Page 142 - DCAP403_Operating System

Unit 7: Memory Management

virtual memory is the single-user system. Consider a single-user system with 128 KB memory Notes
composed of pages of size 1 KB. Thus, there are 128 frames. The operating system may take 35
KB, leaving 93 frames for the user process. Under pure demand paging, all 93 frames would
initially be put on the free-frame list. When a user process started execution, it would generate

a sequence of page faults. The first 93 page faults would all get free frames from the free-frame
list. When the free-frame list was exhausted, a page replacement algorithm would be used to
select one of the 93 in-memory pages to be replaced with the ninety-fourth, and so on. When the
process terminated, the 93 frames would once again be placed on the free-frame list.
There are many variations on this simple strategy. You can require that the operating system
allocate all its buffer and table space from the free-frame list. When this space is not in use by the
operating system, it can be used to support user paging. You can try to keep three free frames
reserved on the free-frame list at all times. Thus, when a page fault occurs, there is a free frame
available to page into. While the page swap is taking place, a replacement can be selected, which
is then written to the disk as the user process continues to execute.
Other variants are also possible, but the basic strategy is clear. The user process is allocated any
free frame.

Task Least frequency is calculated throughout the reference stream, and its value can
be calculated in a various ways. Discuss those ways.

7.12 Thrashing

Thrashing happens when a hard drive has to move its heads over the swap area many times
due to the high number of page faults. This happens when memory accesses are causing page
faults as the memory is not located in main memory. The thrashing happens as memory pages
are swapped out to disk only to be paged in again soon afterwards. Instead of memory access
happening mainly in main memory, access is mainly to disk causing the processes to become
slow as disk access is required for many memory pages and thus thrashing.
The OS can reduce the effects of thrashing and improve performance by choosing a more suitable
replacement strategy for pages. Having a replacement strategy that does not cause memory

areas to be written to disk that have not modified since been retrieved reduces thrashing. Using
replacement strategies that allow little used rarely accessed pages to remain in memory while the
most required pages are swapped in and out.
Thrashing is a situation where large amounts of computer resources are used to do a minimal
amount of work, with the system in a continual state of resource contention. Once started,
thrashing is typically self-sustaining until something occurs to remove the original situation that
led to the initial thrashing behavior.
Usually thrashing refers to two or more processes accessing a shared resource repeatedly
such that serious system performance degradation occurs because the system is spending a
disproportionate amount of time just accessing the shared resource. Resource access time may
generally be considered as wasted, since it does not contribute to the advancement of any process.
This is often the case when a CPU can process more information than can be held in available
RAM; consequently the system spends more time preparing to execute instructions than actually
executing them.

Concept of Thrashing

If the number of frames allocated to a low priority process is lower than the minimum number
required by the computer architecture then in this case we must suspend the execution of this

LOVELY PROFESSIONAL UNIVERSITY 135