Operating Systems | Segmentation

A Memory Management technique in which memory is divided into variable sized chunks which can be allocated to processes. Each chunk is called a Segment.

A table stores the information about all such segments and is called Segment Table.

Segment Table: It maps two dimensional Logical address into one dimensional Physical address.

It’s each table entry has

  • Base Address: It contains the starting physical address where the segments reside in memory.
  • Limit: It specifies the length of the segment.

Translation of Two dimensional Logical Address to one dimensional Physical Address.

Address generated by the CPU is divided into:

  • Segment number (s): Number of bits required to represent the segment.
  • Segment offset (d): Number of bits required to represent the size of the segment.

Advantages of Segmentation:

  • No Internal fragmentation.
  • Segment Table consumes less space in comparison to Page table in paging.

Disadvantage of Segmentation:

  • As processes are loaded and removed from the memory, the free memory space is broken into little pieces, causing External fragmentation.

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Deadlock Prevention And Avoidance

Deadlock Characteristics
Deadlock has following characteristics.

Mutual Exclusion.
Hold and Wait.
No preemption.
Circular wait.

Deadlock Prevention

We can prevent Deadlock by eliminating any of the above four condition.

Eliminate Mutual Exclusion 
It is not possible to dis-satisfy the mutual exclusion because some resources, such as the tap drive and printer, are inherently non-shareable.

Eliminate Hold and wait
1. Allocate all required resources to the process before start of its execution, this way hold and wait condition is eliminated but it will lead to low device utilization. for example, if a process requires printer at a later time and we have allocated printer before the start of its execution printer will remained blocked till it has completed its execution.

2. Process will make new request for resources after releasing the current set of resources. This solution may lead to starvation.

Eliminate No Preemption
Preempt resources from process when resources required by other high priority process.

Eliminate Circular Wait
Each resource will be assigned with a numerical number. A process can request for the resources only in increasing order of numbering.
For Example, if P1 process is allocated R5 resources, now next time if P1 ask for R4, R3 lesser than R5 such request will not be granted, only request for resources more than R5 will be granted.

Deadlock Avoidance

Deadlock avoidance can be done with Banker’s Algorithm.

Banker’s Algorithm

Bankers’s Algorithm is resource allocation and deadlock avoidance algorithm which test all the request made by processes for resources, it check for safe state, if after granting request system remains in the safe state it allows the request and if their is no safe state it don’t allow the request made by the process.

Inputs to Banker’s Algorithm
1. Max need of resources by each process.
2. Currently allocated resources by each process.
3. Max free available resources in the system.

Request will only be granted under below condition.
1. If request made by process is less than equal to max need to that process.

2. If request made by process is less than equal to freely availbale resource in the system.

Example

Total resources in system:
A B C D
6 5 7 6
Available system resources are:
A B C D
3 1 1 2
Processes (currently allocated resources):
    A B C D
P1  1 2 2 1
P2  1 0 3 3
P3  1 2 1 0
Processes (maximum resources):
    A B C D
P1  3 3 2 2
P2  1 2 3 4
P3  1 3 5 0
Need = maximum resources - currently allocated resources.
Processes (need resources):
    A B C D
P1  2 1 0 1
P2  0 2 0 1
P3  0 1 4 0

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Operating System | Paging

Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory. This scheme permits the physical address space of a process to be non — contiguous.

  • Logical Address or Virtual Address (represented in bits): An address generated by the CPU
  • Logical Address Space or Virtual Address Space( represented in words or bytes): The set of all logical addresses generated by a program
  • Physical Address (represented in bits): An address actually available on memory unit
  • Physical Address Space (represented in words or bytes): The set of all physical addresses corresponding to the logical addresses

The mapping from virtual to physical address is done by the memory management unit (MMU) which is a hardware device and this mapping is known as paging technique.

  • The Physical Address Space is conceptually divided into a number of fixed-size blocks, called frames.
  • The Logical address Space is also splitted into fixed-size blocks, called pages.
  • Page Size = Frame Size

Let us consider an example:

  • Physical Address = 12 bits, then Physical Address Space = 4 K words
  • Logical Address = 13 bits, then Logical Address Space = 8 K words
  • Page size = frame size = 1 K words (assumption)

Address generated by CPU is divided into

  • Page number(p): Number of bits required to represent the pages in Logical Address Space or Page number
  • Page offset(d): Number of bits required to represent particular word in a page or page size of Logical Address Space or word number of a page or page offset.

Physical Address is divided into

  • Frame number(f): Number of bits required to represent the frame of Physical Address Space or Frame number.
  • Frame offset(d): Number of bits required to represent particular word in a frame or frame size of Physical Address Space or word number of a frame or frame offset.

The hardware implementation of page table can be done by using dedicated registers. But the usage of register for the page table is satisfactory only if page table is small. If page table contain large number of entries then we can use TLB(translation Look-aside buffer), a special, small, fast look up hardware cache.

  • The TLB is associative, high speed memory.
  • Each entry in TLB consists of two parts: a tag and a value.
  • When this memory is used, then an item is compared with all tags simultaneously.If the item is found, then corresponding value is returned.

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Deadlock Detection And Recovery

In the previous post, we have discussed Deadlock Prevention and Avoidance. In this post, Deadlock Detection and Recovery technique to handle deadlock is discussed.

Deadlock Detection

1. If resources have single instance:
In this case for Deadlock detection we can run an algorithm to check for cycle in the Resource Allocation Graph. Presence of cycle in the graph is the sufficient condition for deadlock.

In the above diagram, resource 1 and resource 2 have single instances. There is a cycle R1–>P1–>R2–>P2. So Deadlock is Confirmed.

2. If there are multiple instances of resources:
Detection of cycle is necessary but not sufficient condition for deadlock detection, in this case system may or may not be in deadlock varies according to different situations.

Deadlock Recovery
Traditional operating system such as Windows doesn’t deal with deadlock recovery as it is time and space consuming process. Real time operating systems use Deadlock recovery.

Recovery method

1. Killing the process.

killing all the process involved in deadlock.
     
     Killing process one by one. After killing each 
     process check for deadlock again keep repeating 
     process till system recover from deadlock.

2. Resource Preemption
Resources are preempted from the processes involved in deadlock, preempted resources are allocated to other processes, so that their is a possibility of recovering the system from deadlock. In this case system go into starvation.

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Operating System | Process Management | Deadlock Introduction

A process in operating systems uses different resources and uses resources in following way.
1) Requests a resource
2) Use the resource
2) Releases the resource

Deadlock is a situation where a set of processes are blocked because each process is holding a resource and waiting for another resource acquired by some other process.
Consider an example when two trains are coming toward each other on same track and there is only one track, none of the trains can move once they are in front of each other. Similar situation occurs in operating systems when there are two or more processes hold some resources and wait for resources held by other(s). For example, in the below diagram, Process 1 is holding Resource 1 and waiting for resource 2 which is acquired by process 2, and process 2 is waiting for resource 1.

Deadlock can arise if following four conditions hold simultaneously (Necessary Conditions) 
Mutual Exclusion: One or more than one resource are non-sharable (Only one process can use at a time)
Hold and Wait: A process is holding at least one resource and waiting for resources.
No Preemption: A resource cannot be taken from a process unless the process releases the resource.
Circular Wait: A set of processes are waiting for each other in circular form.

Methods for handling deadlock
There are three ways to handle deadlock
1) Deadlock prevention or avoidance: The idea is to not let the system into deadlock state.

2) Deadlock detection and recovery: Let deadlock occur, then do preemption to handle it once occurred.

3) Ignore the problem all together: If deadlock is very rare, then let it happen and reboot the system. This is the approach that both Windows and UNIX take.

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Operating System | Process Synchronization | Introduction

On the basis of synchronization, processes are categorized as one of the following two types:

  • Independent Process : Execution of one process does not affects the execution of other processes.
  • Cooperative Process : Execution of one process affects the execution of other processes.

Process synchronization problem arises in the case of Cooperative process also because resources are shared in Cooperative processes.

Critical Section Problem

Critical section is a code segment that can be accessed by only one process at a time. Critical section contains shared variables which need to be synchronized to maintain consistency of data variables.

do{

entry section

critical section

exit section

remainder section

}while(TRUE);

In the entry section, the process requests for entry in the Critical Section.

Any solution to the critical section problem must satisfy three requirements:

  • Mutual Exclusion : If a process is executing in its critical section, then no other process is allowed to execute in the critical section.
  • Progress : If no process is in the critical section, then no other process from outside can block it from entering the critical section.
  • Bounded Waiting : A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted.

Peterson’s Solution
Peterson’s Solution is a classical software based solution to the critical section problem.

In Peterson’s solution, we have two shared variables:

  • boolean flag[i] :Initialized to FALSE, initially no one is interested in entering the critical section
  • int turn : The process whose turn is to enter the critical section.

do{

flag[i]=TRUE;

turn=j;

while(flag[i] && turn==j);

critical section

flag[i]=FALSE;

remainder section

}while(TRUE);

Peterson’s Solution preserves all three conditions :

  • Mutual Exclusion is assured as only one process can access the critical section at any time.
  • Progress is also assured, as a process outside the critical section does not blocks other processes from entering the critical section.
  • Bounded Waiting is preserved as every process gets a fair chance.
  • It involves Busy waiting
  • It is limited to 2 processes.

Binary Semaphores : They can only be either 0 or 1. They are also known as mutex locks, as the locks can provide mutual exclusion. All the processes can share the same mutex semaphore that is initialized to 1. Then, a process has to wait until the lock becomes 0. Then, the process can make the mutex semaphore 1 and start its critical section. When it completes its critical section, it can reset the value of mutex semaphore to 0 and some other process can enter its critical section.

Counting Semaphores : They can have any value and are not restricted over a certain domain. They can be used to control access a resource that has a limitation on the number of simultaneous accesses. The semaphore can be initialized to the number of instances of the resource. Whenever a process wants to use that resource, it checks if the number of remaining instances is more than zero, i.e., the process has an instance available. Then, the process can enter its critical section thereby decreasing the value of the counting semaphore by 1. After the process is over with the use of the instance of the resource, it can leave the critical section thereby adding 1 to the number of available instances of the resource.

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Operating System | Critical Section

Critical Section:

In simple terms a critical section is group of instructions/statements or region of code that need to be executed atomically, such as accessing a resource (file, input or output port, global data, etc.).

In concurrent programming, if one thread tries to change the value of shared data at the same time as another thread tries to read the value (i.e. data race across threads), the result is unpredictable.

The access to such shared variable (shared memory, shared files, shared port, etc…) to be synchronized. Few programming languages have built in support for synchronization.

It is critical to understand the importance of race condition while writing kernel mode programming (a device driver, kernel thread, etc.). since the programmer can directly access and modifying kernel data structures.

A simple solution to critical section can be thought as shown below,

acquireLock();
Process Critical Section
releaseLock();

A thread must acquire a lock prior to executing critical section. The lock can be acquired by only one thread. There are various ways to implement locks in the above pseudo code.

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Operating System | Process Management | CPU Scheduling

Scheduling of processes/work is done to finish the work on time.

Below are different time with respect to a process.

Arrival Time:       Time at which the process arrives in the ready queue.
Completion Time:    Time at which process completes its execution.
Burst Time:         Time required by a process for CPU execution.
Turn Around Time:   Time Difference between completion time and arrival time.          
     Turn Around Time = Completion Time - Arrival Time
Waiting Time(W.T): Time Difference between turn around time and burst time.
     Waiting Time = Turn Around Time - Burst Time

Why do we need scheduling?
A typical process involves both I/O time and CPU time. In a uniprogramming system like MS-DOS, time spent waiting for I/O is wasted and CPU is free during this time. In multiprogramming systems, one process can use CPU while another is waiting for I/O. This is possible only with process scheduling.

Objectives of Process Scheduling Algorithm

Max CPU utilization [Keep CPU as busy as possible]
Fair allocation of CPU.
Max throughput [Number of processes that complete their execution per time unit]
Min turnaround time [Time taken by a process to finish execution]
Min waiting time [Time a process waits in ready queue]
Min response time [Time when a process produces first response]

Different Scheduling Algorithms

First Come First Serve (FCFS): Simplest scheduling algorithm that schedules according to arrival times of processes.

Shortest Job First(SJF): Process which have the shortest burst time are scheduled first.

Shortest Remaining Time First(SRTF): It is preemptive mode of SJF algorithm in which jobs are schedule according to shortest remaining time.

Round Robin Scheduling: Each process is assigned a fixed time in cyclic way.

Priority Based scheduling (Non Preemptive): In this scheduling, processes are scheduled according to their priorities, i.e., highest priority process is schedule first. If priorities of two processes match, then schedule according to arrival time.

Highest Response Ratio Next (HRRN) In this scheduling, processes with highest response ratio is scheduled. This algorithm avoids starvation.

Response Ratio = (Waiting Time + Burst time) / Burst time

Multilevel Queue Scheduling: According to the priority of process, processes are placed in the different queues. Generally high priority process are placed in the top level queue. Only after completion of processes from top level queue, lower level queued processes are scheduled.

Multi level Feedback Queue Scheduling: It allows the process to move in between queues. The idea is to separate processes according to the characteristics of their CPU bursts. If a process uses too much CPU time, it is moved to a lower-priority queue.

Some useful facts about Scheduling Algorithms:
1) FCFS can cause long waiting times, especially when the first job takes too much CPU time.

2) Both SJF and Shortest Remaining time first algorithms may cause starvation. Consider a situation when long process is there in ready queue and shorter processes keep coming.

3) If time quantum for Round Robin scheduling is very large, then it behaves same as FCFS scheduling.

4) SJF is optimal in terms of average waiting time for a given set of processes,i.e., average waiting time is minimum with this scheduling, but problems is, how to know/predict time of next job.

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Operating System | Process Scheduler

There are three types of process scheduler.

1. Long Term or job scheduler It bring the new process to the ‘Ready State’. It controls Degree of Multi-programming, i.e., number of process present in ready state at any point of time.

2. Short term ot CPU scheduler: It is responsible for selecting one process from ready state for scheduling it on the running state. Note: Short term scheduler only selects the process to schedule it doesn’t load the process on running.
Dispatcher is responsible for loading the selected process by Short Term scheduler on the CPU (Ready to Running State) Context switching is done by dispatcher only. A dispatcher does following:
1) Switching context.
2) Switching to user mode.
3) Jumping to the proper location in the newly loaded program.

3. Medium term scheduler It is responsible for suspending and resuming the process. It mainly does swapping (moving processes from main memory to disk and vice versa).

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Largest Sum contiguous subarray

Largest Sum Contiguous Subarray Write an efficient C program to find the sum of contiguous subarray within a one-dimensional array of numbers which has the largest sum. Kadane’s Algorithm: Initialize: max_so_far = 0 max_ending_here = 0 Loop for each element of the array (a) max_ending_here = max_ending_here + a[i] (b) if(max_ending_here < 0) max_ending_here =

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