Shared and Preemptable Resources. A few resources may be sharable. Anyone can write the system log.

Transcription

1 Shared and Preemptable Resources A few resources may be sharable Anyone can write the system log Deadlock Ch 3 Some resources are preemptable They can be safely taken away Memory may be preempted by swapping a job out to disk Deadlock generally involves exclusive, nonpreemptable resources 1 3 Deadlock Deadlock Requires Circular Wait When two or more processes are each waiting on resources held by another in such a way that none can progress until another one does A Process A holds Resource R S Process B waits for Resource S T D U R B C (a) (b) (c) 2 4

2 Two Processes, Two Resources This Order Deadlocks Process C Release U Release T Process D Release T Release U Process C Wait for T Process D Wait For U Order Matters 5 7 This Order Is Okay Possible Execution Orders Process C Release U Release T Process D Wait for T Release T Release U Printer Plotter I 8 I 7 I 6 I 5 B p r q Printer I 1 t s I 2 I 3 I 4 Plotter u (Both processes finished) A 6 8

3 More Complex Deadlock Dealing with Deadlock P1 P4 Res F Res E P7 Ignorance Hope it doesn t happen Detection and Recovery Detect and eliminate deadlocks as they occur P2 Res A Res C Res B P3 Res D P5 Res G P6 Dynamic avoidance Refuse risky allocations Prevention Eliminate one of the four conditions P8 Res H 9 11 Conditions for Deadlock All four conditions must hold for a deadlock to exist: Mutual Exclusion Hold and wait No preemption Circular wait Ignorance What if you just ignore the problem? Deadlocked processes will eventually be killed by whoever ran them Deadlock is rare If the frequency of deadlock is less than the frequency of power failure, or O/S hangs, it may not be worth the trouble to avoid This is actually the most common approach 10 12

4 Deadlock Detection Detect deadlock and eliminate Graph cycles work when there is only one resource of each type We use a model of system resources and allocations Invariant Every resource is allocated or available n C ij + A j = E j, for each j, 1 j m i= Deadlock Detection System state described by four matrices: Existing resources: (E 1, E 2,, E m ) Available (free) resources: (A 1, A 2,, A m ) Current Allocation: Current Request: C 11 C 12 C 1m C 21 C 22 C 2m C n1 C n2 C nm R 11 R 12 R 1m R 21 R 22 R 2m R n1 R n2 R nm 14 Deadlock Detection Algorithm For vectors, X Y means X i Y i for each i, 1 i m usable (A 1, A 2,, A m ); remaining {1, 2,, n} // All process ids while remaining repeat find some p remaining, such that R p usable if such a p exists usable usable + C p ; remaining remaining {p} else return remaining deadlocked; end; return no deadlock; 16

5 Deadlock Detection Algorithm There is some order of the processes such that each can: fill its outstanding request from the free pool finish and return its all its resources for use by the next Formally, there exists some order of the processes p 1, p 2,, p n, such that When to Run Detection After ever allocation Periodically When CPU is idle too often i 1 R pi A + C pj, j=1 1 i n Text Example No Deadlock E = (4, 2, 3, 1) A = (2, 1, 0, 0) C = R = Has Deadlock When R = Recovery Preempt resources Not usually possible Rollback a process Requires checkpoints Kill deadlocked processes until the deadlock vanishes Prefer ones that can be restarted Compiles are usually okay Database updates usually are not 18 20

6 Avoidance Avoid situations where there is a risk of deadlock System states are classified as safe or unsafe Safe states cannot deadlock Unsafe states might deadlock Refuse any allocation which enters an unsafe state Safe State Determination usable (A 1, A 2,, A m ); remaining {1, 2,, n} // All process ids while remaining repeat find some p remaining, such that M p C p usable if such a p exists usable usable + C p ; remaining remaining {p} else return unsafe; end; return safe; System State for Avoidance Maximum which might be wanted instead of current request Existing resources: (E 1, E 2,, E m ) Safe State Unsafe states are those which would deadlock if all processes asked for their maximum resources Available (free) resources: (A 1, A 2,, A m ) Current Allocation: Max Resources: C 11 C 12 C 1m C 21 C 22 C 2m C n1 C n2 C nm M 11 M 12 M 1m M 21 M 22 M 2m M n1 M n2 M nm 22 Safe when there is some order of the processes such that each can: reach its maximum its allocation from the free pool finish and return its resources for use by the next Formally, there exists some order of the processes p 1, p 2,, p n, such that M pi A + i C pj, j=1 1 i n 24

7 Allocation Procedure Action taken upon a resource request: If the new total would exceed the maximum claim, abort the process If the requested resources are not free, suspend the process Otherwise: Pretend to grant, and find the new state of the system If that state is safe, grant If not, suspend Banker s Algorithm Problems With Avoidance Requires processes to declare their maximum resources A bother Often a guess; often very wrong Usually exaggerated to avoid termination on overallocation Conservative: May deny allocations which do not actually lead to deadlock Expensive: Must run the algorithm for every resource request No one actually uses this algorithm Elegant Famous Too Expensive A Safe State E = (5, 11, 3, 2) A = (1, 2, 1, 1) C = M = Some Requests To Process Process 2 requests (0, 0, 0, 1) Process 3 requests (0, 2, 0, 0) Process 1 requests (0, 0, 1, 0) 26 Prevention Elimination of one condition of deadlock makes it impossible Mutual Exclusion Spooling does this; not generally applicable Hold and wait Allocate all resources at start Release and reallocate Twophase locking is a form of this No preemption Must be able to roll back processes Circular wait Allocation must be performed in a fixed order 28

8 Problems with Prevention Can force a process to allocate sooner than needed Low utilization of resources Preemption requires rollback Rollback requires checkpointing Policies Swap space: Allocate all at start (Prevention) Process resources: Avoidance Main memory: Preemption by swapping (Prevention) Internal resource: Ordering (Prevention) An Integrated Approach Group resources into a number of classes Allocate resources from these classes in a fixed order Handle deadlock as appropriate within each class Swap space Process resources: Devices and files Main memory Internal resource: I/O channels Sources Tanenbaum, Modern Operating Systems (Course textbook) The combined approach is due to J Howard, CACM July 1973, cited by Stallings in his Operating Systems 30 32