Resource Management

ReusableDisk blocksFile descriptorsSemaphores

ConsumableMessagesPackets

Resource State

Available Total1 2+(1)+[1] 1 4Owns 2, request 1, remaining-claim 1

2 1Owns 1, cannot ask for any more

3 [2]Owns none, maximum claim is 2

Resource State Diagram Process 1 2 3Resource avail total

1 2+(1)+[1] 1 [2] 0 32 2 1+[1] 1+(1)+[1] 0 23 3 1+(1)[1] 1 0 2

3 RESOURCES 3 PROCESSESProcess 1 owns 2 units of R1, has requested

1 more with a remaining claim of 1 unit. There are no R1 units avail.

Deadlock Guarantees

No Effective Deadlock A request must be granted in finite time even if new requests are accepted.

No Deadlock A request must be granted in finite time assuming that no further requests are accepted.

Effective Deadlock Example

Process 1 2 3Resource avail total 1 1+[1] (2)+[2] 1+[1] 0 2

Process 1 and 3 alternate requests for 1 unit at a time. Process 2’s request for 2 units is never satisfied.

Necessary Conditions for Deadlock

(1) Processes claim exclusive control. (2) Processes hold resources while requesting

additional resources. (3) Resources cannot be forcibly removed

(preempted) from processes. (4) A circular chain of processes exist such that

each process holds one or more of the resources requested by its predecessor in the chain. If not, one of the processes would eventually receive a resource that it needed to complete.

Solution Classes

Prevention Static, works before programs execute

Avoidance Dynamic, check every request for safety Delay request if not safe

Detection/Recovery Check for “stuck” processes at intervals

Prevention

No exclusive control -- share everything No incremental requests

1. Collective requests2. Release all before requesting more

• Allow preemption• Havender’s Ordered Requests

• request a resource in class C(i) only if it holds no resources in any class C(j), j>=i.

• Dijkstra’s Minimal Progress

Havender Ordered Request Example(works for locks too!)

Class Resource Type 6 Printer

5 Disk 1

4 Tape Drives 1 and 2

3 Disk 2

2 Tape Drives 3 and 4

1 Memory

Avoidance - tasks required to provide maximum claim resource counts prior

to executionA request is granted only if it results in a

SAFE resource state.Requires Max Claim information.A safe execution sequence (SES) is an

execution schedule of processes (P1, P2, ... , Pn) such that for each P(i),

when it begins execution, its maximal claims can be satisfied by the available resources.

A Safe State

Process

1 2 3

Resource avail total

1 1+[4] 3+(1)+[4] 3+[3] 3 10

Safe Execution Sequences =(P3, P1, P2) (P3, P2, P1)

An Unsafe State

Process

1 2 3

Resource avail total

1 1+(1)+[4] 4+[3] 3+[3] 2 10

NO Safe Execution Sequence exists!

Banker’s Algorithm

On every request, search for a SES such that Every process can have its MAXIMUM CLAIM

satisfied in at least one execution order

Most Liberal because it gives away all resources if possible at the cost of execution order flexibility

Detection and RecoveryFor single-unit resources

(semaphores/files), a cycle is a necessary and sufficient condition for deadlock.

Detection and Recovery

Process

1 2 3 4

Resource available total

1 1+(1) 1 (1) 1 0 3

2 1+(1) 1 0 0 2

A cycle (P1 <-> P2) exists but no deadlock because P4 can terminate and give a resource back. Note: not single unit resources.

Recovery Options

Terminate operating systemTerminate all processes in a cycleTerminate one process in a cyclePreempt resources