[CS] Operating System

1.Process is an 'Independent House', Thread is a 'Roommate'

To understand how computer programs work, we can imagine them as houses.

🏠 Process = "An Independent House"

• Definition: A basic unit of work that gets resources from the Operating System (OS).
• Features:
  • Independence: It is completely separate from other houses. If the house next door catches fire, your house is safe!
  • Cost: Building a house (creating a process) is heavy work. It takes a lot of land (RAM) and time.

🧔 Thread = "Roommates in a House"

• Definition: A unit of execution that actually does the work inside a process.
• Features:
  • Sharing: Roommates share the living room (Heap memory) and the fridge. Communication is very fast because they live together.
  • Lightweight: Adding one person is much easier and faster than building a whole new house.
  • Risk: If one roommate starts a fire (a critical error), the whole house (the process) burns down.
• Example: This is why if your Chrome browser crashes, your KakaoTalk app still works fine. They are in different houses!

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2. How to Stop Fighting (Synchronization)

When roommates share one fridge (Heap), they might fight. Here are two ways to keep the peace.

🔒 1. Synchronized (Pessimistic Lock)

"The world is dangerous. I will lock the door while I use the fridge!"

• Pros:
  • 100% Safe: Since the door is locked, the data never gets messy.
  • Good for complex tasks: It is safe when you have many steps to do, like taking food out, cooking, and putting it back.
• Cons:
  • Deadlock Risk: Roommate A has the bathroom key, and B has the kitchen key. They both wait for each other forever, and the whole house stops.
  • Slow Speed: If the line is long, other roommates must wait and do nothing.

⚛️ 2. Atomic / CAS (Optimistic Lock)

"I don't think anyone touched it while I was gone. Oh, they did? Then I'll try again!"

• Pros:
  • Very Fast: There is no locking or unlocking. It is great for simple tasks like adding numbers.
  • No Waiting: Roommates don't stand in line. If they fail, they just try again immediately.
• Cons:
  • Infinite Loop: If 100 people try to open the fridge at the same time, they might keep failing and trying again forever. This makes the CPU very busy.
  • The ABA Problem: If a value changes from 100 to 200 and back to 100 while you are away, the system thinks "Oh, nothing changed!" even though it did.

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