Memory Management

• What is Memory Management? #1

  • Memory management is a fundamental role of the operating system, dealing with the allocation and deallocation of the computer's primary memory 
  • When a user opens an application, its data is loaded from storage into active memory so that it can run smoothly  
• Memory Management #2

  • When a user opens a file from the file system, e.g. word document, the CPU loads this file data, as well as application data, into the primary memory 
  • Primary memory is a limited resource in the system, so it needs careful management 
  • Efficient allocation of memory enables a system to continue multitasking 
  • Memory management is made more efficient through 3 techniques: 
  • Paging 
  • Segmentation 
  • Virtual Memory
• Memory Management Image

  
• What is Paging? #1

  • Data stored in memory will lead to the smooth running of applications 
  • Paging is a method of chunking the primary memory into equal-sized blocks 
  • When an application is launched, data will be moved from the hard disk into Pages for faster access 
• Paging #2

  • As users move between applications, memory is dynamically allocated 
  • Pages will be taken away from applications not in active use and granted to applications that are in active use 
  • Paging can lead to internal fragmentation 
  • If a 200KB file is divided into four 64KB Pages, the last Page would have 8KB of unused space 
  • Unused space in a Page is wasteful because other unrelated data cannot be stored in this Page 
  • Over time, more pockets of wasted space will exist across the memory; this process is called internal fragmentation 
• Paging Image

  • The image below shows a single 64KB Page with 4KB of unoccupied space 
  • The box below this shows many Pages, each with varying sizes of internal fragments 

  
• What is Segmentation? #1

  • Segmentation is a method of chunking memory into blocks that correspond to different types of data needed by an application 
  • A video editing application may have a Segment for video data, audio data and special effects 
  • Segments are not all the same size; they are sized depending on their allocated data 
  • Segmentation is space-efficient due to only allocating space depending on the amount an application needs
• Segmentation #2

  • Segmentation can lead to external fragmentation 
  • As Segments fill up the memory, physical gaps reduce the maximum size of new Segments that can be allocated 
  • The arrangement of data in the segment becomes more fragmented over time because as blocks are taken away it's not possible to guarantee a new block will occupy the same amount of space 
• Segmentation image

  • Below (left) shows different application data assigned to a Segment 
  • Below (right) shows a defragmented version of the Segment to highlight the total unused space

  
• Virtual Memory 

  • If a computer is running low on primary memory, it can make secondary storage act as an 'extension' of the main memory 
  • The operating system can offload data from the primary memory into virtual memory 
  • Virtual memory creates an illusion of a larger memory and enables applications to continue to multitask 
  • However, accessing data in virtual memory is considerably slower compared to RAM 
  • Solid-state drives are faster than traditional hard-disk drives, but neither are as fast as RAM  
  • Over-reliance on virtual memory can lead to performance issues 
• Comparing Paging, Segmentation and Virtual Memory Image