How Does Variable Partitioning Manage Memory?

TL;DR

Variable partitioning allocates each process exactly the memory it requests at runtime, eliminating internal fragmentation. It supports more flexible process sizes and has no fixed partition-based limit on process count, but scattered holes cause external fragmentation. Compaction can combine free space, although stopping and relocating processes makes it slow and undesirable.

Transcript

Variable Partitioning. In Variable Partitioning whenever the processes are coming into the RAM only then we are allocating space to the processes. Means we keep the RAM empty for now, when processes come in the RAM then at run time the capacity they need, the space they need, according to that space I will allocate them. But in Fixed Partitioning w... Read More

Key Insights

• Variable partitioning allocates memory at runtime according to each arriving process's requested size. RAM is not divided into predetermined blocks, so a 2MB process receives exactly 2MB and a 4MB process receives exactly 4MB.
• Internal fragmentation is avoided because the allocated partition matches the process size. This differs from fixed partitioning, where a 1KB process placed in a 2KB partition leaves 1KB inside that partition unavailable to other processes.
• The degree of multiprogramming is not restricted by a predetermined partition count. If RAM capacity is sufficient, additional processes can continue receiving dynamically created partitions instead of being limited to the number of partitions established beforehand.
• Process size is not limited by the largest predefined partition under variable partitioning. A process requesting 64MB can receive 64MB when that much contiguous free memory is available, whereas the fixed example cannot accept a process larger than its 16KB partition.
• A hole is a free memory region created when an allocated process exits. Holes appear dynamically as processes terminate or leave memory, and their different locations and sizes make subsequent memory allocation decisions more complicated.
• External fragmentation occurs when total free memory is sufficient but divided among noncontiguous holes. Two separate 4MB holes provide 8MB collectively, but they cannot hold one 8MB process because contiguous allocation prevents splitting that process across both locations.
• Compaction reduces external fragmentation by moving active processes together and collecting free space into one area. The method resembles copying processes from their current addresses and pasting them elsewhere so that separate holes become a single contiguous block.
• Compaction is undesirable because processes must be stopped before relocation, and copying their contents takes considerable time. Allocation and deallocation are also more complex because both processes and holes appear dynamically, requiring mechanisms such as bitmaps and linked lists for management.

Questions & Answers

Q: What is variable partitioning in memory management?

Variable partitioning is a contiguous memory allocation technique in which partitions are created at runtime rather than before processes arrive. When a process requests memory, it receives a contiguous block matching its required size. For example, processes requesting 2MB, 4MB, and 8MB are allocated blocks of exactly 2MB, 4MB, and 8MB, provided sufficient suitable memory is available.

Q: How does variable partitioning differ from fixed partitioning?

Fixed partitioning divides RAM into predetermined blocks before processes arrive, such as partitions of 2KB, 4KB, 8KB, and 16KB. Variable partitioning keeps memory unpartitioned until runtime and creates a block according to each process's demand. Consequently, it avoids wasted space inside oversized fixed partitions and is not restricted by a predetermined number of partitions.

Q: Why does variable partitioning avoid internal fragmentation?

Variable partitioning avoids internal fragmentation because each allocated block is made to match the requesting process's size. A process requiring 2MB receives exactly 2MB, leaving no unused capacity inside its partition. By comparison, placing a 1KB process inside a predetermined 2KB fixed partition wastes the remaining 1KB, which is internal fragmentation.

Q: What advantages does variable partitioning provide?

Variable partitioning provides three stated advantages. It eliminates internal fragmentation by matching allocation size to process demand. It removes the fixed partition-based limitation on the degree of multiprogramming when RAM is sufficient. It also removes the restriction imposed by the largest predefined partition, allowing a larger process to enter when enough contiguous free memory exists.

Q: What is external fragmentation in variable partitioning?

External fragmentation occurs when enough free memory exists collectively, but the free space is separated into holes that are not contiguous. For example, two 4MB holes contain 8MB in total, yet an 8MB process cannot use them because the process must occupy one continuous block. The process therefore waits despite the sufficient combined free capacity.

Q: Why can a process not use multiple separate memory holes?

A process cannot use multiple separate holes in variable partitioning because the technique belongs to contiguous allocation. The entire process must be placed in one continuous memory region. An 8MB process therefore cannot be divided into two halves and stored across two separate 4MB holes, even though those holes contain the required 8MB collectively.

Q: How does compaction remove external fragmentation?

Compaction removes external fragmentation by relocating active processes so they occupy one side of memory while the separate empty regions are gathered on the other side. This combines multiple holes into a larger contiguous free area. The procedure operates like copying a process from its current address and pasting it at another address, making room for larger allocations.

Q: Why are compaction and allocation management difficult?

Compaction is undesirable because running processes must be stopped before they are moved, and copying process contents to new addresses takes considerable time. Allocation and deallocation are also complex because the numbers of processes and holes change dynamically. Bitmaps and linked lists can manage which memory areas contain processes and which areas remain available as holes.

Summary & Key Takeaways

• Variable partitioning leaves available RAM unpartitioned until processes arrive. Each process receives a contiguous block matching its requested size, such as 2MB, 4MB, or 8MB. Because the allocated block equals the process demand, no unused space remains inside the partition, so internal fragmentation does not occur under this approach.

• Unlike fixed partitioning, variable partitioning does not restrict the number of resident processes to a predetermined number of partitions. It also does not limit a process to the largest predefined partition. The practical constraint remains the available RAM, and every process must still occupy one contiguous area of memory.

• Terminated processes leave holes that can become scattered throughout memory. Their combined capacity may be sufficient for a new process, yet allocation can fail when no single hole is large enough. Compaction joins the free space by relocating processes, but it requires stopping running processes and copying substantial amounts of data.