Process and Memory Management in Operating Systems

Posted on Jan 13, 2025 in Computers

Resource Management

Processes and Flows

  • Def Process: Program in execution.
  • Homepage More names:
    • Control-Flow
    • Tasks
    • Threads (I will comment with a Java language example)
    • Thread (of execution)
  • The Operating System (OS) assigns a data structure (BCP) to all processes.
  • What does the BCP contain?
    1. Current status
    2. Process Identifier
    3. Process priority
    4. Location in memory
    5. Resources used

Thread and States of Processes

  • Def Thread: Point of execution of a process (a process can have one or more threads).
  • Example: Word running (1 process) opening 1 document (1st thread), opening a 2nd document (2nd thread).
  • States of a process:
    1. Running on CPU
    2. Ready (waiting, active)
    3. Blocked (withheld), for example, waiting for a resource to be free (DVD player, …)
  • Additional Comments:
    • Any activity has a Process ID (PID). (already discussed in paragraph 3.1)
    • A running process may need (create) other processes (children). It has a Parent Process ID (PPID).

Transition of Processes

  • Question: Can a process change state? Answer: Yes
  • Consider the changes (transitions) in Figure 3.2 on page 56. (Study it, the explanatory text is next to its lower part).
  • Additional comments:
    • If a process (or thread) changes state, it’s called a context switch.
    • If the change is between threads of the same process, it’s a partial context switch (if between threads of different processes, it’s a complete context switch).
    • State changes are directly related to the priorities between processes.

Who handles all this?

Answer: The scheduler, using a scheduling process execution mechanism. Priority assignment is done by a scheduling algorithm. -> Quantum.

BCP

Read about BCP.

Scheduling Algorithms

  1. Round-Robin Algorithm:
    • Each process is assigned the same amount of run time (quantum).
    • The choice between processes is FIFO (First In, First Out).
    • Example: CPU P1 -> P2 -> P3 -> quantum = 5 ms
  2. FCFS Algorithm (First Come, First Serve):
    • It also uses a FIFO queue.
    • It starts a process, and only when it finishes, it starts the next one, and so on.
  3. STR Algorithm (Shortest Time Remaining):
    • Prioritizes processes that are short, with the idea of finishing them soon.
  4. SRTF Algorithm (Shortest Remaining Time First):
    • Prioritizes the one that has the least time remaining to finish.

RAM and Virtual Memory

Virtual Memory: Concept introduced in 1961 (Fotheringham). It consists of using part of the secondary memory as RAM (virtualization).

Problem: The lower speed of secondary memory causes slowdowns.

Question: Can I assign all (almost all) of the secondary memory as RAM?

Exchange

  • If a process needs to enter into memory:
    • The scheduler requests memory from the Memory Manager.

Fixed partitions, Variable partitions. Techniques: Paging, External Segmentation.

Paging, Segmentation, Swapping

Paging

  • Divide:
    • RAM into frames
    • Processes into pages
  • We place the various pages in frames.
  • We will need:
    1. A page table
    2. A page manager

Segmentation

  • We define blocks of varying size but with a maximum allowable size.
  • Technique used in processor architecture (design), which divides the running program into 4 blocks:
    • CS (Code Segment)
    • DS (Data Segment) Each segment corresponds to an internal CPU register.
    • ES (Extra Segment)
    • SS (Stack Segment)

Classification of Programs

(Depending on how, when, and where they are located in memory)

  1. Relocatable: They can change position in RAM if necessary.
  2. Re-entrant: If they can, they don’t need to go to disk (virtual memory).
  3. Resident: Permanently stay in memory (e.g., antivirus).
  4. Reusable: Used by more than one user at a time.