5)This  algorithm   increases  the  complexity  and  also  leads  to  the   low  utilization.         

 

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Deadlock prevention

 Deadlock Prevention:

  • That the system never enters a deadlock state.
  • It provides a set of methods to make sure that at least one of the four necessary conditions for a deadlock is never satisfied.
  • Mutual Exclusion
  •  this condition is needed to be checked for non-sharable resources (e.g. Printer)
  • Hold and Wait
  •  It requires a process to request a resource and get allocated before execution or allow process to request resources when the process has none.
  • No preemption
  •  If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released
  • Circular Wait
  • Every process holds a resource needed by the next process. We can impose a total ordering of all resource types, and ask that each process requests resources in an increasing order of enumeration.Disadvantage is that it can lead to low device utilization and reduced system throughput.

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