In this post, we implement the Round Robin (RR) CPU scheduling algorithm in C++. Round Robin is the most widely used algorithm in time-sharing systems because it gives every process a fair, equal slice of CPU time. Each process gets to run for a fixed duration called the time quantum, after which it is preempted and moved to the back of the ready queue.
What is Round Robin Scheduling?
All processes are arranged in a circular queue. The CPU runs each process for at most Q (time quantum) units. If the process finishes within Q units, it leaves the queue. If it doesn’t finish, it is interrupted and placed at the back to wait for its next turn. This cycle repeats until all processes complete.
Time Quantum (Q) — The maximum CPU time a process gets per round. A smaller Q improves response time but increases context-switch overhead. A very large Q degrades to FCFS behavior.
Waiting Time (WT) — Total time the process spent waiting across all rounds, not including its own execution time.
In this post, we implement a simulation of the Producer-Consumer Problem in C++ using a circular bounded buffer. This is one of the most classic synchronization problems in operating systems, modeling the coordination between a process that generates data (producer) and a process that consumes data (consumer) through a shared buffer of fixed capacity.
What is the Producer-Consumer Problem?
The Producer-Consumer problem (also called the Bounded Buffer problem) defines these rules:
The producer generates items and places them into the buffer — but only if space is available. If the buffer is full, the producer must wait (sleep).
The consumer retrieves and processes items from the buffer — but only if items exist. If the buffer is empty, the consumer must wait (sleep).
Only one entity should access the buffer at a time (mutual exclusion).
This simulation runs 20 steps. At each step, a random number determines whether the producer or consumer acts. A circular buffer is used so the array can be reused efficiently without shifting elements.
In this post, we implement the Priority Scheduling CPU scheduling algorithm in C++. Each process is assigned a priority number, and the process with the highest priority is scheduled first. This algorithm is used in real-time and embedded systems where certain tasks are more urgent than others and must preempt routine work.
What is Priority Scheduling?
In this implementation, a higher priority number means higher urgency (e.g., priority 5 executes before priority 1). The algorithm is non-preemptive — once a process starts, it runs to completion. All processes are assumed to arrive at time 0.
The algorithm works by sorting processes in descending order of priority and then computing scheduling metrics in that order:
Waiting Time (WT): WT[i] = WT[i-1] + BurstTime[i-1] (since all arrive at time 0)
Turnaround Time (TT): TT[i] = WT[i] + BurstTime[i]
The main concern with priority scheduling is starvation — low-priority processes may never get the CPU. The solution is aging: gradually increasing a process’s priority the longer it waits.
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.
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).
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.
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.
