CS 162
Discussion Section
Week 4
9/30 - 10/4
Today’s Section
●
●
●
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Project administrivia
Quiz
Lecture Review
Worksheet and Discussion
Project 1
● Autograder is up
submit proj1-test
● Due 10/8 (Tuesday) at 11:59 PM
submit proj1-code
● Due 10/9 (Wednesday) at 11:59 PM
Final design doc & Project 1 Group Evals
● Questions?
Quiz…
1. Name the conditions causing deadlock, given definitions (0.5 points
each):
a. thread holding at least one resource waits to acquire other
resources held by other threads
b. only one thread at a time can use a resource
c. resource only released voluntarily by the thread holding it after
done using the resource
d. chain of n threads T(i); T(i) depends on T(i+1); T(n) depends on
T(0)
(True/False)
2. Starvation implies deadlock
3. Deadlock implies starvation (that a thread waits indefinitely)
4. To find general deadlocks, it is enough to look for loops in the
resource allocation graph.
Lecture Review
Starvation vs Deadlock
• Starvation vs. Deadlock
– Starvation: thread waits indefinitely
» Example, low-priority thread waiting for resources constantly
in use by high-priority threads
– Deadlock: circular waiting for resources
» Thread A owns Res 1 and is waiting for Res 2
Thread B owns Res 2 and is waiting for Res 1
Owned
By
Thread
A
Res 2
Res 1
Wait
For
Wait
For
Owned
By
Thread
B
– Deadlock  Starvation but not vice versa
» Starvation can end (but doesn’t have to)
» Deadlock can’t end without external intervention
9/25/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 7.7
Four requirements for Deadlock
• Mutual exclusion
– Only one thread at a time can use a resource
• Hold and wait
– Thread holding at least one resource is waiting to acquire
additional resources held by other threads
• No preemption
– Resources are released only voluntarily by the thread holding
the resource, after thread is finished with it
• Circular wait
– There exists a set {T1, …, Tn} of waiting threads
»
»
»
»
9/25/13
T1 is waiting for a resource that is held by T2
T2 is waiting for a resource that is held by T3
…
Tn is waiting for a resource that is held by T1
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 7.8
Deadlock Detection Algorithm
• Only one of each type of resource  look for loops
• More General Deadlock Detection Algorithm
– Let [X] represent an m-ary vector of non-negative
integers (quantities of resources of each type):
[FreeResources]:
[RequestX]:
[AllocX]:
Current free resources each type
Current requests from thread X
Current resources held by thread X
– See if tasks can eventually terminate on their own
[Avail] = [FreeResources]
Add all nodes to UNFINISHED
do {
done = true
Foreach node in UNFINISHED {
if ([Requestnode] <= [Avail]) {
remove node from UNFINISHED
[Avail] = [Avail] + [Allocnode]
done = false
}
}
} until(done)
R1
T1
T2
T3
R2
T4
– Nodes left in UNFINISHED  deadlocked
9/25/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 7.9
Banker’s Algorithm for Preventing
Deadlock
• Toward right idea:
– State maximum resource needs in advance
– Allow particular thread to proceed if:
(available resources - #requested)  max
remaining that might be needed by any thread
• Banker’s algorithm (less conservative):
– Allocate resources dynamically
» Evaluate each request and grant if some
ordering of threads is still deadlock free afterward
» Keeps system in a “SAFE” state, i.e. there exists a sequence {T1,
T2, … Tn} with T1 requesting all remaining resources, finishing, then
T2 requesting all remaining resources, etc..
– Algorithm allows the sum of maximum resource needs of all
current threads to be greater than total resources
9/25/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 7.10
Banker’s Algorithm
• Technique: pretend each request is granted, then run
deadlock detection algorithm, substitute
([Requestnode] ≤ [Avail])  ([Maxnode]-[Allocnode] ≤ [Avail])
[FreeResources]:
[MaxX]:
[AllocX]:
Current free resources each type
Max resources requested by thread X
Current resources held by thread X
[Avail] = [FreeResources]
Add all nodes to UNFINISHED
do {
done = true
Foreach node in UNFINISHED {
if ([Maxnode]–[Allocnode] <= [Avail]) {
remove node from UNFINISHED
[Avail] = [Avail] + [Allocnode]
done = false
}
}
} until(done)
9/25/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 7.11
Summary: Deadlock
• Starvation vs. Deadlock
– Starvation: thread waits indefinitely
– Deadlock: circular waiting for resources
• Four conditions for deadlocks
– Mutual exclusion
» Only one thread at a time can use a resource
– Hold and wait
» Thread holding at least one resource is waiting to acquire
additional resources held by other threads
– No preemption
» Resources are released only voluntarily by the threads
– Circular wait
»  set {T1, …, Tn} of threads with a cyclic waiting pattern
• Deadlock preemption
• Deadlock prevention (Banker’s algorithm)
9/25/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 7.12
Scheduling Metrics
• Waiting Time: time the job is waiting in the ready queue
– Time between job’s arrival in the ready queue and launching
the job
• Service (Execution) Time: time the job is running
• Response (Completion) Time:
– Time between job’s arrival in the ready queue and job’s
completion
– Response time is what the user sees:
» Time to echo a keystroke in editor
» Time to compile a program
Response Time = Waiting Time + Service Time
• Throughput: number of jobs completed per unit of time
– Throughput related to response time, but not same thing:
» Minimizing response time will lead to more context switching than
if you only maximized throughput
9/25/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 7.13
Scheduling Policy Goals/Criteria
•Minimize Response Time
–Minimize elapsed time to do an operation (or job)
•Maximize Throughput
–Two parts to maximizing throughput
»Minimize overhead (for example, context-switching)
»Efficient use of resources (CPU, disk, memory, etc)
•Fairness
–Share CPU among users in some equitable way
–Fairness is not minimizing average response time:
»Better average response time by making system less fair
9/30/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 8.
Summary
•Scheduling: selecting a process from the ready queue and
allocating the CPU to it
•FCFS Scheduling:
–Run threads to completion in order of submission
–Pros: Simple (+)
–Cons: Short jobs get stuck behind long ones (-)
•Round-Robin Scheduling:
–Give each thread a small amount of CPU time when it
executes; cycle between all ready threads
–Pros: Better for short jobs (+)
–Cons: Poor when jobs are same length (-)
9/30/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 8.
Summary (cont’d)
•Shortest Job First (SJF)/Shortest Remaining Time First
(SRTF):
–Run whatever job has the least amount of computation to
do/least remaining amount of computation to do
–Pros: Optimal (average response time)
–Cons: Hard to predict future, Unfair
•Multi-Level Feedback Scheduling:
–Multiple queues of different priorities
–Automatic promotion/demotion of process priority in order to
approximate SJF/SRTF
•Lottery Scheduling:
–Give each thread a number of tokens (short tasks  more
tokens)
–Reserve a minimum number of tokens for every thread to
ensure forward progress/fairness
9/30/13
Anthony D. Joseph and John Canny
CS162
©UCB Fall 2013
Lec 8.
Worksheet…
1. Name the conditions causing deadlock, given definitions (0.5 points
each):
a. chain of n threads T(i); T(i) depends on T(i+1); T(n) depends on
T(0)
b. thread holding at least one resource waits to acquire other
resources held by other threads
c. resource only released voluntarily by the thread holding it after
done using the resource
d. only one thread at a time can use a resource
(True/False)
2. Deadlock implies starvation (that a thread waits indefinitely)
3. Starvation implies deadlock
4. To find general deadlocks, it is enough to look for loops in the
resource allocation graph.