Deadlock Ezio Bartocci Institute for Computer Engineering

Transcription

1 TECHNISCHE UNIVERSITÄT WIEN Fakultät für Informatik Cyber-Physical Systems Group Deadlock Ezio Bartocci Institute for Computer Engineering

2 Deadlock Permanent blocking of a set of processes, which compete for a common resource or communicate with each other Cyclic resource conflict between two or more processes: 1. Each process holds one resource and 2. Waits on a resource that is hold by another process There is no universal solution for this problem

3 Example Roundabout Roundabout; car from right has priority Cars only drive forward Cars ó Processes Street segments ó Resources Deadlock: each car blocks a street segment an tries to move to the segment of the upcoming car

4 Example Processes Processes access common resources; Mutual Exclusion P1: Get B Get A Release B Release A... P2: Get A Get B Release B Release A... P3: Get A Release A Get B Release B...

5 Joint Progress Diagram P1 Release A Release B Get A Get B P3 Get A Release A Get B Release B

6 Joint Progress Diagram P1 Release A Release B Get A Get B Deadlock inevitable P2 Get A Get B Release B Release A

7 The 4 Deadlock-Conditions Three requirements for the system: 1. Mutual Exclusion Exclusive access to resources (on process holds the resource, no other process can use it) 2. Hold and Wait Process can hold resources while waiting for other resources 3. No Preemption Held resources cannot be forcibly removed from a process holding it

8 The 4 Deadlock-Conditions Sequence of events may occur that leads to: 4. Circular Wait Closed chain of processes, such that each process holds at least one resource needed by another process in the chain Process P1 requests held by Resource A Resource B held by requests Process P2

9 Deadlock-Conditions Deadlock = unresolvable circular wait Circular Wait cannot be resolved when conditions 1 to 3 are true ð Conditions 1 to 4 are necessary and sufficient for a deadlock

10 Treatment of Deadlocks Deadlock Prevention Avoid one of the four Deadlock-Conditions Deadlock Avoidance Resource reservation/allocation which may lead to a deadlock are not allowed/granted Deadlock Detection Resource allocation always allowed Periodic check if a deadlock exists and recovery from the deadlock if any

11 Indirect Deadlock Prevention Mutual Exclusion This is a goal; cannot be prevented Hold and Wait Processes can acquire all resources at once Blocks until all resources available Ø Long time waiting, delay of processes Ø Inefficient use of allocated resources Ø Process must know in advance what resources are needed

12 Indirect Deadlock Prevention No Preemption a) Process releases held resources, if the request for other resources is denied; requests all resources later again b) A request of a resource of one process may require another process to release the held/needed resource Applicable for resources, which state can be easily saved and restored if necessary (cf. processor state)

13 Direct Deadlock Prevention Protocol to avoid the Circular Wait Strict linear ordering O of resource types, e.g., O(Tape Drives) = 2, O(Disk Drives) = 4 First Requirement: process requests resources of type R i (all in one step) Next: process is only allowed to acquire resources of type R k such that O(R k ) > O(R i )

14 Prevention of Circular Wait ind. Proof: protocol avoids Circular Wait Assumption: there is a Circular Wait of P 0 P n s.t. for all i: P i holds R i and waits on R (i+1) mod n W.r.t. the protocol it is valid that: O(R 0 ) < O(R 1 ) < < O(R n ) < O(R 0 ) ð which is not possible according to the protocol The protocol prevents a deadlock Inefficiency: possibly unnecessarily denying resources due to the ordering

15 Deadlock Avoidance Conditions 1 to 3 allowed, selective assignment of resources Process Initiation Denial: a process is not started if its demands can lead to a deadlock Resource Allocation Denial: resource-demands will be refused if it can lead to a deadlock Higher parallelism than deadlock prevention Requirement: resource demands of process must be known

16 Deadlock Avoidance Notation n Processes m Resource categories Vectors Resource = (R 1, R 2,, R m ) Total amount of resources in the system Available = (V 1, V 2,, V m ) Resources that are not held/allocated to any process

17 Deadlock Avoidance Notation Matrices Claim = C 11, C 12,, C 1m C n1, C n2,, C nm C_ij.. requirement of process i for resource j Allocation = A 11, A 12,, A 1m A n1, A n2,, A nm A_ij.. current allocation to process i of resource j

18 Process Initiation Denial A process P n+1 is only started, if its resource demands cannot lead to a deadlock, that is: R i C (n+1)i + n C ki k=1 Disadvantage: assumes the worst that all processes will make their maximal claim together ð restricts parallelism

19 Resource Allocation Denial - Banker s Algorithm State: current allocation of resources to processes, characterized by vectors and matrices Safe State: at least one sequence of resource allocations exists that leads to no deadlock, i.e., all processes complete their execution (marked as finished ) Unsafe State: no such sequence exists

20 Banker s Algorithm - Notation Additional matrix Required = N 11, N 12,, N 1m N n1, N n2,, N nm N ki = C ki - A ki Before a resource allocation: check if allocation leads to a safe state Safe State ð assignment of resources?? Unsafe State ð no resource assignments??

21 Banker s Algorithm Initialization: mark all processes as unfinished Work vector W i = V i, for all i loop search P k with unfinished (P k ) and N ki <= W i for all i no process found ð goto END else ð mark processes as finished and release its resources: W i = W i + A ki for all i end loop END: all processes marked finished ð Safe State else ð Unsafe State

22 Banker s Alg. - Application Test, if current resource request Q ki of process P k shall be granted 1. Test: Q ki <= N ki for all i true ð next false ð error (requirement too high) 2. Test: Q ki <= V i for all i true ð next false ð wait (resource not available) 3. Tentative grant the demand and Check for Safe State (see continuation on next slide)

23 Banker s Alg. - Application Continuation 3. for all i: V i := V i - Q ki A ki := A ki + Q ki N ki := N ki - Q ki 4. Test: Is the current?? State a Safe State? true ð allocate resources Q ki to P k false ð delay the request to later

24 Banker s Algorithm - Example 3 resources, 4 processes Initial state: R = (9 3 6) C = A = V = (1 1 2)

25 Banker s Algorithm - Example Demand of P 2 : Q 2 = (1 0 1) C = A = V = (0 1 1) Safe State: sequence of execution P 2, P 1, P 3, P 4 allows completion of all processes (after execution of P 2 is W = (6 2 3) ) Resource assignment to P 2 granted

26 R: (9 3 6) V: (1 1 2) Q 2 : (1 0 1) Banker s 3 2 Algorithm Beispiel C: A: N: W: (0 1 1) C: A: N: W: (6 2 3) C: A: N: W: (7 2 3) C: A: N:

27 Banker s Algorithm - Beispiel2 Demand of P 1 : Q 1 = (1 0 1) C = A = V = (0 1 1) Unsafe State: each process needs at least one unit of resource R 1 No assignment of resources to P 1

28 R: (9 3 6) V: (1 1 2) Q 1 : (1 0 1) C: A: N: W: (0 1 1) C: A: N: Unsafe State!!!

29 Banker s Alg. - Bemerkungen Safe State ð no deadlock possible Unsafe State ð deadlock possible, but may not happen Processes do not need the resources during their overall execution time Maximal demands/requirements in the resource categories do not occur simultaneously Strategies assume independent processes for deadlock avoidance (no conditional synchronization between processes)

30 Deadlock Detection Requested resources are granted whenever it is possible Necessary OS arrangements: Algorithm for deadlock detection Algorithm for recovery from a deadlock Deadlock check, e.g., on every resource request ð CPU-intensive

31 Deadlock Detection Algorithm Mark deadlock-free processes Initialization: set all processes P k to unmarked mark all processes P k with A ki = 0 for all i set W i := V i for all i loop search unmarked P k mit Q ki <= W i for all i Test: exists such an unmarked process P k? true ð mark P k ; set W i := W i + A ki for all i false ð exit loop end loop -- unmarked processes are in a deadlock

32 Deadlock Detection - Example Q = A = V = ( ) mark P 4 (uses no resources) W:= ( ) Q 3i (demand of P 3 ) <= W i, therefore mark P3 and set W := W + ( ) à ( ) Algorithm terminates, returns deadlock of P 1 and P 2

33 Deadlock Detection - Notes Optimistic assumption: processes do not need additional resources for completion Violation of this assumptions may lead to a deadlock later on ð Will be detected at the next call of the Deadlock Detection Algorithm

34 Deadlock Recovery Resolve the detected deadlock Abort all processes in the deadlock chain (typically used) Rollback of all involved processes to a specific state (deadlock may occur again) Abort individual processes of the chain until the deadlock is resolved (repeatedly call the detection algorithm)

35 Deadlock Recovery (2) Resources are gradually withdrawn and assigned to other processes until the deadlock is resolved Reset processes which resources have been withdrawn to a point before the resource acquisition Selection of the process to withdraw a resource: Least CPU-time used? Least amount of acquired resources? Least progress?

36 Integrated Deadlock-Strategy Combine the approaches Group the resources into classes and order the classes - Swappable space (secundary memory) - Process resources (I/O-Geräte, Files, etc.) - Main memory Circular wait prevention between classes Use strategy that fits best for an individual class (e.g., main memory - prevention: preemption, process resources - avoidance)

37 Summary 4 Conditions: Mutual Exclusion Hold and Wait No Preemption Circular Wait Deadlock Prevention Deadlock Avoidance Deadlock Detection and Recovery