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12. Memory Management

31/10/22

Memory Management​

Memory Hierarchies​

  • Computers typically have memory hierarchies:
    • Registers, L1/l2/L3 cache
    • Main Memory (RAM)
    • Disks
  • "Higher memory" is faster, more expensive and volatile 'lower memory' is slower, cheaper and non-volatile
  • OS provides a memory abstractions
  • Memory can be seen as one linear array of bytes/words

OS Responsibilities​

  • Allocate/de-allocate memory when requested by process. keep track of used/unused memory
  • Distribute memory between processes and simulate a 'larger than available' memory space
  • Control access when multi programming is applied
  • Transparently move data from memory to disk and vice versa

Partitioning​

  • Contiguous memory management models - allocate memory in one single block without any holes or gaps
  • Non-contiguous memory management models - capable of allocating memory in multiple blocks, or segments, which may be placed anywhere in physical memory

Contiguous Approaches​

  • Mono-programming - one single partition for user processes
  • Multi-programming with fixed partitions
    • Fixed equal sized partitions
    • Fixed non-equal sized partitions
  • Multi-programming with dynamic partitions

Mono-Programming​

  • Only one single user process is in memory/executed at any point in time
  • A fixed region of memory is allocated to the OS, the remaining memory is reserved for a single process
  • This process has direct access to physical memory
  • Process is allocated contiguous block of memory
  • One process is allocated the entire memory space, and the process is always located in the same address space
  • Overlays allow the program to control the memory. Programmer can use more memory than available

Shortcomings​

  • Direct access to the physical memory means it may have access to OS memory
  • OS can be seen as a process - have two processes anyway
  • Low utilisation of hardware resources
  • Mono programming is unacceptable as multi programming is expected on modern machines

Direct memory access/mono programming are common in basic embedded systems, microwaves, washing machines etc

Simulating​

  • Simulate multi-programming through swapping
    • Swap process out to the disk and load a new one
    • Apply threads within the same process

Multi-Programming​

  • There are nn processes in memory
  • A process spends pp percent of its time waiting for I/O
  • CPU utilisation is calculated as 1 minus the time that all processes are waiting for. 1−p1-p
  • Probability that all nn processes are waiting for I/O is pnp^n

Caveats​

  • This model assumes that all processes are independent, not true
  • More complex models could be built queuing theory, but we can still use this simplistic model to make approximate predictions

Partitioning​

Fixed partitions of equal size​

  • Divide memory into static, contiguous and equal sized partitions that have a fixed size and fixed location
    • Any process can take any partition
    • Allocation of fixed equal sized partitions to process is trivial
    • Very little overhead and simple implementation
    • The OS keeps a track of which partitions are being used and which ones are free

Disadvantages​

  • Low memory utilisation and internal fragmentation: partition may be unnecessarily large
  • Overlays must be used if a program does not fit into a partition

Fixed partitions of non-equal size​

  • Divide memory into static and non-equal sized partitions that have a fixed size and fixed location.
  • Reduces internal fragmentation
  • The allocation of process to partitions must be carefully considered

Fixed Partitions (Allocation Methods)​

One private queue per partition

  • Assigns each process to the smallest partition that it would fit in
  • Reduces internal fragmentation
  • Can reduce memory utilisation and result in starvation

A single shared queue for all partitions can allocate small process to large partitions but result in increased internal fragmentation