Resource Allocation Graphs

Recall (System) Resource Allocation Graphs (SRAGs).
    They can be used to demonstrate deadlock.

Now consider some larger SRAGs:

This SRAG shows processes P0, P1, P3, P4, and P5, with (single-instance) resources R0, R1, R3, R4, R5, and R6.

A request edge, P_i --> R_j, means that the process P_i needs the resource R_j.

An allocation edge, R_j --> P_i, means that the process P_i has been allocated the resource R_j.

A cycle in a Resource Allocation Graph suggests that there might be circular wait in the system, i.e. deadlock.

However, this may happen without deadlock being present.

Examine the SRAG above.

• Is there a cycle?
• Is the system in a safe state?
• Is the system deadlocked?

This SRAG shows processes P1, P3, P4, P5, and P6, with (single-instance) resources R1 and R4, along with with (double-instance) resources R0, R5, P6, and R7.

In other words, there are two each of resources R0, R5, P6, and R7.

Examine this SRAG.

• Is there a cycle?
• Is the system in a safe state?
• Is the system deadlocked?

Resources for SRAGs can have any number of instances.

Here's one more example of a SRAG with multiple resource instances.

Again, examine this SRAG.

• Is there a cycle?
• Is the system in a safe state?
• Is the system deadlocked?

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