chapter7

Synchronization Algorithms and Concurrent Programming

Gadi Taubenfeld

Chapter 7

Multiple resources

The dinning philosophers problem

Chapter 7

Version: June 2014


Gadi Taubenfeld © 2014

1


ISBN: 0131972596, 1 st edition

A note on the use of these ppt slides:

I am making these slides freely available to all (faculty, students, readers).

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 That you mention their source, after all, I would like people to use my book!

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Thanks and enjoy!

Gadi Taubenfeld

All material copyright 2014

Gadi Taubenfeld, All Rights Reserved

Chapter 7

To get the most updated version of these slides go to: http://www.faculty.idc.ac.il/gadi/book.htm



2

Chapter 7

Multiple Resources

2.1

Deadlocks

2.2

Deadlock Prevention

2.3

Deadlock Avoidance

2.4

The Dining Philosophers

2.5

Hold and Wait Strategy

2.5

Wait and Release Strategy

2.6

Randomized algorithms

Chapter 7 Synchronization Algorithms and Concurrent Programming


3

Deadlocks

Section 7.1



4

Deadlocks

A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause.



5

Multiple resources

How to avoid deadlock?

account A account B

Transferring money between two bank accounts deadlock semaphores A and B, initialized to 1

P

0 down(A); down(B);

P

1 down(B) down(A)



6

Multiple resources

How to avoid deadlock?

Bridge crossing





On the bridge traffic only in one direction.

The resources are the two entrances.



7

Chapter 7

Two Simple Questions

Question: A system has 2 processes and 3 identical resources. Each process needs a maximum of 2 resources. Is deadlock possible?

Question: Consider a system with X identical resources. The system has 15 processes each needing a maximum of 15 resources. What is the smallest value for X which makes the system deadlock-free (without the need to use a deadlock avoidance algorithm)?

15 × 14+1 = 211

No



8

Chapter 7

Question

Question: Two processes, P1 and P2 each need to hold five records 1,2,3,4 and 5 in a database to complete. If

P1 asks for them in the order 1,2,3,4,5 and P2 asks them in the same order, deadlock is not possible.

However, if P2 asks for them in the order 5,4,3,2,1 then deadlock is possible. With five resources, there are 5! or 120 possible combinations each process can request the resources. Hence there are 5!

× 5! different algorithms. What is the exact number of algorithms

(out of 5!

× 5!) that is guaranteed to be deadlock free?

5!(4!

× 4!) = (5!

× 5!)/5



9

Chapter 7

Strategies for dealing with Deadlocks









Just ignore the problem altogether



UNIX and Windows take this approach.

Detection and recovery



Allow the system to enter a deadlock state and then recover.

Avoidance



By careful resource allocation, ensure that the system will never enter a deadlock state.

Prevention



The programmer should write programs that never deadlock. This is achieved by negating one of the four necessary conditions for deadlock to occur

(mentioned in the next slide.)



10

Deadlock Prevention

Section 7.2



11

Chapter 7

Deadlock Prevention

Attacking one of the following conditions for deadlock









Mutual exclusion condition

 one process at a time can use the resource.

Hold and wait condition

 a process can request (and wait for) a resource while holding another resource.

No preemption condition



A resource can be released only voluntarily by the process holding it.

Circular wait condition

 must be a cycle involving several processes, each waiting for a resource held by the next one.



12

Attacking the mutual exclusion condition



Some devices (such as printer) can be spooled

 only the printer daemon uses printer resource, thus deadlock for printer eliminated

Not all devices can be

 spooled

 attack is not useful in general

Attacking the no preemption condition



Many resources (such as printer) should not be preempted

 can not take the printer from a process that has not finished printing yet

 attack is not useful in general



13

Attacking the Hold and Wait Condition

Processes may request all the resources they need in advance.



Problems







May not know all required resources in advance.

Inefficient : ties up resources other processes could be using.

Starvation is possible.



14

Two-Phase Locking

(Notice similarity to requesting all resources at once)





Phase one



The process tries to lock all the resources it

 currently needs, one at a time if needed record is not avaliable, release and start over

Phase two: when phase one succeeds,



 performing updates releasing locks

“livelock” is possible.



15

The time-stamping ordering technique

Time stamps:



Before a process starts locking a unique new timestamp is associated with that process.





If a process has been assigned timestamp T and later a new process has assigned timestamp T j then T i

<T j

.

We associate with each resource a timestamp value, which is the timestamp of the process that is currently holding that resource.

i



16

The time-stamping ordering technique

Phase one: the process tries to lock all the resources it currently needs, one at a time.



If a needed resource is not available and the timestamp value is smaller than that of the process,

 release all the resources,

 waits until the resource with the smaller timestamp is released,



 and starts over.

Otherwise, if the timestamp of the resource is not smaller,

 waits until the resource is released and locks it.

Phase two: when phase one succeeds,

 performing updates; releasing locks.

Chapter 7

Prevents deadlock and starvation.



17

Attacking the Circular Wait Condition

Impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration.

1 2 3 time

4 5 6 7

Chapter 7 account A account B

Solves transferring money between two bank accounts

18

Deadlock Avoidance

Section 7.3



19

safe unsafe deadlock

Deadlock Avoidance

Safe and Unsafe States time



20

Basic Facts



If a system is in safe state  no deadlock.



If a system is in unsafe state  deadlock now or in the future.



Deadlock Avoidance  ensure that a system will never enter an unsafe state.



21

Example: Prove that the state below is safe

P1 1 9

P2 4 5

P3 2 8 available : 2



22

Proof

P1 1 9

P2 4 5


P1 9 9

P2 0 --

P3 0 -available : 0

Chapter 7

P1 1 9

P2 5 5


P1 1 9

P2 0 -


P1 1 9

P2 0 -


P1 0 --

P2 0 --

P3 0 -available : 9 time



P1 1 9

P2 0 -

P3 0 available : 8

23

Example: safe and unsafe

P1 1 9

P2 4 5


P1 2 9

P2 4 5


Chapter 7



If process P1 requests one (out of the 2 avaliable), resources the Banker will not allocated it .

unsafe state



24

The Banker’s Algorithm



When there is a request for an available resource, the banker must decide if immediate allocation leaves the system in a safe state.



If the answer is positive, the resource is allocated, otherwise the request is temporarily denied.



A state is safe if there exists a sequence of all processes <P

1

, P

2

, …, P n such that for each P i resources that P i

>

, the can still request can be satisfied by currently available resources + resources held by all the P j

, with j < i.



25

The Banker’s Algorithm: commensts









Can handle Multiple instances.

Each process must a priori claim maximum use -- a disadvantage.

When a process requests a resource it may have to wait.

When a process gets all its resources it must return them in a finite amount of time.



26

The Dinning Philosophers Problem

Section 7.4



27

Dining Philosophers









Philosophers

 think



 take forks eat

 put forks

Eating needs 2 forks

Pick one fork at a time

How to prevent deadlock



28

Chapter 7

An incorrect solution

( means “waiting for this forks”)

L

L L

L

L

L



29

An inefficient solution using mutual exclusion



30

Proving deadlock-freedom & starvation-freedom

Impose a total ordering of all forks, and require that each philosopher requests resources in an increasing order.

1 2

R

Chapter 7

L R

6

R

R

R

5 4



3

31

The Hold and Wait Strategy

Section 7.5



32

Chapter 7

The LR Solution

L

R R

L

L

R



33

Chapter 7

The LR Solution


1 4

R

L L

6

R

R

L

3 5



2

34

Concurrency

How many can eat simultaneously?



35

At most

half

can eat simultaneously



36

Only one third can eat simultaneously

Any algorithm is at most  n/3  -concurrent



37

In LR only one forth can eat simultaneously

The LR algorithm is at most  n/4  -concurrent



38

L

LR

If all want to eat, there is a case where only  n/4  will be able to eat simultaneously.

( means “waiting for this forks”)

R L

R free



39

Robustness

k-robust: if all except k consecutive philosophers fail, then one will not starve.



Any algorithm is at most

 n/3  -robust.



The LR algorithm is not 4-robust.





The LR algorithm is 5-robust iff n is even .

There is no 4-robust algorithm using a hold and wait strategy.



40

The LLR Solution

Chapter 7

L

R L

L

R

L



41

Chapter 7

LLR


3 2

L

R L

0 1

L

R

L

5 6



42

In LLR one third can eat simultaneously

The LLR algorithm is  n/3  -concurrent

A tight bound



43

Robustness

k-robust: if all except k consecutive philosophers fail, then one will not starve.



The LLR algorithm is not 4robust.



The LLR algorithm is 5-robust iff n  0 mod 3 .



44

The Hold and Release Strategy

Section 7.6



45

The

LR

wait/release Algorithm

The LR wait/release algorithm is:

 deadlock-free but not starvation-free.



 n/3  -concurrent.



3-robust iff n is even .



Recall: There is no 4-robust algorithm using a hold and wait strategy.

R

L

L

R

L

R



46

Randomized Algorithms

Section 7.7



47

Two Randomized Algorithms

The Free Philosophers algorithm is:

 deadlock-free with probability 1, but is not starvation-free and is not 2-concurrent.



3-robust with probability 1 .

The Courteous Philosophers algorithm is:

 starvation-free with probability

1, but is not 2-concurrent and is not (n-1)-robust.



48

chapter7

Deadlocks

Deadlocks

Two Simple Questions

Question

Deadlock Prevention

Deadlock Prevention

Deadlock Avoidance

Deadlock Avoidance

The Dinning Philosophers Problem

Dining Philosophers

The Hold and Wait Strategy

Concurrency

At most

can eat simultaneously

LR

Robustness

LLR

Robustness

The Hold and Release Strategy

The

wait/release Algorithm

Randomized Algorithms

Two Randomized Algorithms

Related documents

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chapter7

Deadlocks

Deadlocks

Two Simple Questions

Question

Deadlock Prevention

Deadlock Prevention

Deadlock Avoidance

Deadlock Avoidance

The Dinning Philosophers Problem

Dining Philosophers

The Hold and Wait Strategy

Concurrency

At most

can eat simultaneously

LR

Robustness

LLR

Robustness

The Hold and Release Strategy

The

wait/release Algorithm

Randomized Algorithms

Two Randomized Algorithms

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