Thrashing

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

window.

§ if  too small , it will not encompass entire locality

§ if  too large ,it will encompass several localities

§ if  =   it will encompass entire program

D=WSSi

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

Prepaging

§ 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.