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cs604 operating systems - lecture 23: memory management - page replacement algorithms

this lecture focuses on **page replacement algorithms**, a crucial component of virtual memory management. when a page fault occurs (the requested page is not in physical memory), the operating system needs to decide which page currently residing in memory should be replaced to make room for the new page. the goal of these algorithms is to minimize the number of page faults, thereby improving system performance.

*why are page replacement algorithms important?*

*limited physical memory:* physical ram is a limited resource. virtual memory allows processes to address more memory than physically available. this necessitates swapping pages between physical memory and secondary storage (e.g., hard disk).
*performance impact:* page faults are expensive. swapping a page to disk takes considerable time (millions of clock cycles). minimizing page faults significantly boosts system responsiveness.
*fairness:* a good page replacement algorithm should be fair to all processes, preventing one process from monopolizing physical memory and causing excessive page faults for others.

*key concepts to understand:*

*page fault:* occurs when a process tries to access a page that is not currently in ram. the os must retrieve the page from disk.
*page frame:* a fixed-size block of physical memory where a page resides.
*reference string:* a sequence of page numbers representing the order in which a process accesses pages in memory. used to evaluate the performance of page replacement algorithms. example: `7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1`
*frame allocation:* deciding how many page frames to allocate to each process. this is discussed in other lectures but impacts the performance of any replacement algorithm.
*thrashing:* a situation where the system spends more time swapping pages in and out than executing instructions, leading to ...

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operating systems
process management
memory management
scheduling algorithms
deadlock prevention
synchronization
interprocess communication
system calls
virtual memory
file systems
kernel architecture
resource allocation
concurrency control
performance metrics
OS security