Consider a system that has three processes and three identical resources. Each process needs a maximum of two resources. Is deadlock possible in this system?
It it my understanding that deadlock is possible if the four conditions hold simultaneously:
Mutual exclusion, hold and wait, no pre-emption, and circular wait.
If each process is allocated one resource, then all three resources will be held. There is no forth resource available.
How do I go about proving that deadlock is not possible, and how would I calculate how many resources should be available to make the system deadlock free?
In such cases the trick is to evaluate the CIRCULAR WAIT condition and see if it holds or not. 3 Processes and 3 identical resources. Let us give 1 to each of them. 0 resources left, yet no process requirement is complete(as each needs 2),which means that every process is waiting for some other process to release the resources. Circular wait condition satisfied. Therefore the given scenario can lead to deadlock.
Suppose we have n processes and m identical resources with maximum demands as d1,d2,d3......dn.
If m > (d1-1) + (d2-1) + (d3-1)......(dn-1)., then its deadlock free,
otherwise it can lead to deadlock
Consider a system with m resources of the same type being shared by n
processes. Resources can be requested and released by processes only on at a
time. The system is deadlock free if and only if the sum of all max needs is < m+n.
For example:
A system has 3 processes sharing 4 resources. If each process needs a maximum of 2 units, then:
To make a system deadlock free, assign each process with one less then their max need. After doing so if we are left with one or more resources then there is no deadlock.
Assign 1 resource (max need -1) to each process.
allocated resources=1+1+1=3
we are still left with 1 resource to avoid deadlock.
so deadlock can never occur.
In Priority based scheduling, I came across the problem of Priority inversion which is a higher priority process is forced to wait for the lower priority task.
One possible scenario is, consider three process L,M,H with order of priority L < M < H .
L is running in CS ; H also needs to run in CS ; H waits for L to come out of CS ; M interrupts L and starts running ; M runs till completion and relinquishes control ; L resumes and starts running till the end of CS ; H enters CS and starts running.
Here, my question is, regarding the statement M interrupts L and starts running i.e., can a process executing in Critical section be interrupted or pre-empted.
Here, my question is, regarding the statement M interrupts L and starts running i.e., can a process executing in Critical section be interrupted or pre-empted.
It depends on how the critical section is implemented.
In operating system code you will frequently find critical sections implemented where interrupts are blocked. In this kind of implementation, a process will always execute the entire critical section without interruption.
In user code that uses critical sections implemented through system services, the process invariably can be interrupted. If the were not the case a process could take over the system by putting all its code in a critical section.
You are describing one of the reasons process priorities should be consistent. Unless you are doing real time processing or background batch processing, all processes should generally have the same base priority.
The old DECUS tapes used to be filled with "fair share" applications that would lower the priory of processes with high CPU usage and that would wreak havoc with system scheduling.
The answer is simple and yes.
If someother process with a higher priority in a preemptive system doesn't need to run in critical section, i.e. doesn't need to aquire a lock which is held by a lower priority process, then it can preempt the lower priority process regardless of what it is executing.
Even if M needs the CS, it will preempt L, run, get blocked and switched out for L to continue execution.
When multiple processors are working, the processes are working concurrently. Race condition happens when multiple threads accessing some common data area, one may overwrite the other value.
So, if it is a single processor and single core environment, can it prevent the race condition from happening?
Help me clarify this confusion, Thank you.
A race condition could happen in Single processor environment. As per Wiki Race Condition occurs when output is dependent on the sequence or timing of other uncontrollable events
Single processor environment could support multiple threads of the same process of different process that might be waiting for another thread to yield on a resource. Deadlocks can happen in single processor environments too.
Scenario:
T1: Wants add an employee record to file "employee.txt"
T2: Wants to compute average salary for "legal dept"
T3: Wants to remove an employee who left
T4: Wants to list number of employees working in each dept
If all the above threads are waiting at time=0 and submitted to single processor, it would decide which thread goes first, second and so on. The order in which the Threads are prioritised and yielded differs on different platform, scenarios etc. Thus T2 and T4 might not give consistent result.
this is a problem I faced in my operating system's exam. I could not figure out the right answer for it. Can someone help.Given is a code for synchronization where many threads are trying to access a global counter g using lock-
if(lock==1)
wait(); //sleep this thread until some other thread wakes up this thread
else
lock=1; //enter in protected area
//access global counter g//
lock=0;
//wake up some other thread which is waiting for the lock to be released
What is the problem in above synchronization? Choose anyone of the options given below
The synchronization is fine and will run correctly.
Will only run on uni-processor systems but not on multiprocessor systems.
Will not run on any system
Can’t say. Need more data
The answer is 3. This code fails both at safety and liveness as long as threads can be preempted. For safety, consider the following interleaving of operations with two threads t1 and t2:
t1 checks lock, skips to the else statement
OS preempts t1 and schedules t2
t2 checks lock, skips to the else statement
And we have two threads in the critical section. This is why you need some sort of atomic test-and-set operation, or the ability to disable preemption, to do it properly.
For liveness, consider the following interleaving of operations with two threads t1 and t2:
t1 checks lock, skips to the else statement
t1 sets lock to 1
OS preempts t1 and schedules t2
t2 checks lock, finds 1
OS preemtps t2 and schedules t1
t1 sets lock to 0
t1 finds no thread waiting and does nothing else
OS schedules t2 again
t2 starts waiting...
And thus t2 is (potentially) waiting forever. The solution is for the synchronization primitive to keep track of wake-ups (e.g., a semaphore) or require that testing the condition and waiting is done atomically (e.g., mutexes and condition variables).
I've heard the phrase 'priority inversion' in reference to development of operating systems.
What exactly is priority inversion?
What is the problem it's meant to solve, and how does it solve it?
Imagine three (3) tasks of different priority: tLow, tMed and tHigh. tLow and tHigh access the same critical resource at different times; tMed does its own thing.
tLow is running, tMed and tHigh are presently blocked (but not in critical section).
tLow comes along and enters the critical section.
tHigh unblocks and since it is the highest priority task in the system, it runs.
tHigh then attempts to enter the critical resource but blocks as tLow is in there.
tMed unblocks and since it is now the highest priority task in the system, it runs.
tHigh can not run until tLow gives up the resource. tLow can not run until tMed blocks or ends. The priority of the tasks has been inverted; tHigh though it has the highest priority is at the bottom of the execution chain.
To "solve" priority inversion, the priority of tLow must be bumped up to be at least as high as tHigh. Some may bump its priority to the highest possible priority level. Just as important as bumping up the priority level of tLow, is dropping the priority level of tLow at the appropriate time(s). Different systems will take different approaches.
When to drop the priority of tLow ...
No other tasks are blocked on any of the resources that tLow has. This may be due to timeouts or the releasing of resources.
No other tasks contributing to the raising the priority level of tLow are blocked on the resources that tLow has. This may be due to timeouts or the releasing of resources.
When there is a change in which tasks are waiting for the resource(s), drop the priority of tLow to match the priority of the highest priority level task blocked on its resource(s).
Method #2 is an improvement over method #1 in that it shortens the length of time that tLow has had its priority level bumped. Note that its priority level stays bumped at tHigh's priority level during this period.
Method #3 allows the priority level of tLow to step down in increments if necessary instead of in one all-or-nothing step.
Different systems will implement different methods depending upon what factors they consider important.
memory footprint
complexity
real time responsiveness
developer knowledge
Hope this helps.
Priority inversion is a problem, not a solution. The typical example is a low priority process acquiring a resource that a high priority process needs, and then being preempted by a medium priority process, so the high priority process is blocked on the resource while the medium priority one finishes (effectively being executed with a lower priority).
A rather famous example was the problem experienced by the Mars Pathfinder rover: http://www.cs.duke.edu/~carla/mars.html, it's a pretty interesting read.
Suppose an application has three threads:
Thread 1 has high priority.
Thread 2 has medium priority.
Thread 3 has low priority.
Let's assume that Thread 1 and Thread 3 share the same critical section code
Thread 1 and thread 2 are sleeping or blocked at the beginning of the example. Thread 3 runs and enters a critical section.
At that moment, thread 2 starts running, preempting thread 3 because thread 2 has a higher priority. So, thread 3 continues to own a critical section.
Later, thread 1 starts running, preempting thread 2. Thread 1 tries to enter the critical section that thread 3 owns, but because it is owned by another thread, thread 1 blocks, waiting for the critical section.
At that point, thread 2 starts running because it has a higher priority than thread 3 and thread 1 is not running. Thread 3 never releases the critical section that thread 1 is waiting for because thread 2 continues to run.
Therefore, the highest-priority thread in the system, thread 1, becomes blocked waiting for lower-priority threads to run.
It is the problem rather than the solution.
It describes the situation that when low-priority threads obtain locks during their work, high-priority threads will have to wait for them to finish (which might take especially long since they are low-priority). The inversion here is that the high-priority thread cannot continue until the low-priority thread does, so in effect it also has low priority now.
A common solution is to have the low-priority threads temporarily inherit the high priority of everyone who is waiting on locks they hold.
[ Assume, Low process = LP, Medium Process = MP, High process = HP ]
LP is executing a critical section. While entering the critical section, LP must have acquired a lock on some object, say OBJ.
LP is now inside the critical section.
Meanwhile, HP is created. Because of higher priority, CPU does a context switch, and HP is now executing (not the same critical section, but some other code). At some point during HP's execution, it needs a lock on the same OBJ (may or may not be on the same critical section), but the lock on OBJ is still held by LP, since it was pre-empted while executing the critical section. LP cannot relinquish now because the process is in READY state, not RUNNING. Now HP is moved to BLOCKED / WAITING state.
Now, MP comes in, and executes its own code. MP does not need a lock on OBJ, so it keeps executing normally. HP waits for LP to release lock, and LP waits for MP to finish executing so that LP can come back to RUNNING state (.. and execute and release lock). Only after LP has released lock can HP come back to READY (and then go to RUNNING by pre-empting the low priority tasks.)
So, effectively it means that until MP finishes, LP cannot execute and hence HP cannot execute. So, it seems like HP is waiting for MP, even though they are not directly related through any OBJ locks. -> Priority Inversion.
A solution to Priority Inversion is Priority Inheritance -
increase the priority of a process (A) to the maximum priority of any
other process waiting for any resource on which A has a resource lock.
Let me make it very simple and clear. (This answer is based on the answers above but presented in crisp way).
Say there is a resource R and 3 processes. L, M, H. where p(L) < p(M) < p(H) (where p(X) is priority of X).
Say
L starts executing first and catch holds on R. (exclusive access to R)
H comes later and also want exclusive access to R and since L is holding it, H has to wait.
M comes after H and it doesn't need R. And since M has got everything it wants to execute it forces L to leave as it has high priority compared to L. But H cannot do this as it has a resource locked by L which it needs for execution.
Now making the problem more clear, actually the M should wait for H to complete as p(H) > p(M) which didn't happen and this itself is the problem. If many processes such as M come along and don't allow the L to execute and release the lock H will never execute. Which can be hazardous in time critical applications
And for solutions refer the above answers :)
Priority inversion is where a lower priority process gets ahold of a resource that a higher priority process needs, preventing the higher priority process from proceeding till the resource is freed.
eg:
FileA needs to be accessed by Proc1 and Proc2.
Proc 1 has a higher priority than Proc2, but Proc2 manages to open FileA first.
Normally Proc1 would run maybe 10 times as often as Proc2, but won't be able to do anything because Proc2 is holding the file.
So what ends up happening is that Proc1 blocks until Proc2 finishes with FileA, essentially their priorities are 'inverted' while Proc2 holds FileA's handle.
As far as 'Solving a problem' goes, priority inversion is a problem in itself if it keeps happening.
The worst case (most operating systems won't let this happen though) is if Proc2 wasn't allowed to run until Proc1 had. This would cause the system to lock as Proc1 would keep getting assigned CPU time, and Proc2 will never get CPU time, so the file will never be released.
Priority inversion occurs as such:
Given processes H, M and L where the names stand for high, medium and low priorities,
only H and L share a common resource.
Say, L acquires the resource first and starts running. Since H also needs that resource, it enters the waiting queue.
M doesn't share the resource and can start to run, hence it does. When L is interrupted by any means, M takes the running state since it has higher priority and it is running on the instant that interrupt happens.
Although H has higher priority than M, since it is on the waiting queue, it cannot acquire the resource, implying a lower priority than even M.
After M finishes, L will again take over CPU causing H to wait the whole time.
Priority Inversion can be avoided if the blocked high priority thread transfers its high priority to the low priority thread that is holding onto the resource.
A scheduling challenge arises when a higher-priority process needs to read or modify kernel data that are currently being accessed by a lower-priority process—or a chain of lower-priority processes. Since kernel data are typically protected with a lock, the higher-priority process will have to wait for a lower-priority one to finish with the resource. The situation becomes more complicated if the lower-priority process is preempted in favor of another process with a higher priority. As an example, assume we have three processes—L, M, and H—whose priorities follow the order L < M < H. Assume that process H requires resource R,which is currently being accessed by process L.Ordinarily,process H would wait for L to finish using resource R. However, now suppose that process M becomes runnable, thereby preempting process L. Indirectly, a process with a lower priority—process M—has affected how long process H must wait for L to relinquish resource R. This problem is known as priority inversion.It occurs only in systems with more than two priorities,so one solution is to have only two priorities.That is insufficient for most general-purpose operating systems, however. Typically these systems solve the problem by implementing a priority-inheritance protocol. According to this protocol, all processes that are accessing resources needed by a higher-priority process inherit the higher priority until they are finished with the resources in question.When they are finished,their priorities revert to their original values. In the example above, a priority-inheritance protocol would allow process L to temporarily inherit the priority of process H,thereby preventing process M from preempting its execution. When process L had finished using resource R,it would relinquish its inherited priority from H and assume its original priority.Because resource R would now be available, process H—not M—would run next.
Reference :ABRAHAM SILBERSCHATZ
Consider a system with two processes,H with high priority and L with low priority. The scheduling rules are such that H is run whenever it is in ready state because of its high priority. At a certain moment, with L in its critical region, H becomes ready to run (e.g., an I/O operation completes). H now begins busy waiting, but since L is never scheduled while H is running, L never gets the chance to leave the critical section. So H loops forever.
This situation is called Priority Inversion. Because higher priority process is waiting on lower priority process.