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Memory Management Techniques: Contiguous Allocation, Fragmentation, and MMU Dynamics

This text explores the concepts of memory management within computer systems, particularly focusing on the Memory Management Unit (MMU) and various allocation strategies. It discusses contiguous memory allocation, fragmentation types, and dynamic relocation using a relocation register. The challenges of external and internal fragmentation are detailed, along with methods to mitigate these issues, such as compaction. The discussion includes different multiprogramming strategies, namely fixed and dynamic partitions, shedding light on how operating systems handle memory for optimal process management.

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Memory Management Techniques: Contiguous Allocation, Fragmentation, and MMU Dynamics

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  1. ScribeMar 26, 2014(Regular) • Memory-Management Unit (MMU) • Contiguous Memory Allocation • Fragmentation By BandiSumanth 11CS30006

  2. Memory-Management Unit • MMU -> Hardware device that maps virtual address to physical address • Consider a simple MMU scheme, which uses a relocation register • The value in the relocation address is added to every address generated by a user process at the time , it is sent to memory • For eg: if the relocation register contains addree 14000 • User access to address location 0 is dynamically relocated to location 14000 • Similarly, access to location 346 is mapped to location 14346

  3. Memory-Management Unit • The user process deals with logical addresses (o to max) • It never sees the real physical addresses (R+0 to R+ max) • In this form of execution-time binding, the final location of a referenced memory address is not determined until the reference is made • The concept of a logical address space that is bound to a separate physical address space is centrary to proper memory managaement

  4. Dynamic Relocation using a relocation register

  5. Contiguous Memory Allocation • Multiple processes reside in memory • Each process is contained in single contiguous section of memory

  6. Contiguous Memory Allocation • Main memory usually divided into two partitions • Resident operating system, usually held in low memory with interrupt vector • User processes then held in high memory

  7. Multiprogramming with Fixed Partitions • Divide memory into several Fixed size partition • Each partition stores one process • Degree of multiprogramming limited by number of partitions • If a partition is free, load process from job queue • The OS keeps a table indicating which parts of memory are available and which are occupied • This method is was used by the IBM OS/360 operating sytem (called MFT - Multiprogramming with a Fixed number of Tasks)

  8. Process 1 F r a g m e n t F r a g m e n t Process 3 F r a g m e n t Process 2 OS Partition 1 Partition 2 Partition 3 Multiprogramming with Fixed Partitions

  9. Multiprogramming with Dynamic Partitions • Variable partition scheme • Hole – block of available memory; holes of various size are scattered throughout memory • Keeps a table of free memory • When a process arrives, it is allocated memory from a hole large enough to accommodate it • Process exiting frees its partition, adjacent free partitions combined • Operating system maintains information about:a) allocated partitions b) free partitions (hole)

  10. Multiprogramming with Dynamic Partitions

  11. Dynamic Storage-Allocation Problem • How to satisfy a request of size n from a list of free holes • First-fit: Allocate the first hole that is big enough (fast, but leads to fragments) • Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size (slow, but small leads to fragments). • Worst-fit: Allocate the largest hole; must also search entire list (slow, but leaves large holes) • First-fit and best-fit better than worst-fit in terms of speed and storage utilization

  12. Hardware Support for Relocation and Limit Registers • Relocation-register scheme used to protect user processes from each other, and from changing operating-system code and data. • Relocation register contains value of smallest physical address; limit register contains range of logical addresses – each logical address must be less than the limit register • Context switch • MMU maps logical address dynamically

  13. Fragmentation • Both the first-fit and best-fit strategies for memory allocation suffer from external fragmentation • As processes are loaded and removed from memory, the free memory space is broken into little pieces • External Fragmentation exists when there is enough total memory space to satisfy a request, but the available spaces are not contiguous; storage is fragmented into a large number of small holes • First fit analysis reveals that given N blocks allocated, another 0.5 N blocks will be lost to fragmentation • That is, one-third of memory may be unusabel! This property is known as 50-percent rule

  14. External Fragmentation • Reduce external fragmentation by compaction • Shuffle memory contents to place all free memory together in one large block. • Compaction is possible only if relocation is dynamic (i.e., registers can be updated), and is done at execution time

  15. Internal Fragmentation • Internal Fragmentation– allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used. • Occurs when memory is allocated in fixed size pieces

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