Memory Allocation in OS

Ever wondered how your computer manages multiple apps without crashing? It’s like a librarian organizing books efficiently! When you run programs, the OS (Operating System) allocates memory space. Let’s explore four strategies it uses: First Fit, Next Fit, Best Fit, and Worst Fit.

What’s Memory Allocation?

Imagine a parking lot with different-sized spaces. Programs (cars) need space to “park” in memory. The OS finds spots using these strategies, balancing speed and space efficiency.

Key Problems

• Fragmentation: Wasted space (“Small gaps between parked cars”).
  • Internal: Allocated space is slightly bigger than needed (like a toddler in a SUV space).
  • External: Free spaces are scattered, too small for new requests.

First Fit: The Eager Librarian

How It Works

The OS checks memory blocks in order and picks the first available space that fits. Ex: At a buffet, you take the first empty seat that fits your group.

Example

Memory Blocks: [100KB, 500KB, 200KB, 300KB, 600KB] Process Request: 450KB

1. Check 100KB → Too small.
2. Check 500KB → Fits!
   • Allocate 450KB here, leaving 50KB free.

Resulting Blocks: [100KB, 450KB (allocated), 50KB, 200KB, 300KB, 600KB]

Pros & Cons

• Fast (minimal searching).
• May waste large blocks early (e.g., 500KB used for 450KB).

Next Fit: The Circular Explorer

How It Works

Like First Fit, but starts searching from where it last left off, looping back if needed. Ex: A taxi driver circles the block, picking up the next passenger they see.

Example

Memory Blocks: [100KB, 500KB, 200KB, 300KB, 600KB] Process Request: 450KB Last Allocation Ended at 500KB:

1. Start after 500KB → Check 200KB (Too small).
2. Check 300KB (Too small).
3. Check 600KB → Fits!
   • Allocate 450KB here, leaving 150KB free.

Resulting Blocks: [100KB, 500KB, 200KB, 300KB, 450KB (allocated), 150KB]

Pros & Cons

• Reduces early memory wear (avoids always starting at the beginning).
• Might miss larger blocks earlier in memory.

Best Fit: The Perfectionist

How It Works

Finds the smallest available block that fits the request. Ex: Choosing the tightest-fitting suitcase to save space in your trunk.

Example

Memory Blocks: [100KB, 500KB, 200KB, 300KB, 600KB] Process Request: 450KB

1. Check all blocks:
   • 500KB is the smallest block ≥450KB.
2. Allocate 450KB in 500KB block → Leaves 50KB.

Resulting Blocks: [100KB, 450KB (allocated), 50KB, 200KB, 300KB, 600KB]

Pros & Cons

• Minimizes wasted space in the allocated block.
• Leaves tiny fragments (e.g., 50KB), causing external fragmentation.

Worst Fit: The Big Spender

How It Works

Picks the largest available block, hoping leftovers stay useful. Ex: Using the biggest box to pack a lamp, leaving medium boxes for other items.

Example

Memory Blocks: [100KB, 500KB, 200KB, 300KB, 600KB] Process Request: 450KB

1. Largest block: 600KB.
2. Allocate 450KB here → Leaves 150KB.

Resulting Blocks: [100KB, 500KB, 200KB, 300KB, 450KB (allocated), 150KB]

Pros & Cons

• Leaves larger leftover blocks.
• Often wastes more space (e.g., 150KB leftover vs. 50KB in Best Fit).

Comparison

Summary

• First Fit: Good balance for most cases.
• Next Fit: Better for systems where memory wears out (e.g., SSDs).
• Best Fit: Use when memory is scarce but causes fragmentation.
• Worst Fit: Rarely used; helps delay fragmentation.

Think of these strategies as different “parking styles” for your computer’s memory! 🚗💻