Thrashing and Causes of Thrashing in Operating System

Thrashing in os, causes of thrashing, working set model, what is thrashing, thrashing in operating system

What is thrashing ?

Today I am going to introduce about thrashing in os. So what is thrashing in os ? Thrashing is a state which occurs when a process is spending more time in paging or swapping than its execution. In thrashing state CPU is so much busy in swapping that it can not respond to user program as much as it required.

Causes of Thrashing

Thrashing is not good from performance point of view. Now I will tell about the causes of thrashing , actually Initially when the CPU utilization is low, then process scheduling mechanism, loads multiple processes into the memory at the same time in order to increase the degree of multi programming.
                                            
So now in this situation we have more number of processes in memory as compare to the available number of frames in memory. So allocating a limited amount of frames to each process.

When any higher priority process arrive in memory and if frame is not freely available at that time then the other process that occupied the frame which is resides in frame will move to secondary storage and this free frame is now allocated to higher priority process 



                            
causes of thrashing



In other words we can say that as the memory fills up, process starts to spend a lot of time for the required pages to be swapped in, again leading to low CPU utilization because most of the processes are waiting for pages.

How to overcome thrashing?

In previous section we found the causes of thrashing. Now in this section we will see how to eliminate thrashing. Here we will study about the technique or methodology used to reduce the thrashing. In order to prevent thrashing in operating system at first we need to know that how many frames as they really is needed by a process at any time. There is a technique known as working-set model which is used to reduce the causes of thrashing in operating system.

Thrashing technique starts by looking at how frames a process is actually using. This approach defines the locality Model of process execution. According to locality model when a process executes, it moves from locality to locality. Here the term locality represents a set of pages that are actively used together.

Working set model in thrashing

Now I will discuss about working set model explanation which is a technique to reduce the thrashing in os. Working set model  is based on assumption of Locality. This model used a parameter Δ defines the working-set window. 

The idea is to examine the most recent Δ page references. Working set is a set of pages available in the most recent Δ  or sometime also known as  an approximation of the program's locality.

If the page is in active use it will be in the working set. If it is no longer being used it will dropped from the working set Δ time units after its last reference. So working set is an approximation of program’s locality.

Working set model example

In this section we will understand the concept of  working set model with the help of an example consider the sequence of memory references shown in following Figure.



how to eliminate thrashing
If 6 = 10 memory references, then the working set at time t1 is {1, 2, 5,6, 7}. By time t2, the working set has changed to {3, 4}.

The accuracy of the working set depends on the selection of 6. Because assume that 6 is too small,then it will not encompass the whole locality; but if 6 is too large, then it may overlap several localities.

The most important property of the working set, then, is its size. If we compute the working-set size, WSS for each process in the system, we can then consider that where D is the total demand for frames. Each process is actively using the pages in its working set. Thus, process i needs WSS; frames.

If the total demand is greater than the total number of available frames (D > m), then it will cause thrashing to occur, because in this case some processes will not have enough frames. Once . has been selected, use of the working-set model is simple.

Here the role of operating system is that the operating system monitors the working set of each process and after that it allocates to that working set enough frames to provide it with its working-set size.

If  we have sufficient extra frames, then another process can be initiated. Here it is important to remember that If the sum of the working-set sizes increases and it is exceeding the total number of available frames then operating system selects a process to suspend.

The process's pages are swapped out, and its frames are reallocated to other processes and the suspended process can be restarted later.

Conclusion

So as a conclusion we can say that  working-set model prevents thrashing in os while keeping the degree of multi programming as high as possible. In this way it optimizes CPU utilization.

Main problem with the working-set model in os is keeping track of the working set. Because working-set window is a moving window and at each memory reference, a new reference appears at one end and the oldest reference drops off the other end.A page is in the working set if it is referenced anywhere in the working-set window.

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Computer Science Junction: Thrashing and Causes of Thrashing in Operating System
Thrashing and Causes of Thrashing in Operating System
Thrashing in os, causes of thrashing, working set model, what is thrashing, thrashing in operating system
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