Prerequisite – Virtual Memory
Thrashing is a condition or a situation when the system is spending a major portion of its time in servicing the page faults, but the actual processing done is very negligible.
The basic concept involved is that if a process is allocated too few frames, then there will be too many and too frequent page faults. As a result, no useful work would be done by the CPU and the CPU utilisation would fall drastically. The long-term scheduler would then try to improve the CPU utilisation by loading some more processes into the memory thereby increasing the degree of multiprogramming. This would result in a further decrease in the CPU utilization triggering a chained reaction of higher page faults followed by an increase in the degree of multiprogramming, called Thrashing.
Locality Model –
A locality is a set of pages that are actively used together. The locality model states that as a process executes, it moves from one locality to another. A program is generally composed of several different localities which may overlap.
For example when a function is called, it defines a new locality where memory references are made to the instructions of the function call, it’s local and global variables, etc. Similarly, when the function is exited, the process leaves this locality.
Techniques to handle:
- Working Set Model –
This model is based on the above-stated concept of the Locality Model.
The basic principle states that if we allocate enough frames to a process to accommodate its current locality, it will only fault whenever it moves to some new locality. But if the allocated frames are lesser than the size of the current locality, the process is bound to thrash.
According to this model, based on a parameter A, the working set is defined as the set of pages in the most recent ‘A’ page references. Hence, all the actively used pages would always end up being a part of the working set.
The accuracy of the working set is dependant on the value of parameter A. If A is too large, then working sets may overlap. On the other hand, for smaller values of A, the locality might not be covered entirely.
If D is the total demand for frames and is the working set size for a process i,
Now, if ‘m’ is the number of frames available in the memory, there are 2 possibilities:
- (i) D>m i.e. total demand exceeds the number of frames, then thrashing will occur as some processes would not get enough frames.
- (ii) D<=m, then there would be no thrashing.
If there are enough extra frames, then some more processes can be loaded in the memory. On the other hand, if the summation of working set sizes exceeds the availability of frames, then some of the processes have to be suspended(swapped out of memory).
This technique prevents thrashing along with ensuring the highest degree of multiprogramming possible. Thus, it optimizes CPU utilisation.
- Page Fault Frequency –
A more direct approach to handle thrashing is the one that uses Page-Fault Frequency concept.
The problem associated with Thrashing is the high page fault rate and thus, the concept here is to control the page fault rate.
If the page fault rate is too high, it indicates that the process has too few frames allocated to it. On the contrary, a low page fault rate indicates that the process has too many frames.
Upper and lower limits can be established on the desired page fault rate as shown in the diagram.
If the page fault rate falls below the lower limit, frames can be removed from the process. Similarly, if the page fault rate exceeds the upper limit, more number of frames can be allocated to the process.
In other words, the graphical state of the system should be kept limited to the rectangular region formed in the given diagram.
Here too, if the page fault rate is high with no free frames, then some of the processes can be suspended and frames allocated to them can be reallocated to other processes. The suspended processes can then be restarted later.