Swapping

Figure 4.6: Swapping.

• Two general approaches to memory management can be used, depending (in part) on the available hardware
• The simplest strategy, called swapping, consists of bringing in each process in its entirety, running it for a while, then putting it back on the disk
• The other strategy, called virtual memory, allows programs to run even they are only partially in main memory
• The basic idea of swapping is to treat main memory as a preemptable resource
• A high-speed swapping device is used as the backing storage of the preempted processes
• Fragmentation refers to the unused memory that the memory management system cannot allocate
  • Internal fragmentation; Waste of memory within a partition, caused by the difference between the size of a partition and the process loaded. Severe in static partitioning schemes (Multiprogramming with Fixed Partitions (MFT)).
  • External fragmentation; Waste of memory between partitions, caused by scattered noncontiguous free space. Severe in dynamic partitioning schemes (Multiprogramming with Variable Partitions (MVT), swapping).
• Compaction (aka relocation) is a technique that is used to overcome external fragmentation
• The responsibilities of a swapper include:
  • Selection of processes to swap out criteria: suspended/blocked state, low priority, time spent in memory
  • Selection of processes to swap in criteria: time spent on swapping device, priority
  • Allocation and management of swap space on a swapping device. Swap space can be:
    • system wide
    • dedicated (e.g., swap partition or disk)