6.5.2: Safe States

Avoiding deadlocks given some extra knowledge.

• Not surprisingly, the resource manager knows how many units of each resource it had to begin with.
• Also it knows how many units of each resource it has given to each process
• It would be great to see all the programs in advance and thus know all future requests, but that is asking for too much.
• Instead, each process when it starts gives its maximum usage. That is each process at startup states, for each resource, the maximum number of units it can possibly ask for. This is called the claim of the process.
  • If during the run the process asks for more than its claim, abort it.
  • If it claims more than it needs, the result is that the resource manager will be more conservative than need be and there will be more waiting.

Give an example of all four possibilities. A state that is

1. Safe and deadlocked
2. Safe and not deadlocked
3. Not safe and deadlocked
4. Not safe and not deadlocked

A manager can determine if a state is safe.

• Since the manager know all the claims, it can determine the maximum amount of additional resources each process can request.
• The manager knows how many units of each resource it has left.

The manager then follows the following procedure called Banker's Algorithms discovered by Dijkstra

1. If there are no processes remaining, the state is safe.
   
   
2. Seek a process P whose max additional requests is less than what remains (for each resource).
   • If no such process can be found, then the state is not safe.
   • The banker (manager) knows that if it refuses all requests excepts those from P, then it will be able to satisfy all of P's requests. Why?
     Look at how P was chosen.
   
   
3. The banker now pretends that P has terminated (since the banker knows that it can guarantee this will happen). Hence the banker pretends that all of P's currently held resources are returned. This makes the banker richer and hence perhaps a process that was not eligible to be chosen as P previously, can now be chosen.
   
   
4. Repeat these steps.

Example 1

• One resource type R with 22 unit
• Three processes X, Y, and Z with claims 3, 11, and 19 respectively.
• Currently the processes have 1, 5, and 10 units respectively.
• Hence the manager currently has 6 units left.
• Also note that the max additional needs for processes are 2, 6, 9
• So the manager cannot assure (with its current remaining supply of 6 units that Z can terminate. But that is not the question.
• This state is safe
  1. Use 2 units to satisfy X; now the manager has 7 units.
  2. Use 6 units to satisfy Y; now the manager has 12 units.
  3. Use 9 units to satisfy Z; done!

Example 2

Assume that Z now requests 2 units and we grant them.

• Currently the processes have 1, 5, and 12 units respectively.
• The manager has 4 units.
• The max additional needs are 2, 6, and 7.
• This state is unsafe
  1. Use 2 unit to satisfy X; now the manager has 5 units.
  2. Y needs 6 and Z needs 7 so we can't guarantee satisfying either
• Note that we were able to find a process that can terminate (X) but then we were stuck. So it is not enough to find one process. Must find a sequence of all the processes.

6.5.3: The Banker's Algorithm (Dijkstra) for a Single Resource

The algorithm is simple: Stay in safe states.

• Check before any process starts that the state is safe (this means that no process claims more than the manager has). If not, then this process is trying to claim more than the system has so cannot be run.
• When the manager receives a request, it pretends to grant it and checks if the resulting state is safe. If it is safe the request is granted, if not the process is blocked.
• When a resource is returned, the manager checks to see if any of the pending requests can be granted (i.e., if the result would now be safe). If so the request is granted and the manager checks to see if another can be granted.

6.5.4: The Banker's Algorithm for Multiple Resources

At a high level the algorithm is identical: Stay in safe states.

• What is a safe state?
• The same definition (if processes are run in a certain order they will all terminate).
• Checking for safety is the same idea as above. The difference is that to tell if there enough free resources for a processes to terminate, the manager must check that for all resources, the number of free units is at least equal to the max additional need of the process.