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Chapter: Operating Systems : Storage Management


High paging activity is called thrashing.



ü High paging activity is called thrashing.


ü If a process does not have enough pages, the page-fault rate is very high. 

ü This leads to:

o low CPU utilization

o operating system thinks that it needs to increase the degree of multiprogramming

o another process is added to the system

 When the CPU utilization is low, the OS increases the degree of multiprogramming.

 If global replacement is used then as processes enter the main memory they tend to steal frames belonging to other processes.

 Eventually all processes will not have enough frames and hence the page fault rate becomes very high.

 Thus swapping in and swapping out of pages only takes place. This is the cause of thrashing.



ü To limit thrashing, we can use a local replacement algorithm.


ü To prevent thrashing, there are two methods namely ,


·       Working Set Strategy


·       Page Fault Frequency


1. Working-Set Strategy


v It is based on the assumption of the model of locality.


v Locality is defined as the set of pages actively used together.


v Working set is the set of pages in the most recent   page references is the working set




§ if  too small , it will not encompass entire locality


§ if  too large ,it will encompass several localities


§ if  =   it will encompass entire program



SSi is the working set size for process i.


   D is the total demand of frames


ü if D > m then Thrashing will occur.


2. Page-Fault Frequency Scheme


v If actual rate too low, process loses frame


v If actual rate too high, process gains frame.




Other Issues




§ To  reduce the large number of  page faults that  occurs at  process startup


§  Prepage all or some of the pages a process will need, before they are referenced


§  But if prepaged pages are unused, I/O and memory are wasted


Page Size


Page size selection must take into consideration:


·        fragmentation


·        table size


·        I/O overhead


·        locality


TLB Reach


• TLB Reach - The amount of memory accessible from the TLB


• TLB Reach = (TLB Size) X (Page Size)


• Ideally, the working set of each process is stored in the TLB.


• Otherwise there is a high degree of page faults.


• Increase the Page Size. This may lead to an increase in fragmentation as not all applications require a large page size


• Provide Multiple Page Sizes. This allows applications that require larger page sizes the opportunity to use them without an increase in fragmentation.


I/O interlock


 Pages must sometimes be locked into memory

 Consider I/O. Pages that are used for copying a file from a device must be locked from being selected for eviction by a page replacement algorithm.

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Operating Systems : Storage Management : Thrashing |

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