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Thrashing In Operating System

 Thrashing is a condition or a situation when the system is spending a major period of time on servicing the page faults, but the actaul processing done is very negligible.
   "A process is said to be thrashing if it is spending more tim in servicing page fault than executing."

The basic concept involved is that if a process is allocated too few frames, then there will be too many and too few frequent page faults. As a result, no useful work would be done by the CPU and CPU utilization would fall drastically. The long term schedular would then try to improve the CPU utilization by loading some more processes into the memory thereby increasing degree of multiprogramming.

This would further decrease in the CPU utilization trigerring a chain reaction of higher page faults followed by an increase in the degree of multiprogramming called thrashing.
In the above diagram, initially degree of multiprogramming upto some extent of point ( sau lamda), the CPU utilization is very high and the system resource are utilized 100%. But if we further increase the degree of multiprogramming CPU utilization will drastically fall down and the system will spent more time only in the page replacement and time taken to complete execution of process will increase. This is the situation in system is called thrashing.

CAUSE OF THRASHING :-
1. High degree of multiprogramming :-
     It the number of processes keeps on increasing in the memory then number of frames alloted to each process will be decreased. So, less number frames will be available to each process. Due to this, page fault will occur more frequently and more CPU time will be wasted in just swapping in ans out of pages and the utilization will keep on decreasing.

2. Lack of frames :-
    If a process has less number of frames then less pages of that process willbe able to reside in memory and hence more frequent swapping in and out will be required. Hence sufficient amount of frames must be allocated to each process in order to prevent thrashing.

RECOVERY FROM THRASHING :-
  Do not allow system to go int thrashing by instructing long term schedular not to bring the processes into memory after the threshold.

If the system is already in thrashing then instruct the mid term schedular to suspend some of the processes so that we can recover the system from thrashing.


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