Deadlock and Starvation

Transcription

1 Deadlock and Starvation

2 Deadlock Permanent blocking of a set of processes that either compete for system resources or communicate with each other Involve conflicting needs for resources by two or more processes No efficient solution 2

3 Reusable resources Description: Used by one process at a time and not depleted by that use Processes obtain resources that they later release for reuse by other processes Example: Processor time, I/O channels, main and secondary memory, files, databases, and semaphores, printers Deadlock occurs if each process holds one resource and requests another that is held by another process 3

4 Preemptible resources It can be taken away from the owning process with no ill effects Can be restored to the process at a later time Examples: memory processor Cannot cause deadlock 4

5 Nonpreemptible resources It can not be taken away from the owning process without adversely affecting its computation The resource must be voluntarily released by the process Examples: printer tape disk Semaphores Can cause deadlock 5

6 Bridge crossing example On bridge traffic can go in only one direction. Each section of a bridge can be viewed as a resource. If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). Several cars may have to be backed up if a deadlock occurs. Starvation is possible. 6

7 Consumable resources Description: Created (produced) and destroyed (consumed) by a process Examples: Interrupts, signals, messages, and information in I/O buffers Deadlock: May occur if a Receive message is blocking May take a rare combination of events to cause deadlock 7

8 Consumable resource example Deadlock occurs if receive is blocking... Receive(P2);... P1 Send(P2, M1);... Receive(P1);... P2 Send(P1, M2); 8

9 System model - reusable resources Resource types R 1, R 2,..., R m CPU cycles, memory space, I/O devices Each resource type R i has W i instances Each process utilizes a resource as follows: request use release 9

10 Necessary conditions for deadlock Mutual exclusion Only one process may use a resource at a time Hold-and-wait A process holding one resource is waiting to acquire additional resources held by other processes No preemption No resource can be forcibly removed form a process holding it 10

11 Deadlocked condition Circular wait There exists a permanent, circular sequence of processes such that each process is waiting for a resource held by the preceding process 11

12 Resource-Allocation Graph (RAG) A set of Vertices V and a set of Edges E V is partitioned into two types: P = {P 1, P 2,, P n }, the set consisting of all the processes in the system. R = {R 1, R 2,, R m }, the set consisting of all resource types in the system. request edge directed edge P i R j assignment edge directed edge R j P i 12

13 RAG example with a deadlock Q113

14 Cycle but no deadlock 14

15 Basic facts If graph contains no cycles no deadlock. If graph contains a cycle if only one instance per resource type, then deadlock. if several instances per resource type, possibility of deadlock. Q215

16 Approaches to deadlock handling Ignore Deadlock (Ostrich approach) If infrequent enough and result is not serious Deadlock Prevention Prevent one of the necessary/sufficient conditions Deadlock Avoidance Allow the 3 necessary conditions Dynamically make choices to avoid deadlock decide based on knowledge of future requests i.e., find a safe path 16

17 Approaches to deadlock handling Deadlock Detection Periodically run algorithm to detect circular waiting After detecting deadlock, run a recovery algorithm to remove deadlock 17

18 Ostrich approach Deadlocks are annoyances that occur rarely Simply ignore them It is not a reasonable strategy for applications that involve life and death situations, e.g., mission critical applications or business critical applications Used by most operating systems, including Windows and UNIX 18

19 Deadlock prevention Restrain ways a request can be made Direct method: prevent circular wait from occurring Indirect method: prevent any one of the three necessary conditions from occurring Mutual Exclusion Hold and Wait No preemption 19

20 Deadlock prevention: indirect method Mutual Exclusion Not required for sharable resources Must hold for non-sharable resources Never Hold-and-Wait Must guarantee that whenever a process requests a resource, it does not hold any other resources. Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. Low resource utilization; starvation possible. 20

21 Deadlock prevention: indirect method Allow Preemption If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released. Preempted resources are added to the list of resources for which the process is waiting. Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. 21

22 Deadlock prevention: direct method Prevent Circular Wait impose a total ordering of all resource types require that each process requests resources in an increasing order of enumeration 22