OS & Networking: CPU Scheduling, Virtual Memory, and the TCP/IP Stack

To design scalable high-performance backend systems, engineers must understand the low-level operating system internals and network protocols that coordinate CPU runtime execution and transit data packets.

1. Operating System Internals: Processes & CPU Scheduling

An operating system kernel manages how processes gain access to computing execution resources on physical CPU cores.

CPU Scheduling Algorithms:

• First-Come First-Served (FCFS): Non-preemptive scheduling; execute processes in order of arrival.
• Round Robin (RR): Preemptive scheduling; assigns a fixed execution time slice (quantum) to each process, cycling through the ready queue.
• Shortest Job First (SJF): Prioritizes processes requiring the least execution time.

Round Robin (Quantum = 4ms):
Ready Queue: [ P1 (8ms), P2 (3ms) ]
Execution: [ P1 (4ms) ] -> [ P2 (3ms) ] -> [ P1 (4ms) ] -> Completed

Virtual Memory and Paging

Virtual memory decouples application memory from physical RAM. The address space is split into fixed-size pages (usually 4KB), mapped by the Memory Management Unit (MMU) onto physical hardware frames:

  Virtual Address Space           Page Table             Physical RAM
   ┌────────────────┐          ┌─────────────┐        ┌────────────────┐
   │ Page 0 (Code)  ├─────────►│ Page0 -> Fr2├───────►│ Frame 2 (Code) │
   │ Page 1 (Heap)  ├────┐     ├─────────────┤        ├────────────────┤
   │ Page 2 (Stack) ├─┐  │     │ Page1 -> Fr0├───────►│ Frame 0 (Heap) │
   └────────────────┘ │  │     ├─────────────┤        ├────────────────┤
                      │  └────►│ Page2 -> Fr5├───────►│ Frame 5 (Stack)│
                      └────────└─────────────┘        └────────────────┘

2. Network Architectures: The TCP/IP Layer Stack

Computer network communications map to the 4-layer TCP/IP Model:

| Layer | Protocol Examples | Core Function | |---|---|---| | Application | HTTP, HTTPS, SSH, DNS | User application level communication | | Transport | TCP, UDP | Endpoint data transmission and flow control | | Network | IP (IPv4, IPv6), ICMP | Host-to-host packet routing | | Network Interface | Ethernet, Wi-Fi | Local physical frame transit |

TCP vs UDP

• TCP (Transmission Control Protocol): Connection-oriented, guarantees packet delivery, respects network congestions, orders packets.
• UDP (User Datagram Protocol): Connectionless, lightweight speed (no handshakes), packet loss is ignored. Used in VoIP and DNS queries.

3. Command Line Diagnostic Tools

System administrators utilize CLI utilities to debug network routing and interface configurations:

# Display routing paths to target host
tracert google.com

# Trace network ports and active sockets
netstat -ano

# Send ICMP echo requests to verify endpoint reachability
ping 8.8.8.8

# Query DNS server configurations for domain names
nslookup ajitdev.com

Understanding how the OS coordinates memory pages and packets allows you to write highly optimized applications that utilize hardware efficiently.