Deadlock

1. Deadlock COMP346 - Operating Systems Tutorial 5 Edition 1.1, June 15, 2002 Serguei A. Mokhov, mokhov@cs.concordia.ca

2. Topics • Deadlock Debrief • Simplest Example • Necessary Deadlock Conditions • Resource Allocation Graph • Deadlock Handling • Deadlock and Starvation with Semaphores • Examples Serguei A. Mokhov, mokhov@cs.concordia.ca

3. R2 P1 P2 R1 Deadlock • Simplest example: • Two processes require two resources to complete (and release the resources) • There are only two instances of these resources • If they acquire one resource each, they block indefinitely waiting for each other to release the other resource => • DEADLOCK, one of the biggest problems in multiprogramming. Serguei A. Mokhov, mokhov@cs.concordia.ca

4. Necessary Deadlock Conditions • Recall which conditions must hold (p. 245): • ME: at least one process exclusively uses a resource • Hold and wait: a process possesses at least one resources and requires more, which are held by others • No preemption: resources are released only in voluntary manner by processes holding them • Circular wait: P1P2 P3 …  PN P1 Serguei A. Mokhov, mokhov@cs.concordia.ca

5. • • • • ? P1 P2 P3 Resource-Allocation Graph • An easy way to illustrate resource allocation and visually detect the deadlock situation(s) • Example: • N+1 resources • N processes • Every process needs 2 resources • Upon acquiring 2 resources, a process releases them • Is there a deadlock? Serguei A. Mokhov, mokhov@cs.concordia.ca

6. More Examples • Example from Salsa Theory Review • Resource Allocation Graph • Is the following true or false? • Deadlock cannot happen in a system with two processes. • One CPU • Two CPUs Serguei A. Mokhov, mokhov@cs.concordia.ca

7. Handling Deadlocks • Deadlock Ignorance • Deadlock Prevention • Deadlock Avoidance • Deadlock Detection and Recovery Serguei A. Mokhov, mokhov@cs.concordia.ca

8. Deadlock Ignorance • As system designers we may pretend that deadlocks don’t happen :-). • If they do happen, they don’t happen very often. • Most common approach because it’s cheap (in terms of human labor, complexity of the system and system’s performance). • Highest resource utilization. • Sysadmin can simply kill deadlocked processes or restart the system if it’s not responding. Serguei A. Mokhov, mokhov@cs.concordia.ca

9. Deadlock Prevention • Recall necessarily deadlock conditions • Deadlock prevention algorithms assure at least one these conditions do not hold, thus preventing occurrence of the deadlock. • Possible Disadvantages: • Low device utilization • Reduced system’s throughput • [1] p. 250 Serguei A. Mokhov, mokhov@cs.concordia.ca

10. Deadlock Avoidance • Unlike in deadlock prevention, deadlock avoidance algorithms do not watch for necessary deadlock conditions, but use additional a priori info about future resource requests. • This info will help the system to avoid entering the unsafe state. • Disadvantages: again, lower resource utilization (because resources may not be granted sometimes even if they are unused). • [1] P. 253 Serguei A. Mokhov, mokhov@cs.concordia.ca

11. Deadlock Detection and Recovery • Instead of preventing or avoiding deadlocks we allow them to happen and also provide mechanisms to recover. • Problems: • How often do we run detection algorithms? • How do we recover? • What is the cost of detection and recovery? • [1] P. 260, p. 264 Serguei A. Mokhov, mokhov@cs.concordia.ca

12. Deadlocks and Starvation with Semaphores • Semaphores act like resources in this case, which can be acquired and (never) released. • One example as book suggests: • [1] P. 204 Serguei A. Mokhov, mokhov@cs.concordia.ca

13. Deadlocks and Starvation with Semaphores (2) • Starvation, or indefinite blocking, is when a process is waiting on a semaphore where nobody is ever going to release it. • Another example of both deadlock and starvation: For this case assume: mutex = 1 synch = 0 Try starting processes in any order and see where they block. process A { process B { wait(mutex) wait(mutex) ... ... wait(synch) signal(synch) ... ... wait(synch) ... ... ... signal(mutex) signal (mutex) } } Serguei A. Mokhov, mokhov@cs.concordia.ca

14. Deadlocks and Starvation with Semaphores (3) • Yet another example :-) a) semaphore S = -2; P1 { P2 { P3 { <phase1> <phase1> <phase1> V(S) V(S) V(S) P(S) P(S) P(S) <phase2> <phase2> <phase2> } } } b) semaphore S = -2, S1 = 0, S2 = 0; P1 { P2 { P3 { <phase1> <phase1> <phase1> V(S) V(S) V(S) P(S) P(S) P(S) V(S1) P(S1) P(S2) <phase2> V(S2) <phase2> } <phase2> } } Serguei A. Mokhov, mokhov@cs.concordia.ca

15. Deadlocks and Starvation with Semaphores (4) • Things to note: • Assume the P2, P3, P1 order in the starvation example; • Assume the P1, P3, P2 order in the deadlock example; • Blocking means getting out from the running state to the waiting state, so a context switch to the next process in the queue happens; • In these example there are no multiple instances of these processes. Serguei A. Mokhov, mokhov@cs.concordia.ca

16. Starvation scenario (in b): 1) P2: <phase1> 2) V(S) (S = -1) 3) P(S) (blocked->switch to P3) 4) P3: <phase1> 5) V(S) (S = 0) 6) P(S) (blocked->switch to P1) 7) P1: <phase1> 8) V(S) (S = 1) 9) P(S) (S = 0) 10) V(S1) (S1 = 1) 11) <phase2> 12) terminates 13) ??? Deadlock Scenario: 1) P1: <phase1> 2) V(S) (S = -1) 3) P(S) (blocked->switch to P3) 4) P3: <phase1> 5) V(S) (S = 0) 6) P(S) (blocked->switch to P2) 7) P2: <phase1> 8) V(S) (S = 1) 9) P(S1) (blocked) 10) ??? Deadlocks and Starvation with Semaphores (5) Serguei A. Mokhov, mokhov@cs.concordia.ca