1. June 11, 2002Serguei A. Mokhov, mokhov@cs.concordia.ca 1 Deadlock COMP346 - Operating Systems Tutorial 5 Edition 1.1, June 15, 2002

2. June 11, 2002Serguei 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

3. June 11, 2002Serguei A. Mokhov, mokhov@cs.concordia.ca 3 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. R1 P1P2 R2

4. June 11, 2002Serguei 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: P 1  P 2  P 3  …  P N  P 1

5. June 11, 2002Serguei A. Mokhov, mokhov@cs.concordia.ca 5 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? P1P2P3 ?

6. June 11, 2002Serguei 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

7. June 11, 2002Serguei A. Mokhov, mokhov@cs.concordia.ca 7 Handling Deadlocks Deadlock Ignorance Deadlock Prevention Deadlock Avoidance Deadlock Detection and Recovery

8. June 11, 2002Serguei 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.

9. June 11, 2002Serguei 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

10. June 11, 2002Serguei 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

11. June 11, 2002Serguei 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

12. June 11, 2002Serguei 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 P1 wait(sem1) wait(sem2) … signal(sem1) signal(sem2) P2 wait(sem2) wait(sem1) … signal(sem2) signal(sem1)

13. June 11, 2002Serguei 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: process A { process B { wait(mutex) wait(mutex)...... wait(synch) signal(synch)...... wait(synch)......... signal(mutex) signal (mutex) } For this case assume: mutex = 1 synch = 0 Try starting processes in any order and see where they block.

14. June 11, 2002Serguei A. Mokhov, mokhov@cs.concordia.ca 14 Deadlocks and Starvation with Semaphores (3) Yet another example :-) a)semaphore S = -2; P1 {P2 {P3 { V(S) V(S) V(S) P(S) P(S) P(S) }} } b)semaphore S = -2, S1 = 0, S2 = 0; P1 {P2 {P3 { V(S) V(S) V(S) P(S) P(S) P(S) V(S1) P(S1) P(S2) V(S2) }

15. June 11, 2002Serguei 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.

16. June 11, 2002Serguei A. Mokhov, mokhov@cs.concordia.ca 16 Deadlocks and Starvation with Semaphores (5) Starvation scenario (in b): 1) P2: 2) V(S) (S = -1) 3) P(S) (blocked->switch to P3) 4) P3: 5) V(S) (S = 0) 6) P(S) (blocked->switch to P1) 7) P1: 8) V(S) (S = 1) 9) P(S) (S = 0) 10) V(S1) (S1 = 1) 11) 12) terminates 13) ??? Deadlock Scenario: 1) P1: 2) V(S) (S = -1) 3) P(S) (blocked->switch to P3) 4) P3: 5) V(S) (S = 0) 6) P(S) (blocked->switch to P2) 7) P2: 8) V(S) (S = 1) 9) P(S1) (blocked) 10) ???