1 Chapter 7 Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014...

1Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014

Chapter 7

Synchronization Algorithms and Concurrent Programming

Gadi TaubenfeldChapter 7

Multiple resourcesThe dinning philosophers

problem

Version: June 2014


Chapter 7

A note on the use of these ppt slides:I am making these slides freely available to all (faculty, students, readers). They are in PowerPoint form so you can add, modify, and delete slides and slide content to suit your needs. They obviously represent a lot of work on my part. In return for use, I only ask the following:

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

book!

That you note that they are adapted from (or perhaps identical to) my slides, and note my copyright of this material.

Thanks and enjoy! Gadi Taubenfeld

All material copyright 2014Gadi Taubenfeld, All Rights Reserved

A note on the use of these ppt slides:I am making these slides freely available to all (faculty, students, readers). They are in PowerPoint form so you can add, modify, and delete slides and slide content to suit your needs. They obviously represent a lot of work on my part. In return for use, I only ask the following:

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

book!

That you note that they are adapted from (or perhaps identical to) my slides, and note my copyright of this material.

Thanks and enjoy! Gadi Taubenfeld

All material copyright 2014Gadi Taubenfeld, All Rights Reserved

Synchronization Algorithms and Concurrent Programming

ISBN: 0131972596, 1st edition

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


Chapter 7

2.1 Deadlocks2.2 Deadlock Prevention2.3 Deadlock Avoidance2.4 The Dining Philosophers2.5 Hold and Wait Strategy2.5 Wait and Release

Strategy2.6 Randomized algorithms

2.1 Deadlocks2.2 Deadlock Prevention2.3 Deadlock Avoidance2.4 The Dining Philosophers2.5 Hold and Wait Strategy2.5 Wait and Release

Strategy2.6 Randomized algorithms

Chapter 7 Multiple Resources

Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014

4

Deadlocks

Section 7.1

Chapter 7


Chapter 7

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.

Deadlocks


Chapter 7

Multiple resourcesHow to avoid deadlock?

account Aaccount A account Baccount B

Transferring money between two bank accounts

P0 P1

down(A); down(B)down(B); down(A)

semaphores A and B, initialized to 1

deadlock


Chapter 7

Multiple resourcesHow to avoid deadlock?

Bridge crossing

On the bridge traffic only in one direction.

The resources are the two entrances.


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: 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)?

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)?

No

15×14+1 = 211


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?

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


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.)

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.)


11

Deadlock Prevention

Section 7.2

Chapter 7


Chapter 7

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.

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.

Deadlock PreventionAttacking one of the following conditions for deadlock


Chapter 7

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

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


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


Many resources (such as printer) should not be preempted can not take the

printer from a process that has not finished printing yet



Chapter 7

Attacking the Hold and Wait Condition

Problems May not know all required resources in

advance. Inefficient : ties up resources other processes

could be using. Starvation is possible.

Problems May not know all required resources in

advance. Inefficient : ties up resources other processes

could be using. Starvation is possible.

Processes may request all the resources they need in advance.


Chapter 7

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

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.


Chapter 7

Time stamps: Before a process starts locking a unique

new timestamp is associated with that process.

If a process has been assigned timestamp Ti and later a new process has assigned timestamp Tj then Ti <Tj.

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

Time stamps: Before a process starts locking a unique

new timestamp is associated with that process.

If a process has been assigned timestamp Ti and later a new process has assigned timestamp Tj then Ti <Tj.

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

The time-stamping ordering technique


Chapter 7

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.

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.

The time-stamping ordering technique

Prevents deadlock and starvation.


Chapter 7

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.

11 22 33 44 55 66 77

We will see other interesting usage of this observation

time

account A account B

Solves transferring money between two bank accounts


19

Deadlock Avoidance

Section 7.3

Chapter 7


Chapter 7

Safe and Unsafe States

safeunsafedeadloc

k

time

All terminated!

Deadlock Avoidance


Chapter 7

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.

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.


Chapter 7

Maxim

um

Allo

catio

n

available : 2

P1

P2

P3

1 9

4 5

2 8

Example: Prove that the state below is safe


Chapter 7

Maxim

um

Allo

catio

n

Proof

time

available : 8

P1

P2

P3

1 9

0 -

0 -

Maxim

um

Allo

catio

n

available : 0

P1

P2

P3

1 9

0 -

8 8

Maxim

um

Allo

catio

n

available : 6

P1

P2

P3

1 9

0 -

2 8

Maxim

um

Allo

catio

n

available : 1

P1

P2

P3

1 9

5 5

2 8

Maxim

um

Allo

catio

n

available : 2

P1

P2

P3

1 9

4 5

2 8

available : 0

P1

P2

P3

9 9

0 --

0 --

available : 9

P1

P2

P3

0 --

0 --

0 --


Chapter 7

Example: safe and unsafe

Maxim

um

Allo

catio

n

available : 2

P1

P2

P3

1 9

4 5

2 8

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

allocated it.

available : 1

P1

P2

P3

2 9

4 5

2 8

unsafestate


Chapter 7

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.

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 <P1, P2, …,

Pn> such that for each Pi,

the resources that Pi can

still request can be satisfied by currently available resources + resources held by all the Pj, with j < i.

A state is safe if there exists a sequence of all processes <P1, P2, …,

Pn> such that for each Pi,

the resources that Pi can

still request can be satisfied by currently available resources + resources held by all the Pj, with j < i.


Chapter 7

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.

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.

The Banker’s Algorithm: commensts


27

The Dinning Philosophers Problem

Section 7.4

Chapter 7


Chapter 7

Dining Philosophers Philosophers

think take forks eat put forks

Eating needs 2 forks

Pick one fork at a time

How to prevent deadlock

Philosophers think take forks eat put forks

Eating needs 2 forks

Pick one fork at a time

How to prevent deadlock


Chapter 7

( means “waiting for this forks”)An incorrect solution

L

L

L

LL

L


Chapter 7

An inefficient solutionusing mutual exclusion


Chapter 7

Proving deadlock-freedom & starvation-freedom

1 2

3

45

6

R

R

R

R

R

L

Impose a total ordering of all forks, and require thateach philosopher requests resources in an increasing

order.


32

The Hold and Wait Strategy

Section 7.5

Chapter 7


Chapter 7

R

L

L

RR

L

The LR Solution


Chapter 7

1 4

2

53

6

R

L

R

L

R

L

The LR Solution



Chapter 7

Concurrency How many can eat simultaneously?


Chapter 7

At most half can eat simultaneously


Chapter 7

Only one third can eat simultaneouslyAny algorithm is at most n/3-concurrent


Chapter 7

In LR only one forth can eat simultaneouslyThe LR algorithm is at most n/4-concurrent


Chapter 7

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

R

free

RL

L

LR

( means “waiting for this forks”)


Chapter 7

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.



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.


Chapter 7

L

L

R

LR

L

The LLR Solution


Chapter 7

LLR

3 2

1

65

0

L

L

R

L

L

R



Chapter 7

In LLR one third can eat simultaneously

A tight bound

The LLR algorithm is n/3-concurrent


Chapter 7

Robustness



starve. The LLR algorithm is not 4-

robust. The LLR algorithm is 5-robust

iff n 0 mod 3.



starve. The LLR algorithm is not 4-

robust. The LLR algorithm is 5-robust

iff n 0 mod 3.


45

The Hold and Release Strategy

Section 7.6

Chapter 7


Chapter 7

The LR wait/release Algorithm

R

L

L

RR

L

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.

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.


47

Randomized Algorithms

Section 7.7

Chapter 7


Chapter 7

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 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.

The Courteous Philosophers algorithm is:

starvation-free with probability 1, but is not 2-concurrent and is not (n-1)-robust.

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