CS 162 Discussion Section Week 4 9/30 - 10/4

1. CS 162 Discussion SectionWeek 4 9/30 - 10/4

2. Today’s Section • Project administrivia • Quiz • Lecture Review • Worksheet and Discussion

3. 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?

4. Quiz…

5. Name the conditions causing deadlock, given definitions(0.5 points each): thread holding at least one resource waits to acquire other resources held by other threads only one thread at a time can use a resource resource only released voluntarily by the thread holding it after done using the resource chain of n threads T(i); T(i) depends on T(i+1); T(n) depends on T(0) (True/False) Starvation implies deadlock Deadlock implies starvation (that a thread waits indefinitely) To find general deadlocks, it is enough to look for loops in the resource allocation graph.

6. Lecture Review

7. Thread A Wait For Owned By Res 2 Res 1 Owned By Wait For Thread B 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 2Thread B owns Res 2 and is waiting for Res 1 • Deadlock  Starvation but not vice versa • Starvation can end (but doesn’t have to) • Deadlock can’t end without external intervention

8. 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 • 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

9. R1 T2 T1 T3 R2 T4 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]: Current free resources each type[RequestX]: Current requests from thread X[AllocX]: 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) • Nodes left in UNFINISHED  deadlocked

10. 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

11. Banker’s Algorithm • Technique: pretend each request is granted, then run deadlock detection algorithm, substitute ([Requestnode] ≤ [Avail])  ([Maxnode]-[Allocnode] ≤ [Avail]) [FreeResources]: Current free resources each type [MaxX]: Max resources requested by thread X [AllocX]: Current resources held by thread X [Avail] = [FreeResources] Add all nodes to UNFINISHED do { done = trueForeachnode in UNFINISHED { if ([Maxnode]–[Allocnode] <= [Avail]) { remove node from UNFINISHED [Avail] = [Avail] + [Allocnode] done = false } } } until(done)

12. 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)

13. 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

14. 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

15. 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 (-)

16. 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

17. Worksheet…

19. Name the conditions causing deadlock, given definitions(0.5 points each): • chain of n threads T(i); T(i) depends on T(i+1); T(n) depends on T(0) • thread holding at least one resource waits to acquire other resources held by other threads • resource only released voluntarily by the thread holding it after done using the resource • only one thread at a time can use a resource (True/False) • Deadlock implies starvation (that a thread waits indefinitely) • Starvation implies deadlock • To find general deadlocks, it is enough to look for loops in the resource allocation graph.