Page 148 - DCAP103_Principle of operating system

Unit 4: Process Management-III

deadlock. This determination is a policy decision, similar to CPU-scheduling problems. The Notes
question is basically an economic one; we should abort those processes the termination of which
will incur the minimum cost. Unfortunately, the term minimum cost is not a precise one. Many
factors may determine which process is chosen, including:
1. What the priority of the process is?
2. How long the process has computed, and how much longer the process will compute
before completing its designated task?
3. How many and what type of resources the process has used (for example, whether the
resources are simple to preempt)?
4. How many more resources the process needs in order to complete?
5. How many processes will need to be terminated?
6. Whether the process is interactive or batch?
4.6.23 Resource Preemption

To eliminate deadlocks using resource preemption, we successively preempt some resources
from processes and give these resources to other processes until the deadlock cycle is broken.
If preemption is required to deal with deadlocks, then three issues need to be addressed:
1. Selecting a Victim: Which resources and which processes are to be preempted? As in
process termination, we must determine the order of pre-1 emption to minimize cost.
Cost factors may include such parameters as the number of resources a deadlock process
is holding, and the amount of time a deadlocked process has thus far consumed during
its execution.
2. Rollback: If we preempt a resource from a process, what should be done with that process?
Clearly, it cannot continue with its normal execution; it is missing some needed resource.
We must roll back the process to some safe state, and restart it from that state. Since, in
general, it is difficult to determine what a safe state is, them simplest solution is a total
rollback: Abort the process and then restart it. However, it is more effective to rollback
the process only as far as necessary to break the deadlock. On the other hand, this method
requires the system to keep more information about the state of all the running processes.
3. Starvation: How do we ensure that starvation will not occur? That is, how can we guarantee
that resources will not always be preempted from the same process? In a system where
victim selection is based primarily on cost factors, it may happen that the same process is
always picked as a victim. As a result, this process never completes its designated task, a
starvation situation that needs to be dealt with in any practical system. Clearly, we must
ensure that a process can be picked as a victim only a (small) finite number of times. The
most common solution is to include the number of rollbacks in the cost factor.

Write Code that Acquires More Than One Lock

Lock *l1, *l2;
void p() {

l1->Acquire();
l2->Acquire();
code that manipulates data that l1 and l2 protect
l2->Release();
Contd...

LOVELY PROFESSIONAL UNIVERSITY 141