PRINCIPLES OF OPERATING SYSTEMS · PRINCIPLES OF OPERATING SYSTEMS Tutorial : Deadlock !! CPSC 457,...

PRINCIPLES OF OPERATING SYSTEMS

Tutorial : Deadlock

CPSC 457, Spring 2015 June 10/11, 2015

Department of Computer Science, University of Calgary

Deadlock Avoidance §  Safe and Unsafe States §  Banker’s Algorithm

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Safe State § A system is in a safe state … §  Only if there exists a safe sequence <P1, P2, ..., Pn> of all the processes in the systems such that …

§  For each Pi, the resources that Pi can sAll request can be saAsfied by current available resources + resources held by all processes Pj, with j < i.

§  That is: §  If the resources that Pi needs are not immediately available, then Pi can wait unAl all the Pj, with j < i, have finished.

§ When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate.

§ When Pi terminates, Pi+1 can obtain the resources, and so on.

§  If no such sequence exists, then the system state is unsafe. 3

Banker’s Algorithm §  To determine whether a request can be safely granted. §  Resource-‐request algorithm §  Safety algorithm

§  Resource-‐request algorithm §  If request > needed, return error. §  If request > current available, return wait. §  Pretend the allocaAon: update Available, AllocaConi, Needi §  Run Safety algorithm §  If safe state, grant the request. §  Otherwise, wait and restore the values of Available, AllocaConi, Needi

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Banker’s Algorithm

§  Safety algorithm §  Update the current available resources §  Find a process that can be saAsfied by the current available resource

§  Mark the process finished and pretend to release its resources §  Repeat unAl no more process can be found §  If all processes finish, return safe. Otherwise, return unsafe.

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Banker’s Example §  Processes P0, P1, P2, P3, P4 §  Resources A (10 instances), B (5 instances), C (7 instances)

§  At Ame T0:

§  Can the request, , be granted? (by P1)

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Alloca:on Max Need Available A B C A B C A B C A B C

P0 0 1 0 7 5 3 7 4 3 3 3 2 P1 2 0 0 3 2 2 1 2 2 P2 3 0 2 9 0 2 6 0 0 P3 2 1 1 2 2 2 0 1 1 P4 0 0 2 4 3 3 4 3 1

(Request1 = (1, 0, 2))

<= (Need1 = (1, 2, 2)) (Request1 = (1, 0, 2)) <= (Available = (3, 3, 2)) (Request1 = (1, 0, 2)) Preliminary checks

Banker’s Example

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Alloca:on Max Need Available

A B C A B C A B C A B C

P0 0 1 0 7 5 3 7 4 3 3 3 2

P1 2 0 0 3 2 2 1 2 2

P2 3 0 2 9 0 2 6 0 0

P3 2 1 1 2 2 2 0 1 1

P4 0 0 2 4 3 3 4 3 1

Pretend the allocaCon: Available = (3, 3, 2) -‐ (1, 0, 2) = (2, 3, 0) Alloca=on1 = (2, 0, 0) + (1, 0, 2) = (3, 0, 2) Need1 = (1, 2, 2) -‐ (1, 0, 2) = (0, 2, 0)

(Request1 = (1, 0, 2))



P0 0 1 0 7 5 3 7 4 3 2 3 0

P1 3 0 2 3 2 2 0 2 0

P2 3 0 2 9 0 2 6 0 0

P3 2 1 1 2 2 2 0 1 1

P4 0 0 2 4 3 3 4 3 1

Banker’s Example

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P0 0 1 0 7 5 3 7 4 3 5 3 2

P1 3 0 2 3 2 2 0 2 0

P2 3 0 2 9 0 2 6 0 0

P3 2 1 1 2 2 2 0 1 1

P4 0 0 2 4 3 3 4 3 1

Safety Algorithm

Finish = [false, false, false, false, false]

Work = Available = (2,3,0)

Find an index i such that both

a. Finish[i] == false b. Needi <= Work

i = 1

Work = Work + AllocaConi

= (2,3,0) + (3,0,2)

= (5,3,2)

Finish = [false, true, false, false, false]

Banker’s Example

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P0 0 1 0 7 5 3 7 4 3 7 4 3

P1 3 0 2 3 2 2 0 2 0

P2 3 0 2 9 0 2 6 0 0

P3 2 1 1 2 2 2 0 1 1

P4 0 0 2 4 3 3 4 3 1

Safety Algorithm

Finish = [false, true, false, false, false]

Work = (5,3,2)



i = 3


= (5,3,2) + (2,1,1)

= (7,4,3)

Finish = [false, true, false, true, false]

Banker’s Example

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P0 0 1 0 7 5 3 7 4 3 7 5 3

P1 3 0 2 3 2 2 0 2 0

P2 3 0 2 9 0 2 6 0 0

P3 2 1 1 2 2 2 0 1 1

P4 0 0 2 4 3 3 4 3 1

Safety Algorithm

finish = [false, true, false, true, false]

Work = (7,4,3)



i = 0


= (7,4,3) + (0,1,0)

= (7,5,3)

finish = [true, true, false, true, false]

Banker’s Example

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P0 0 1 0 7 5 3 7 4 3 10 5 5

P1 3 0 2 3 2 2 0 2 0

P2 3 0 2 9 0 2 6 0 0

P3 2 1 1 2 2 2 0 1 1

P4 0 0 2 4 3 3 4 3 1

Safety Algorithm


Work = (7,5,3)



i = 0


= (7,5,3) + (3,0,2)

= (10,5,5)

finish = [true, true, true, true, false]

Banker’s Example

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P0 0 1 0 7 5 3 7 4 3 10 5 7

P1 3 0 2 3 2 2 0 2 0

P2 3 0 2 9 0 2 6 0 0

P3 2 1 1 2 2 2 0 1 1

P4 0 0 2 4 3 3 4 3 1

Safety Algorithm


Work = (10,5,5)



i = 0


= (10,5,5) + (0,0,2)

= (10,5,7)

finish = [true, true, true, true, true]

Since, all the processes can finish, the state aJer gran:ng the request is a safe state.

PRINCIPLES OF OPERATING SYSTEMS · PRINCIPLES OF OPERATING SYSTEMS Tutorial : Deadlock !! CPSC 457,...

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