Operating Systems

Definition: A state is safe if there is an ordering of the processes such that: if the processes are run in this order, they will all terminate (assuming none exceeds its claim).

Recall the comparison made above between detecting deadlocks (with multi-unit resources and the banker's algorithm

• The deadlock detection algorithm given makes the most optimistic assumption about a running process: it will return all its resources and terminate normally. If we still find processes that remain blocked, they are deadlocked.
  
  
• In the bankers algorithm we make the most pessimistic assumption about a running process: it immediately asks for all the resources it can (details later on “can”). If, even with such demanding processes, the resource manager can assure that all process terminates, then we can assure that deadlock is avoided.

In the definition of a safe state no assumption is made about the running processes; that is, for a state to be safe termination must occur no matter what the processes do (providing the all terminate and to not exceed their claims).

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

1. Safe and deadlocked--not possible.
   
   
2. Safe and not deadlocked--trivial (e.g., no arcs).
   
   
3. Not safe and deadlocked--easy (any deadlocked state).
   
   
4. Not safe and not deadlocked--interesting.

Is the figure on the right safe or not?

• You can not tell until I give you the claims of the process.
  
  
• Please do not make the unfortunately common exam mistake to give an example involving safe states without giving the claims.
  
  
• For the figure on the right, if the claims are:
  P: 1 unit of R and 2 units of S (written (1,2))
  Q: 2 units of R and 1 units of S (written (2,1))
  the state is NOT safe.
  
  
• But if the claims are instead:
  P: 2 units of R and 1 units of S (written (2,1))
  Q: 1 unit of R and 2 units of S (written (1,2))
  the state IS safe.
  
  
• Explain why this is so.

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, which is part of Banker's Algorithms discovered by Dijkstra, to determine if the state is safe.

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 type).
   • 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?
     Ans: 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

Start with example 1 and 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. We must find a sequence of all the processes.

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

The algorithm is simple: Stay in safe states. Initially, we assume all the processes are present before execution begins and that all claims are given before execution begins. We will relax these assumptions very soon.

• Before execution begins, check that the system is safe. That is, check that no process claims more than the manager has). If not, then the offending process is trying to claim more of some resource than exist in the system has and hence cannot be guaranteed to complete even if run by itself. You might say that it can become deadlocked all by itself.
  
  
• 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 (politely thanks the process and then) checks to see if “the first” pending requests can be granted (i.e., if the result would now be safe). If so the request is granted. The manager checks to see if the next pending request can be granted, etc..

Homework: 13.

3.5.4: 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.

Limitations of the banker's algorithm

• Often users don't know the maximum requests a process will make. They can estimate conservatively (i.e., use big numbers for the claim) but then the manager becomes very conservative.
  
  
• New processes arriving cause a problem (but not so bad as Tanenbaum suggests).
  • The process's claim must be less than the total number of units of the resource in the system. If not, the process is not accepted by the manager.
  • Since the state without the new process is safe, so is the state with the new process! Just use the order you had originally and put the new process at the end.
  • Insuring fairness (starvation freedom) needs a little more work, but isn't too hard either (once an hour stop taking new processes until all current processes finish).
  
  
• A resource becoming unavailable (e.g., a tape drive breaking), can result in an unsafe state.

Homework: 21, 27, and 20. There is an interesting typo in 20: A has claimed 3 units of resource 5, but there are only 2 units in the entire system. Change the problem by having B both claim and be allocated 1 unit of resource 5.

3.7: Other Issues

3.7.1: Two-phase locking

This is covered (MUCH better) in a database text. We will skip it.

3.7.2: Non-resource deadlocks

You can get deadlock from semaphores as well as resources. This is trivial. Semaphores can be considered resources. P(S) is request S and V(S) is release S. The manager is the module implementing P and V. When the manager returns from P(S), it has granted the resource S.

3.7.3: Starvation

As usual FCFS is a good cure. Often this is done by priority aging and picking the highest priority process to get the resource. Also can periodically stop accepting new processes until all old ones get their resources.

3.8: Research on Deadlocks

Skipped.

3.9: Summary

Read.