Concurrency: Principles of Deadlock. Processes and resources. Concurrency and deadlocks. Operating Systems Fall Processes need resources to run

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Gerard Hart
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1 Concurrency: Principles of Deadlock Operating Systems Fall 2002 Processes and resources Processes need resources to run CPU, memory, disk, etc process waiting for a resource cannot complete its execution until the resource becomes available There is only a finite amount of resources E.g., 1 CPU, 1 GB memory, 2 disks Concurrency and deadlocks In a multiprogramming system the total resource demand by all concurrently active processes exceeds by far the total amount of available resources Processes compete for resources process can grab the last instance of a resource and wait for the resource B nother process may hold B and wait for No one can proceed: Deadlock

2 Deadlock Permanent blocking of a set of processes that either compete for system resources or communicate with each other Involves conflicting needs for resources by two or more processes There is no satisfactory solution in the general case Deadlock in the everyday life Deadlock in the everyday life

3 Deadlock when contending for the critical section Process do { P i flag[ i] = true; while( flag[1 i]); criticalsection flag[ i] = false; remainder section } while(1) : Shared : boolean flag[2] Example of Deadlock Progress of Q Release Release B 1 2 P and Q want B Get Get B deadlock 3 inevitable 4 P and Q want B 5 6 Get Get B Release Release B Progress of P B Example of No Deadlock Progress of Q Release Release B Get P and Q want P and Q want B 4 B Get B 5 6 Get Release Get B Release B Progress of P B

4 Resource categories Reusable: Used by one process at a time and not depleted by that use can be reused by other processes,may exist several instances Processors, memory, disks, tapes, etc. Consumable: Created (produced) and destroyed (consumed) by a process Interrupts, signals, messages, and information in I/O buffers Reusable resources and Deadlock Deadlock might occur if each process holds one resource and requests the other E.g., Space is available for allocation of 200K Request 80K bytes; Request 60K bytes; Request 70K bytes; Request 80K bytes; Consumable resources and Deadlock Example: Deadlock occurs if receive is blocking Receive(); Send(); Receive(); Send();

5 Conditions for Deadlock Policy conditions Mutual exclusion Hold-and-wait No preemption Circular wait Requests Resource Held by Process Process Held By Resource B Requests Conditions for Deadlock Mutual exclusion Hold-and-wait No preemption Circular wait DEDLOCK Circular Wait R1 R2 R3

6 No circular wait R1 R2 R3 Coping with Deadlocks Deadlock prevention Deadlock possibility is excluded a priori by the system design Deadlock avoidance Deadlocks are possible in principle but avoided Deadlock detection Deadlocks can occur: detect and solve the problem Deadlock prevention Design system so that it violates one of the four necessary conditions Prevent hold and wait: request all the resources at the outset wait until all the resources are available Prevent circular wait by defining linear ordering of the resource types process holding some resources can request only resource types with higher numbers

7 Preventing circular wait R1 R2 R3 Deadlock prevention: Cons Degraded performance Delayed execution Low parallelism Hold and wait prevention is wasteful Hold resources more than they are needed When might this be reasonable? Deadlock avoidance llocate resources in a way that assures that the deadlock point is never reached The allocation decision is made dynamically based on total amount of resources available currently available processes resource claim processes current resources allocation

8 Banker s algorithm (Dijkstra 65 ) Do not grant an incremental resource request to a process is this allocation might lead to deadlock The system state: is the current allocation of resources to processes Safe state: is a state in which there is at least one sequence in which all processes can be run to completion Unsafe state = NOT safe state Determination of the safe state We have 3 resources types with amount: R(1) = 9, R(2) = 3, R(3) = 6 Is the state S0 below safe? Claim llocated Total vailable Determination of the safe state Claim llocated Total vailable Claim llocated Total vailable 7 2 3

9 Determination of the safe state Claim llocated Total vailable Claim llocated Total S0 is safe: ->->-> vailable Banker s algorithm When a process request resources: ssume the request is granted Update the system state accordingly Determine whether the resulting state is safe If yes: grant the resources Otherwise, block the process until it is safe to grant the resources Banker s algorithm Claim llocated Total vailable requests (1, 0, 1): Grant or not? requests (1, 0, 1): Grant or not?

10 Deadlock detection Banker s algorithm is Pessimistic: always assume that a process will not release the resources until it got m all decreased parallelism Involves complicated checks for each resource allocation request (O(n^2)) Optimistic approach: don t do any checks When deadlock occurs - detect and recover Detection: look for circular waits Practice Most operating systems employ an ostrich algorithm Break hold-and-wait Cannot acquire a resource - fail back to the user: e.g., too many processes, too many open files Quotas Programming discipline: acquire locks (semaphores) in a specific order Dining philosophers problem

11 Dining philosophers problem n abstract problem demonstrating some fundamental limitations of the deadlockfree synchronization There is no symmetric solution Solutions execute different code for odd/even give m another fork allow at most 4 philosophers at the table Randomized (Lehmann-Rabin) Concurrency: summary Critical section is an abstract problem for studying concurrency and synchronization software solutions hardware primitives higher level primitives: semaphores, monitors Deadlocks are inherent to concurrency 4 conditions 3 ways to cope with Next: Memory management

Deadlock. Concurrency: Deadlock and Starvation. Reusable Resources

Deadlock. Concurrency: Deadlock and Starvation. Reusable Resources Concurrency: Deadlock and Starvation Chapter 6 Deadlock Permanent blocking of a set of processes that either compete for system resources or communicate with each other No efficient solution Involve conflicting