Week 5

Published

May 12, 2025

VL 9 - 12.05.25

IPC via Sharem Memory

SM in Posix

  • process first creates a shared memory segment:

    segment_id = shmget(IPC_PRIVATE, size, S_IRUSR | S_IWSUR);

  • processes that want access to this segment have to subscibe / attach to it.

  • after the process is done it detaches from the shared memory with shmdt(shared_memory).

  • Problem: how other subscibers get this value segment_id \(\Rightarrow\) fork()

  • alternative (in windows): Memory-mapped-data (MMD) are used for IPC

  • handles (look up)

Swapping

  • Swapping: the complete memory image of a process is backed up / stored in the primary memory in order to protect the RAM and allow more processes.
  • Problems:
    • when processes are swapped in and out the memory addresses in the RAM change - how to remap or recalculate?
    • swapping takes time - seconds
    • processes that are currently doing I/O can’t be swapped in / out,
      • solution: prioritize swapping in / out the waiting or blocked processes
      • additional process state: blocked and suspended means swapped out
      • additional process state: ready and suspended means ready but still swapped out, needs to be first swapped in
    • internal fragmentation of RAM.

Memory Management

  • In past systems and actual memory and the addressable memory was the same - very limiting.
  • General question how to allocate memory to incoming processes ? Various approaches
    • fixed-sized parititions: obvious problems - partitions are too small or too large
    • variable-sized partition schemes. problems:
      • address relocation: without virtual memory the address of the process must be reculculated in the program if swapped in and out
      • contigious address: without virtual memory the complete process has to have a contingious memory
      • external fragmentation (btw, external fragmentation is worse in primary memory). How to solve this problem:
        • Reorganize the RAM \(\Rightarrow\) compaction. Problems:
          • This is time intensive
          • is I/O is happening problematic
          • works only for code reallocation in runtime.
        • use fitting schemes for memory allocation that allocated the memory well
          • first fit:
          • best fit
          • worst fit (usually worse than two other solutions)

Intro to Virtual Memory - Logical vs Physical Memory

previous twe problems:

  1. P1: illusion of fixed memory address from 0 to max
  2. P2: illusion of contigious memory

are solved by introducing the concept logical memory (mapping physical to logical memory):

  • The cpu and the processes see (or use) completely different addresses from the actual memory addresses from the ram
  • the mapping is implemented directly in hardware by teh memory-management-unit - MMU.(usually inside the processor)
  • These logical addresses seen by the cpu and proccesses are called virtual memory / addresses.
  • The addresses in the RAM are physical memory / addresses
  • how is the mapping implemented? The process and the cpu sees the addresses from 0 to max. If the actual process is located in the memory at some address. MMU simply adds the offset to this address.
  • second problem of contigious memory is solved with a table but the table is not refined up to bytes, rather up to pages (more rough subdivision of a process) \(\Rightarrow\) page table
    • page index (page = Seite)
    • frame index (Frame = Seitenrahmen)

VL 10 - 14.05.25

Memory Management

  • a page index must be translated to a frame index
    • page is the virtual thing seen by the process and the processor,
    • frame is the actual physical location in the RAM.
      • Translated with a function: \(F_{\text{pid}}(p) = f\)
      • p := page index
      • f := frame
    • how does the translation work ? examples in the lecture