Page Replacement Algorithms
Page replacement algorithms are
used by the operating system to decide which pages should be removed from
memory (RAM) when it becomes full, and a new page needs to be loaded from disk.
The goal is to select the page that will not be needed for the longest time in
the future. However, predicting future page references is generally not
possible, so various algorithms have been developed to approximate this. Here
are a few common ones:
- FIFO (First-In, First-Out): This is the
simplest page replacement algorithm. In this scheme, the operating system
keeps track of all the pages in memory in a queue, with the most recent
arrival at the back, and the oldest arrival in front. When a page needs to
be replaced, the page at the front of the queue (the oldest page) is
selected.
- LRU (Least Recently Used): This algorithm
works on the idea that pages that have been most heavily used in the past
will probably be heavily used again in the future. So, it discards the
least recently used pages first. This algorithm is often used in practice
and can be implemented by keeping a linked list of pages, with the most recently
used page at the front and the least recently used page at the back.
- Optimal Page Algorithm: Also known as OPT or
MIN, this algorithm selects for replacement the page that will not be used
for the longest time in the future. While this algorithm is excellent for
minimizing page faults, it's not often used in practice because it
requires perfect knowledge of future page requests, which is typically
impossible.
- Clock Algorithm: This algorithm keeps a
circular list of pages in memory, with a 'hand' (pointer) that points to
the oldest page in the list. When a page fault occurs, the hand checks if
the 'use' bit of the page it points to is set. If it is, it is cleared and
the hand moves to the next page. If it's not set, the page is replaced.
- Least Frequently Used (LFU): This algorithm
uses a counter to keep track of the number of times a page is accessed.
When the page frame is full and a page needs to be replaced, the system
selects the one with the smallest count. It's based on the argument that a
page with the smallest count was probably just brought in and yet to be
used.
Each of these algorithms has its
strengths and weaknesses, and the best choice of algorithm can depend on the
specific characteristics of the system, including the hardware architecture,
the number of page frames available, the memory access patterns of the current
processes, and the performance characteristics that are most important for the
system's workload.
Page Replacement Algorithms
FIFO
The First-In, First-Out (FIFO)
page replacement algorithm is one of the simplest methods for managing the
swapping of pages in and out of memory.
The FIFO algorithm operates
similarly to how a real-world queue operates. It maintains a list of all the
pages in memory in the order they were loaded. The oldest page, the one that
was loaded into memory first, is at the front of the queue, while the newest
page is at the back of the queue.
When a page fault occurs, meaning
the operating system needs to load a page into memory that is not already there,
and all the memory frames are full, the FIFO algorithm selects the page at the
front of the queue, the oldest page, to be removed from memory. The new page is
then added to the back of the queue.
While FIFO is simple and easy to
understand, it does not always yield the best performance, as it doesn't take
into account the frequency or recency of page use. As a result, it could
potentially remove a page from memory that is still frequently used, while a
less frequently used page remains in memory. This behavior can lead to an
increased number of page faults, a situation known as Belady's anomaly.
For these reasons, other page
replacement algorithms, such as Least Recently Used (LRU) or the Clock
algorithm, are often used instead, as they typically offer better performance
in most scenarios. However, the best page replacement algorithm to use can vary
depending on the specific workload and system requirements.
Page Replacement Algorithms LRU
The Least Recently Used (LRU) page
replacement algorithm is a common method for managing the swapping of pages in
and out of memory in an operating system.
The LRU algorithm operates on the
principle that pages which have been most heavily used in the recent past will
likely be used heavily in the near future. Hence, when a page needs to be
replaced (e.g., during a page fault), the LRU algorithm selects the page that
has not been used for the longest amount of time.
LRU maintains a list of pages,
with the most recently used page at the front of the list and the least recently
used page at the back. When a page is referenced, it is moved to the front of
the list. When a page needs to be replaced, the page at the back of the list
(the least recently used page) is chosen.
LRU can be more efficient than
simpler methods like FIFO (First-In, First-Out), as it takes into account the
recency of page usage. However, implementing true LRU can be expensive in terms
of time and resources, as it requires maintaining a linked list of pages and
updating this list every time a page is referenced. Therefore, many systems use
approximations of LRU, such as the Clock or Second Chance algorithm, which
offer a compromise between the efficiency of LRU and the simplicity of FIFO.
Page Replacement Algorithms
Optimal
The Optimal Page Replacement
Algorithm, also known as OPT or MIN, is a page replacement policy that selects
for replacement the page that will not be used for the longest time in the
future. It's a theoretical concept used mainly for comparative studies, because
it requires perfect knowledge of future requests, which is generally impossible
in practice.
Here's how it works:
- When a page needs to be swapped into memory and
there is no free space left, the operating system looks at all the pages
currently in memory.
- For each of these pages, it predicts how long it
will be until the page is needed again, based on the list of future page
requests.
- The OS then selects the page that won't be needed
for the longest time in the future and swaps it out to make room for the
new page.
The optimal algorithm minimizes
the number of page faults and is, by definition, the most efficient algorithm.
However, it's not practical for real systems because it requires advance
knowledge of the sequence of page requests, something that is not typically possible
to know. For this reason, the Optimal Page Replacement Algorithm serves as a
benchmark to compare the performance of other, more practical, page replacement
algorithms such as LRU (Least Recently Used) or FIFO (First-In, First-Out).