1. Introduction
Deadlock prevention eliminates the possibility of deadlock by imposing strict restrictions on resource allocation. While effective, this approach often leads to poor resource utilization and reduced system flexibility.
Deadlock Avoidance takes a more intelligent approach.
Instead of permanently restricting process behavior, the operating system dynamically evaluates each resource request and grants it only if doing so keeps the system in a safe state.
The fundamental goal of deadlock avoidance is:
To ensure that the system never enters a state from which deadlock could potentially occur.
Unlike prevention, avoidance allows all four deadlock conditions to exist. However, it carefully controls resource allocation so that dangerous states are never reached.
This approach achieves a better balance between safety and efficiency, though it requires additional computation and information about process requirements.
2. Core Idea
The basic principle of deadlock avoidance is simple:
Before granting a resource request, the operating system asks:
"If this request is granted,
will the system still remain safe?"
If the answer is:
YES
the request is granted.
If the answer is:
NO
the request is delayed or denied.
Conceptually:
Resource Request
↓
Safety Check
↓
Safe State ?
↓ ↓
Yes No
↓ ↓
Grant Delay
Thus, every allocation decision is based on future system safety rather than immediate availability.
3. Safe State vs Unsafe State
The concepts of safe state and unsafe state form the foundation of deadlock avoidance.
Without understanding these concepts, avoidance algorithms such as Banker’s Algorithm cannot be understood.
3.1 Safe State
Definition
A system is said to be in a safe state if there exists at least one sequence of process execution such that every process can complete successfully.
This sequence is called a:
Safe Sequence
A safe sequence guarantees that all processes can eventually obtain the resources they need and finish execution.
Example
Suppose:
P1 → P2 → P3
is a possible execution order.
If:
P1 can finish
Then release resources
Allow P2 to finish
Then release resources
Allow P3 to finish
then:
Safe Sequence Exists
Therefore:
System is Safe
Important Observation
A safe state does not mean every process can execute immediately.
It only means:
There Exists At Least One Order
In Which All Processes Can Finish
3.2 Unsafe State
Definition
A system is in an unsafe state if no guaranteed safe sequence can be found.
In such a state:
Future Completion Cannot Be Guaranteed
Important Insight
Many students confuse unsafe state with deadlock.
This is incorrect.
Unsafe State ≠ Deadlock
Instead:
Unsafe State
↓
May Lead To
↓
Deadlock
An unsafe state simply means the system is moving into a risky condition.
Example
Suppose available resources become extremely limited and no safe execution order can be determined.
The system may still continue normally.
However:
Future Requests
could eventually create a deadlock.
Therefore:
Unsafe State = Warning Zone
not necessarily deadlock itself.
4. Relationship Between Safe, Unsafe, and Deadlock States
The relationship can be visualized as:
Safe States
|
|
V
Unsafe States
|
|
V
Deadlock States
Important observations:
Every deadlocked state is unsafe.
Not every unsafe state is deadlocked.
Safe states guarantee completion.
Unsafe states provide no guarantee.
This distinction is one of the most frequently tested interview and examination concepts.
5. Key Insight of Deadlock Avoidance
The operating system does not try to eliminate deadlock conditions.
Instead, it ensures:
Never Enter Unsafe State
Because:
Unsafe State
↓
Potential Deadlock
Avoidance algorithms continuously perform safety analysis before every allocation.
Thus:
Safe State Maintained
↓
Deadlock Avoided
6. How Deadlock Avoidance Works
Whenever a process requests resources:
Step 1: Pretend Allocation
The OS temporarily assumes:
Request Granted
and updates resource information.
Step 2: Perform Safety Check
The system checks:
Can All Processes Still Finish?
Step 3: Decision
If Safe
Allocation Confirmed
If Unsafe
Rollback Allocation
and the process waits.
Conceptually:
Request
↓
Temporary Allocation
↓
Safety Test
↓
Safe ?
↓ ↓
Yes No
↓ ↓
Grant Rollback
This trial-allocation approach is the core mechanism behind avoidance algorithms.
7. Requirements for Deadlock Avoidance
Deadlock avoidance requires significantly more information than prevention.
The operating system must know:
7.1 Maximum Resource Requirement
Each process must declare:
Maximum Resources Needed
before execution.
Example:
P1 may need at most:
3 Printers
5 Tape Drives
2 Scanners
7.2 Current Allocation
The OS tracks:
Resources Already Assigned
to each process.
7.3 Available Resources
The OS maintains:
Resources Currently Free
in the system.
Without this information:
Safety Analysis Impossible
8. Resource Allocation Model
Deadlock avoidance relies on a formal resource allocation model.
For every process, the OS maintains:
Maximum Need
Maximum[i]
Resources process i may request.
Allocation
Allocation[i]
Resources currently assigned.
Remaining Need
Need[i]
Computed as:
Need = Maximum − Allocation
Available
Available
Resources currently free.
These data structures form the foundation of avoidance algorithms such as Banker’s Algorithm.
9. Algorithms Used for Deadlock Avoidance
Two major techniques are used.
9.1 Resource Allocation Graph (RAG)
Used when:
Single Instance Per Resource Type
The graph includes:
Processes
Resources
Allocation edges
Request edges
Claim edges
The OS grants requests only if doing so does not create an unsafe situation.
9.2 Banker’s Algorithm (Most Important)
Used when:
Multiple Resource Instances Exist
The algorithm:
Simulates allocation
Performs safety analysis
Grants resources only if the resulting state remains safe
This is the most important deadlock avoidance algorithm and is heavily asked in exams and interviews.
10. Comparison with Deadlock Prevention
| Feature | Deadlock Prevention | Deadlock Avoidance |
|---|---|---|
| Strategy | Break conditions | Avoid unsafe states |
| Resource Allocation | Restricted | Flexible |
| Safety Check | Not required | Required |
| Flexibility | Low | High |
| Utilization | Lower | Better |
| Complexity | Lower | Higher |
| Runtime Overhead | Low | High |
Key Difference
Prevention says:
Never Allow Dangerous Behavior
Avoidance says:
Allow It
Only If Safe
11. Advantages of Deadlock Avoidance
11.1 Better Resource Utilization
Resources are allocated more efficiently than in prevention.
Less Resource Wastage
11.2 Greater Flexibility
Processes are not forced into strict allocation rules.
11.3 Higher Concurrency
More processes can execute simultaneously.
11.4 Deadlock-Free Execution
The system remains safe throughout execution.
12. Disadvantages of Deadlock Avoidance
12.1 Requires Advance Knowledge
Processes must declare:
Maximum Future Need
before execution.
In many systems:
Future Resource Usage Unknown
making avoidance difficult.
12.2 High Computational Overhead
Every request requires:
Safety Analysis
which consumes CPU time.
12.3 Not Suitable for Dynamic Systems
Modern applications often:
Create resources dynamically
Spawn threads dynamically
Change behavior unpredictably
This makes avoidance difficult to implement.
12.4 Increased Complexity
Maintaining:
Need matrix
Allocation matrix
Available resources
Safety checks
adds significant complexity to the operating system.
13. Real-World Analogy
Consider a bank issuing loans.
Before approving a new loan, the bank checks:
Will Enough Funds Remain
For All Customers
To Eventually Receive Their Money?
If yes:
Loan Approved
If no:
Loan Rejected
The bank does not wait for bankruptcy to happen.
Instead, it prevents risky situations beforehand.
This is exactly how deadlock avoidance works.