Internal vs external fragmentation in Operating Systems - Quick Revision & Key Differences

Recall & Review

beginner

What is internal fragmentation in memory management?

Internal fragmentation happens when fixed-size memory blocks are allocated, but the process uses less space than the block size, leaving unused memory inside the allocated block.

beginner

Define external fragmentation.

External fragmentation occurs when free memory is split into small scattered blocks over time, making it hard to find a large enough continuous block for new processes.

intermediate

How does internal fragmentation differ from external fragmentation?

Internal fragmentation wastes space inside allocated blocks, while external fragmentation wastes space outside allocated blocks due to scattered free memory.

beginner

Give a real-life example of internal fragmentation.

Imagine renting a 10-seat bus for 7 people. The 3 empty seats are like internal fragmentation — space reserved but unused.

intermediate

What is a common technique to reduce external fragmentation?

Techniques like memory compaction rearrange memory contents to combine free spaces into larger blocks, reducing external fragmentation.

Which type of fragmentation occurs when allocated memory blocks have unused space inside them?

A Internal fragmentation

B External fragmentation

C Memory leak

D Paging

Internal fragmentation is unused space inside allocated blocks.

External fragmentation is caused by:

A Fixed-size blocks being too large

B Scattered small free memory blocks

C Processes using less memory than allocated

D Memory leaks

External fragmentation happens when free memory is split into small scattered blocks.

Which method helps reduce external fragmentation?

A Increasing block size

B Allocating fixed-size blocks

C Ignoring free space

D Memory compaction

Memory compaction rearranges memory to combine free spaces.

Internal fragmentation wastes memory because:

A Processes use more memory than allocated

B Free memory is scattered

C Allocated blocks are larger than needed

D Memory is not allocated

Internal fragmentation occurs when allocated blocks are larger than the process needs.

Which fragmentation type can be reduced by using variable-sized memory allocation?

A External fragmentation

B Internal fragmentation

C Both internal and external fragmentation

D None

Variable-sized allocation reduces external fragmentation by fitting memory more closely to process size.

Explain the difference between internal and external fragmentation with examples.

Think about wasted space inside allocated blocks vs scattered free space.

Describe one method to reduce external fragmentation and why it works.

Consider how combining small free spaces helps.

Practice

1. Which of the following best describes internal fragmentation in memory management?

easy

A. Wasted space outside allocated memory blocks

B. Wasted space inside allocated memory blocks

C. Memory lost due to hardware failure

D. Memory used by the operating system kernel

Solution

1. Step 1: Understand internal fragmentation

   Internal fragmentation happens when allocated memory blocks have unused space inside them because the block size is larger than the requested memory.

2. Step 2: Compare with external fragmentation

   External fragmentation is about wasted space outside allocated blocks, so it is different from internal fragmentation.

3. Final Answer:

   Wasted space inside allocated memory blocks -> Option B

4. Quick Check:

   Internal fragmentation = wasted space inside blocks [OK]

Hint: Internal = inside allocated blocks wasted space [OK]

Common Mistakes:

• Confusing internal with external fragmentation
• Thinking fragmentation is caused by hardware
• Mixing up allocated and free memory spaces

2. Which of the following statements about external fragmentation is syntactically correct?

easy

A. External fragmentation wastes space inside allocated blocks.

B. External fragmentation is caused by software bugs only.

C. External fragmentation means memory is fully used without gaps.

D. External fragmentation wastes space outside allocated blocks in free memory.

Solution

1. Step 1: Identify correct definition of external fragmentation

   External fragmentation refers to wasted space outside allocated blocks, scattered in free memory.

2. Step 2: Check other options for correctness

   The statement "External fragmentation wastes space inside allocated blocks" describes internal fragmentation. The claim that it is caused by software bugs only is incorrect, as it arises from repeated allocation and deallocation. The idea that memory is fully used without gaps is false, since fragmentation creates unusable gaps in free memory.

3. Final Answer:

   External fragmentation wastes space outside allocated blocks in free memory. -> Option D

4. Quick Check:

   External fragmentation = wasted free memory gaps [OK]

Hint: External = wasted space outside allocated blocks [OK]

Common Mistakes:

• Mixing internal and external fragmentation definitions
• Assuming fragmentation is only due to bugs
• Ignoring free memory gaps in external fragmentation

3. Consider a memory system where 5 blocks of size 10KB are allocated, but each request was for 8KB. What type of fragmentation is present and what is the total wasted memory?

medium

A. Internal fragmentation; 10KB wasted total

B. Internal fragmentation; 2KB wasted

C. Internal fragmentation; 10KB wasted inside each block

D. External fragmentation; 10KB wasted

Solution

1. Step 1: Calculate wasted space per block

   Each block is 10KB but only 8KB is used, so 2KB is wasted inside each block.

2. Step 2: Calculate total wasted memory

   With 5 blocks, total wasted = 5 blocks * 2KB = 10KB wasted inside allocated blocks, which is internal fragmentation.

3. Final Answer:

   Internal fragmentation; 10KB wasted total -> Option A

4. Quick Check:

   Internal fragmentation = wasted inside blocks total 10KB [OK]

Hint: Wasted inside allocated blocks = internal fragmentation [OK]

Common Mistakes:

• Confusing total wasted with per block wasted
• Mixing internal with external fragmentation
• Ignoring multiplication by number of blocks

4. A system has free memory blocks of sizes 5KB, 3KB, and 4KB scattered around. A process requests 6KB. Which problem is this an example of, and what is the main issue?

medium

A. External fragmentation; no single free block large enough

B. Internal fragmentation; wasted space inside allocated blocks

C. External fragmentation; all free blocks are too large

D. Internal fragmentation; process size is too small

Solution

1. Step 1: Analyze free memory blocks

   The free blocks are 5KB, 3KB, and 4KB. The process needs 6KB, but no single block is large enough (maximum 5KB < 6KB).

2. Step 2: Identify fragmentation type and issue

   Free memory total is 12KB > 6KB, but scattered into small non-contiguous blocks with none large enough, causing allocation failure. This is external fragmentation.

3. Final Answer:

   External fragmentation; no single free block large enough -> Option A

4. Quick Check:

   External fragmentation = scattered free blocks too small [OK]

Hint: External fragmentation = scattered free blocks too small [OK]

Common Mistakes:

• Confusing internal with external fragmentation
• Assuming process size is too small causes fragmentation
• Ignoring block sizes when allocating memory

5. A system uses fixed-size memory blocks of 4KB. A program requests 10KB. How does internal and external fragmentation affect this allocation, and what is the total wasted memory?

hard

A. Internal fragmentation wastes 4KB; external fragmentation wastes 2KB

B. Internal fragmentation wastes 2KB; external fragmentation wastes 4KB

C. Internal fragmentation wastes 2KB; external fragmentation wastes 0KB

D. Internal fragmentation wastes 0KB; external fragmentation wastes 2KB

Solution

1. Step 1: Calculate blocks needed and internal fragmentation

   10KB requires 3 blocks of 4KB each (total 12KB). The extra 2KB (12KB - 10KB) inside allocated blocks is internal fragmentation.

2. Step 2: Consider external fragmentation

   Since blocks are fixed and allocated contiguously, no free space outside allocated blocks is wasted here, so external fragmentation is 0KB.

3. Final Answer:

   Internal fragmentation wastes 2KB; external fragmentation wastes 0KB -> Option C

4. Quick Check:

   Internal fragmentation = 2KB; external = 0KB [OK]

Hint: Internal = leftover in allocated blocks; external = gaps outside [OK]

Common Mistakes:

• Miscounting blocks needed for allocation
• Confusing internal and external fragmentation amounts
• Assuming external fragmentation always occurs