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Implementation of LRU Page Replacement Algorithm in C++

In this post, we implement the LRU (Least Recently Used) Page Replacement Algorithm in C++. When a page fault occurs and all frames are occupied, LRU evicts the page that has not been used for the longest time. Unlike FIFO which only tracks load order, LRU tracks usage history, making it significantly more effective in practice. What is LRU Page Replacement? LRU exploits the principle of temporal locality — recently used pages are likely to be used again soon. By evicting the page that hasn’t been touched for the longest time, LRU keeps the most actively used pages in memory. Page Hit — Referenced page is already in a frame (its last-used time is implicitly updated). Page Fault — Referenced page is not in memory. On a fault with full frames, the frame whose page was accessed least recently is selected for replacement. LRU identification — For each frame, scan the page history backwards from the current reference to find the most recent access of that frame’s page. The frame with the oldest last-access is the LRU victim. LRU does not suffer from Bélady’s Anomaly (unlike FIFO) and closely approximates the optimal (OPT) algorithm.

May 26, 2015 · 1 min read

A Ankur in OS

Implementing First Fit Algorithm in C++

In this post, we implement the First Fit Memory Allocation Algorithm in C++. When a new process (job) needs memory, First Fit scans the list of available memory blocks from the beginning and allocates the first block that is large enough to hold the job. It is the fastest of the three classic memory placement strategies — First Fit, Best Fit, and Worst Fit. What is First Fit Memory Allocation? Memory is divided into fixed-size blocks (also called partitions). Each block can hold at most one job at a time. When a job arrives: Scan memory blocks from block 1 onwards. Allocate the first free block whose size ≥ job size. Mark that block as occupied. Stop scanning (no need to look at remaining blocks). If no block fits, the job remains unallocated. The leftover space inside an allocated block (block size − job size) is called internal fragmentation. Over time, First Fit tends to leave small, unusable gaps in the lower portion of memory (external fragmentation).

May 25, 2015 · 1 min read

A Ankur in OS

Implementing FCFS Scheduling Algorithm in C++

In this post, we implement the First Come First Serve (FCFS) CPU scheduling algorithm in C++. FCFS is the simplest scheduling strategy — the process that arrives first in the ready queue gets the CPU first. It is non-preemptive: once a process starts, it runs to completion without interruption. What is FCFS Scheduling? FCFS works exactly like a real-world queue: the first person in line is served first. The CPU picks the first process from the ready queue, runs it completely, then picks the next. No reordering happens — processes execute strictly in arrival order. The key metrics computed are: Waiting Time (WT) — Time a process spends waiting before the CPU is allocated to it. Formula: WT[i] = WT[i-1] + BurstTime[i-1] - (ArrivalTime[i] - ArrivalTime[i-1]) Turnaround Time (TT) — Total time from arrival to completion. Formula: TT[i] = WT[i] + BurstTime[i] A well-known drawback is the Convoy Effect — a long process can block many short ones behind it, inflating average waiting time.

May 24, 2015 · 1 min read

A Ankur in OS

Implementing Best Fit Algorithm in C++

In this post, we implement the Best Fit Memory Allocation Algorithm in C++. Unlike First Fit which takes the first block large enough for a job, Best Fit searches through all free blocks and selects the one whose size is closest to the job’s size — the tightest possible fit. The goal is to minimize wasted space inside allocated blocks (internal fragmentation) and preserve large blocks for future large jobs. What is Best Fit Memory Allocation? Best Fit works by sorting memory blocks in ascending order of size before allocation. By scanning the sorted list and taking the first block that fits, we automatically pick the smallest block that still accommodates the job. This is the defining characteristic of Best Fit: among all free blocks that are large enough, it selects the one with the smallest size, leaving the rest of the available space in larger blocks undisturbed for future use. Sort all memory blocks in ascending order of size. For each job, scan the sorted blocks and allocate the first free block whose size ≥ job size. Because blocks are sorted by ascending size, the first fitting block is automatically the smallest one that fits — i.e., the best fit. After all allocations are done, re-sort blocks by their original block number so the output table is readable. Compared to First Fit, Best Fit produces less internal fragmentation per allocation because it picks the tightest available block. However, it tends to leave behind many tiny leftover fragments that are too small to be reused — a form of external fragmentation. It is also slower than First Fit because it must consider all blocks before allocating.

May 23, 2015 · 2 min read