Chapter 2: Process and Thread Management

2.1: Processes

Definition: A process is a program in execution.

2.1.1: The Process Model

Even though in actuality there are many processes running at once, the OS gives each process the illusion that it is running alone.

Virtual time and virtual memory are examples of abstractions provided by the operating system to the user processes so that the latter “sees” a more pleasant virtual machine than actually exists.


Operating Systems

2.1.2: Process Creation

From the users or external viewpoint there are several mechanisms for creating a process.

  1. System initialization, including daemon (see below) processes.
  2. Execution of a process creation system call by a running process.
  3. A user request to create a new process.
  4. Initiation of a batch job.

But looked at internally, from the system's viewpoint, the second method dominates. Indeed in unix only one process is created at system initialization (the process is called init); all the others are children of this first process.

Why have init? That is why not have all processes created via method 2?
Ans: Because without init there would be no running process to create any others.

Definition of daemon

Many systems have daemon process lurking around to perform tasks when they are needed. Here is a definition:

daemon: /day'mn/ or /dee'mn/ n. [from the mythological meaning, later rationalized as the acronym `Disk And Execution MONitor'] A program that is not invoked explicitly, but lies dormant waiting for some condition(s) to occur. The idea is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon). For example, under {ITS}, writing a file on the LPT spooler's directory would invoke the spooling daemon, which would then print the file. The advantage is that programs wanting (in this example) files printed need neither compete for access to nor understand any idiosyncrasies of the LPT. They simply enter their implicit requests and let the daemon decide what to do with them. Daemons are usually spawned automatically by the system, and may either live forever or be regenerated at intervals. Daemon and demon are often used interchangeably, but seem to have distinct connotations. The term `daemon' was introduced to computing by CTSS people (who pronounced it /dee'mon/) and used it to refer to what ITS called a dragon; the prototype was a program called DAEMON that automatically made tape backups of the file system. Although the meaning and the pronunciation have drifted, we think this glossary reflects current (2000) usage.

2.1.3: Process Termination

Again from the outside there appear to be several termination mechanism.

  1. Normal exit (voluntary).
  2. Error exit (voluntary).
  3. Fatal error (involuntary).
  4. Killed by another process (involuntary).

And again, internally the situation is simpler. In Unix terminology, there are two system calls kill and exit that are used. Kill (poorly named in my view) sends a signal to another process. If this signal is not caught (via the signal system call) the process is terminated. There is also an “uncatchable” signal. Exit is used for self termination and can indicate success or failure.

2.1.4: Process Hierarchies

Modern general purpose operating systems permit a user to create and destroy processes.

Old or primitive operating system like MS-DOS are not fully multiprogrammed, so when one process starts another, the first process is automatically blocked and waits until the second is finished.

2.1.5: Process States and Transitions

The diagram on the right contains much information.

One can organize an OS around the scheduler.

2.1.6: Implementation of Processes

The OS organizes the data about each process in a table naturally called the process table. Each entry in this table is called a process table entry (PTE) or process control block.

2.1.6A: An addendum on Interrupts

In a well defined location in memory (specified by the hardware) the OS stores an interrupt vector, which contains the address of the (first level) interrupt handler.

Assume a process P is running and a disk interrupt occurs for the completion of a disk read previously issued by process Q, which is currently blocked. Note that disk interrupts are unlikely to be for the currently running process (because the process that initiated the disk access is likely blocked).

Actions by P prior to the interrupt:

  1. Who knows??
    This is the difficulty of debugging code depending on interrupts, the interrupt can occur (almost) anywhere. Thus, we do not know what happened just before the interrupt.

Executing the interrupt itself:

  1. The hardware saves the program counter and some other registers (or switches to using another set of registers, the exact mechanism is machine dependent).

  2. Hardware loads new program counter from the interrupt vector.
  3. As with a trap, the hardware automatically switches the system into privileged mode. (It might have been in supervisor mode already, that is an interrupt can occur in supervisor mode).

Actions by the interrupt handler (et al) upon being activated

  1. An assembly language routine saves registers.

  2. The assembly routine sets up new stack. (These last two steps are often called setting up the C environment.)

  3. The assembly routine calls a procedure in a high level language, often the C language (Tanenbaum forgot this step).

  4. The C procedure does the real work.
  5. The scheduler decides which process to run (P or Q or something else). This loosely corresponds to g calling other procedures in the simple f calls g case we discussed previously. Eventually the scheduler decides to run P.

Actions by P when control returns

  1. The C procedure (that did the real work in the interrupt processing) continues and returns to the assembly code.

  2. Assembly language restores P's state (e.g., registers) and starts P at the point it was when the interrupt occurred.

Properties of interrupts