[OSTEP] 2. The Process

CH4: The Abstraction: The Process

Virtualizing the CPU: Time sharing

• technique running one process, then stopping it and running another, and so forth.
• allows users to run many concurrent processes
• to implement this, the OS will need
  • low-level machinery: mechanisms
    • i.e., context switch
  • high-level intelligence

1. The Abstraction: A Process

process

• simply, a running program
• constituted by:
  • Memory
    • instructions lie in memory
    • data that running program reads and writes sits in memory
    • thus memory that the process can address (address space) is part of process
  • Registers
    • program counter (PC) or instruction pointer (IP)
      • tells us which instruction of the program will execute next
    • stack pointer
      • manage the stack for function parameters, local variables, and return addresses

2. Process API

• must be included in OS.

• are available on any modern OS

• Create

  • create new process

• Destroy

  • destroy processes forcefully

• Wait

  • stop running

• Miscellaneous Control

  • suspend a process and then resume it

• Status

  • get some status information about a process
  • i.e., how long it has run for, what state it is in...

3. Process Creation

how programs are transformed into processes?

1. load its code and any static data into memory, into the address space of the process.
   • programs initially reside on disk in some kind of executable format
   • the OS read those bytes from disk and place them in memory somewhere
   • In early (or simple) OS : loading process is done eagerly
     • i.e., all at once before running the program
   • In modefn OS: perform the process lazily
     • i.e., by loading pieces of code of data only as they are needed during program execution
     • to unserstand this, concept of paging and swapping is needed
2. allocate the program's run-time stack (or just stack)
   • the OS allocates memory for local variables, function parameters, and return addresses
   • the OS initialize the stack with arguments: argc and the argv array of main() function
3. allocate memory for the program's heap
   • heap is used for explicitly requested dynamically-allocated data
     • requested by malloc(), freed by free()
     • linked lists, hash tables, trees, etc.
4. other initialization tasks
   • I/O setup
     • each process by default has three open file descriptors(standard input, output, error)
5. start the program running at the entry point(main())

4. Process States

a process can be in one of three states:

1. Running
   • process is running on a processor.
   • processor is executing instructions.
2. Ready
   • process is ready to run but for some reason, the OS has chosen not to run it at this given moment.
3. Blocked
   • process has performed some kind of operation that makes it not ready to run until some other event takes place.
   • i.e., I/O request to a disk

• Being moved from ready to running : process has been scheduled

• Being moved from running to ready : process has been descheduled

• initial: Sometimes a system will have an initial state that the process is in when it is being created

• final: a process could be placed in a final state where it has exited but has not yet been cleaned up

  • (in UNIX-based systems, this is called the zombie state

Example

• Figure 4.3: Tracing Process State: CPU Only

• Figure 4.4: Tracing Process State: CPU and I/O
  • Process 0 issues an I/O. At that point, process 0 is blocked, giving proceses 1 a chance to run.

5. Data Structures

The OS is a program. Like any program, the OS has some key data structures

• process list
  • to track the state of each process
    • currently running process
    • processes in ready
    • blocked processes
  • in xv6 kernel

    // the registers xv6 will save and restore
    // to stop and subsequently restart a process
    struct context {
    	int eip;
    	int esp;
    	int ebx;
    	int ecx;
    	int edx;
    	int esi;
    	int edi;
    	int ebp;
    };
    // the different states a process can be in
    enum proc_state {
    	UNUSED, EMBRYO, SLEEPING,
    	RUNNABLE, RUNNING, ZOMBIE
    };
    // the information xv6 tracks about each process
    // including its register context and state
    struct proc {
    	char* mem; // Start of process memory
    	uint sz; // Size of process memory
    	char* kstack; // Bottom of kernel stack
    	// for this process
    	enum proc_state state; // Process state
    	int pid; // Process ID
    	struct proc* parent; // Parent process
    	void* chan; // If !zero, sleeping on chan
    	int killed; // If !zero, has been killed
    	struct file* ofile[NOFILE]; // Open files
    	struct inode* cwd; // Current directory
    	struct context context; // Switch here to run process
    	struct trapframe* tf; // Trap frame for the
    	// current interrupt
    };
    

  • register context
    • for a stopped process
    • registers of stopped process will be saved to this memory location