[mem] Memory management

Memory management:

• allocate / deallocate

• protection: no access to a process's mem space from outside the given process

• shared mem management: (between 2 & more procs;) shared libs, shared mem segments (via shmget()), etc.

Approaches:

• contiguous mem allocation

• mono-contiguous mem allocation

• virtual memory

With multi-programming, multiple options for managing mem:

• (early) partitions: each partition is a block of contiguous mem

Degree of multiprogramming:

• how many processes could be run at once?
  • depend on: process size, partition size & mem available

A logical address references some location within a given process address space: the os must map every logical address(generated by some compiler ? assembler) to a physical address

Contiguous memory allocation:

• fit the entire process address space into physical mem in one contiguous block
  • each block has: start point, LIMIT (size in bytes)
• partitioned mem can be fixed / dynamic

Memory Allocation Schemes

Fixed Partitioning Scheme:

• N * {Size} (different sizes)
• Each process having exactly one partition
• OS manage the mapping between used partition - proc, keep a list of the free partitions

Dynamic Partition Scheme:

• size of a proc ---> size of the partition
  • on-demand partitioning
• cause memory fragmentation

|*********|___(finished)__(fragment)_|*********| 

• if a process arrives and requires more than the largest free partition, looking for defragmentation:
  • move some proc mem physical addresses upstream
  • new process on waiting

algorithms for determine newly arrived proc placement

• first-fit
  scan from the top until found

  • might cause more fragments

• next fit
  scan from the end until found

  • will cause less fragments (provide larger contiguous partition upstream)

• best-fit
  fit into the smallest possible partition

• worst-fit
  fit into the largest possible partition