Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld Chapter 7 Multiple resources The dinning philosophers problem Version June 2014 Chapter 7 Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld 2014 ID: 815758
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Slide1
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Synchronization Algorithms and Concurrent Programming
Gadi Taubenfeld
Chapter 7 Multiple resourcesThe dinning philosophers problem
Version:
June 2014
Slide2Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
A note on the use of these
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Gadi TaubenfeldAll material copyright 2014Gadi Taubenfeld, All Rights Reserved
Synchronization Algorithms and Concurrent ProgrammingISBN: 0131972596, 1
st
edition
To get the most updated version of these slides go to:
http://www.faculty.idc.ac.il/gadi/book.htm
Slide3Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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
Slide4Deadlocks
Section 7.1
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Slide5Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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
Slide6Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Multiple resources
How to avoid deadlock?account A
account B
Transferring money between two bank accounts
P
0
P
1
down(A); down(B)
down(B); down(A)
semaphores
A
and
B
, initialized to 1
deadlock
Slide7Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Multiple resources
How to avoid deadlock?
Bridge crossing
On the bridge traffic only in one direction.
The resources are the two entrances.
Slide8Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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)?
No
15
×
14+1 = 211
Slide9Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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
Slide10Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Strategies for dealing with Deadlocks
Just ignore the problem altogetherUNIX and Windows take this approach.Detection and recovery
Allow the system to enter a deadlock state and then recover.AvoidanceBy 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.)
Slide11Deadlock Prevention
Section 7.2
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Slide12Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Mutual exclusion condition
one process at a time can use the resource.Hold and wait conditiona process can request (and wait for) a resource while holding another resource.
No preemption conditionA resource can be released only voluntarily by the process holding it.Circular wait conditionmust 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
Slide13Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Attacking the mutual exclusion condition
Some devices (such as printer) can be spooledonly the printer daemon uses printer resource, thus deadlock for printer eliminatedNot all devices can be spooled attack is
not useful in generalAttacking the no preemption condition
Many resources (such as printer) should not be preemptedcan not take the printer from a process that has not finished printing yet
attack is not useful in general
Slide14Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Attacking the Hold and Wait Condition
ProblemsMay 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.
Slide15Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Two-Phase Locking
(Notice similarity to requesting all resources at once)Phase oneThe process tries to lock all the resources it currently needs, one at a timeif needed record is not avaliable, release and start overPhase two: when phase one succeeds,performing updatesreleasing locks
“livelock” is possible.
Slide16Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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
Slide17Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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 techniquePrevents deadlock and starvation.
Slide18Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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
234
56
7
We will see other interesting usage of this observation
time
account A
account B
Solves transferring money between two bank accounts
Slide19Deadlock Avoidance
Section 7.3
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Slide20Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Safe and Unsafe States
safe
unsafe
deadlock
time
All terminated!
Deadlock Avoidance
Slide21Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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.
Slide22Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Maximum
Allocation
available : 2
P1
P2
P3
1
9
4
5
2
8
Example: Prove that the state below is safe
Slide23Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Maximum
Allocation
Proof
time
available : 8
P1
P2
P3
1
9
0
-
0
-
Maximum
Allocation
available : 0
P1
P2
P3
1
9
0
-
8
8
Maximum
Allocation
available : 6
P1
P2
P3
1
9
0
-
2
8
Maximum
Allocation
available : 1
P1
P2
P3
1
9
5
5
2
8
Maximum
Allocation
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
--
Slide24Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Example: safe and unsafe
Maximum
Allocation
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
unsafe
state
Slide25Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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 <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.
Slide26Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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
Slide27The Dinning Philosophers Problem
Section 7.4
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Slide28Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Dining Philosophers
Philosophersthinktake forkseatput forksEating needs 2 forksPick one fork at a time How to prevent deadlock
Slide29Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
(
means “waiting for this forks”)
An incorrect solutionL
L
L
L
L
L
Slide30Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
An inefficient solution
using mutual exclusion
Slide31Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Proving deadlock-freedom & starvation-freedom
1
2
3
4
5
6
R
R
R
R
R
L
Impose a total ordering of all forks, and require that
each philosopher requests resources in an increasing order.
Slide32The Hold and Wait Strategy
Section 7.5
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Slide33Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
R
L
L
R
R
L
The LR Solution
Slide34Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
1
4
2
5
3
6
R
L
R
L
R
L
The LR Solution
Proving deadlock-freedom & starvation-freedom
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Concurrency
How many can eat simultaneously?
Slide36Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
At most
half can eat simultaneously
Slide37Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Only one third can eat simultaneously
Any algorithm is at most
n
/3
-concurrent
Slide38Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
In LR only one forth can eat simultaneously
The LR algorithm is at most
n
/4
-concurrent
Slide39Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
If all want to eat, there is a case where only n/4 will be able to eat simultaneously.
R
free
R
L
L
LR
(
means “waiting for this forks”)
Slide40Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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.
Slide41Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
L
L
R
L
R
L
The LLR Solution
Slide42Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
LLR
3
2
1
6
5
0
L
L
R
L
L
R
Proving deadlock-freedom & starvation-freedom
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
In LLR one third can eat simultaneously
A tight bound
The LLR algorithm is
n
/3
-concurrent
Slide44Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Robustness
k-robust: if all except k consecutive
philosophers fail, then one will not
starve.
The LLR algorithm is not 4-robust.
The LLR algorithm is 5-robust iff n
0 mod 3
.
Slide45The Hold and Release Strategy
Section 7.6
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Slide46Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
The
LR wait/release Algorithm
RL
LR
R
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.
Slide47Randomized Algorithms
Section 7.7
Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
Slide48Chapter 7
Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2014
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.