In operating systems, what is another major drawback of MVT (Multiprogramming with a Variable number of Tasks) besides external fragmentation?

Introduction / Context:
MVT, or Multiprogramming with a Variable number of Tasks, uses variable sized partitions to allocate contiguous memory regions to processes. While this approach improves flexibility compared to fixed partition schemes, it introduces serious external fragmentation. To mitigate fragmentation, the operating system may resort to compaction, which is itself a significant drawback. Understanding this additional disadvantage helps in fully evaluating MVT.

Given Data / Assumptions:

• In MVT, memory is divided dynamically into variable sized contiguous regions for processes.
• Over time, processes terminate and new ones are loaded, leaving free holes scattered throughout memory.
• External fragmentation can prevent allocation of large processes even when total free memory is sufficient.
• The question asks about another drawback of MVT related to managing this fragmentation.

Concept / Approach:
To combat external fragmentation in MVT, the operating system may perform compaction, which involves moving processes in memory to combine scattered free spaces into larger contiguous blocks. Compaction can restore the ability to allocate big processes, but it is computationally expensive. It requires copying large amounts of process memory and updating relocation information, which consumes CPU time and may temporarily pause or slow down affected processes. This need for occasional, costly compaction is a significant drawback of MVT beyond fragmentation itself.

Step-by-Step Solution:
Step 1: Recognize that MVT suffers from external fragmentation because processes of different sizes leave holes of varying sizes in memory.Step 2: Understand that when no single hole is big enough for a new process, the system may still have enough total free memory spread across many small holes.Step 3: To solve this, the operating system can perform compaction, moving processes so that free memory regions are merged into one larger block.Step 4: Note that compaction involves copying process memory contents, updating relocation data, and possibly pausing processes during the operation.Step 5: Conclude that this overhead makes compaction and therefore MVT more complex and potentially slower, which is an important drawback.

Verification / Alternative check:
Operating systems texts describe compaction as a mechanism to alleviate external fragmentation by shifting processes toward one end of memory and merging free space at the other end. They emphasize that compaction is time consuming and may not be feasible frequently on busy systems. Because MVT requires contiguous allocation, it is particularly vulnerable to the need for compaction, unlike paging systems where noncontiguous allocation is natural. This confirms that the compaction requirement is a core disadvantage of MVT.

Why Other Options Are Wrong:
Option B is incorrect because MVT explicitly supports multiprogramming by allowing multiple processes in memory; it does not restrict the system to a single process. Option C is wrong because forcing processes to be the same size is a feature of fixed partition schemes, not MVT with variable partitions. Option D is incorrect because swapping between memory and disk can still be used with MVT; MVT does not inherently prohibit swapping.

Common Pitfalls:
Students often focus only on external fragmentation as the drawback of MVT and forget the costs of any strategy used to mitigate it. Another pitfall is to conflate MVT with paging and assume that noncontiguous allocation makes compaction unnecessary. In reality, MVT uses contiguous allocation and therefore may require compaction, which can be expensive and disruptive.

Final Answer:
Beyond external fragmentation, a major drawback of MVT is that it can require costly compaction operations that move processes in memory to merge scattered free spaces into larger contiguous blocks.