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The Process Abstraction

Operating System Abstractions

Abstractions simplify application design by:
  • hiding undesirable properties,

  • adding new capabilities, and

  • organizing information.

Abstractions provide an interface to application programmers that separates policy — what the interface commits to accomplishing—​from mechanism — how the interface is implemented.

Example Abstraction: File

What undesirable properties do files hide?
  • Disks are slow!

  • Chunks of storage are actually distributed all over the disk.

  • Disk storage may fail!

What new capabilities do files add?
  • Growth and shrinking.

  • Organization into directories.

What information do files help organize?
  • Ownership and permissions.

  • Access time, modification time, type, etc.

Preview of Coming Abstractions

  • Threads abstract the CPU.

  • Address spaces abstract memory.

  • Files abstract the disk.

  • We will return to these abstractions. We are starting with an organizing principle.

thread

addressspace

file

The Process

Processes are the most fundamental operating system abstraction.
  • Processes organize information about other abstractions and represent a single thing that the computer is "doing."

  • You know processes as app(lication)s.

Organizing Information

Unlike threads, address spaces and files, processes are not tied to a hardware component. Instead, they contain other abstractions.

Processes contain:
  • one or more threads,

  • an address space, and

  • zero or more open file handles representing files.

process
Figure 2. The Process

Process as Protection Boundary

The operating system is responsible for isolating processes from each other.

  • What you do in your own process is your own business but it shouldn’t be able to crash the machine or effect other processes—or at least processes started by other users.

  • Thus: safe intra-process communication is your problem; safe inter-process communication is an operating system problem.

Intra-Process Communication: Easy

intraprocess
  • Communication between multiple threads in a process is usually accomplished using shared memory.

  • Threads within a process also share open file handles and both static and dynamically-allocated global variables.

  • Thread stacks and thus thread local variables are typically private.

Intra-Process Communication: Easy…​ Maybe

intraprocess
  • Sharing data requires synchronization mechanisms to ensure consistency.

  • We will return to this later.

Inter-Process Communication: Harder

interprocess
  • A variety of mechanism exist to enable inter-process communication (IPC), including shared files or sockets, exit codes, signals, pipes and shared memory.

  • All require coordination between the communicating processes.

  • Most have semantics limiting the degree to which processes can interfere with each other.

    • A process can’t just send a SIGKILL to any other process running on the machine!

$ wait # IPC: return codes

returncode
  • Simplest and most-limited form of IPC.

  • Allows processes to return a single int to the process that created them.

  • 0 typically indicates success; non-0, failure.

  • Analogous to older computers that would transform a set of punch cards into a "result."

  • bash exposes return codes as $?:

returncode bash

$ p | pes # IPC: pipes

pipes
  • Pipes create a producer-consumer buffer between two processes.

  • Allows the output from one process to be used as the input to another.

  • The operating system manages a queue for each pipe to accommodate different input and output rates.

  • Facilitates the canonical chaining together of small UNIX utilities to do more sophisticated processing.

$ kill -9 ubshuttle # IPC: signals

signals

  • Signals are a limited form of asynchronous communication between processes.

  • Processes can register a signal handler to run when a signal is received.

  • Users can send signals to processes owned by them; the super-user can send a signal to any process.

  • Processes can ignore most signals.

    • SIGKILL is a notable exception; used for non-graceful termination.

    • SIGTERM is used for graceful shutdown.

We Will Return to IPC

  • return codes — we will discuss again when we learn about wait() and _exit().

    • You will implement return codes for ASST2.

  • pipes — we will discuss again when we learn about fork().

  • signals — are difficult to use and implement and not something we will return to.

  • shared memory — we will discuss again when we learn about address spaces and virtual memory.

Processes v. Threads

  • Potential confusing due to overlapping terminology—​we can describe both a process and a thread as running.

  • Terminology can be helpful for remembering the distinction:

    • A computing process requires multiple resources: the CPU, memory, files, etc.

    • A thread of execution abstracts CPU state.

  • Processes contain threads; threads belong to a process.

    • Only one exception: the kernel may have threads of execution not associated with any user process

    • (Except the kernel process, which is a process, remember?)

  • A process is considered to be running when one or more of its threads are running.

  • Different operating systems use different terminology, but share common ideas.

Process Example: Firefox

Firefox has multiple threads. What are they doing?
  • Waiting for and processing interface events: mouse clicks, keyboard input, etc.

  • Redrawing the screen as necessary in response to user input, web page loading, etc.

  • Loading web pages—​usually multiple parts in parallel to speed things up.

Firefox is using memory. For what?
  • Firefox.exe: the executable code of Firefox itself.

  • Shared libraries for web page parsing, security, etc.

  • Stacks storing local variables for running threads.

  • A heap storing dynamically-allocated memory.

Firefox has files open. Why?
  • Configuration files.

  • Fonts.

$ top # more process information

top

Process Example: bash

  • Let’s do this for real using standard Linux system utilities.

Finding bash

finding bash
  • ps aux gives me all process, then grep for the one I’m after.

  • …​or, do it all in one shot using pgrep.

  • …​or, if I know it’s running in my current session a bare ps will do.

bash

process bash 3

$ ps -Lf # thread information

threads bash
What are:
  • UID: user the process is running as.

  • PID: process ID.

  • PPID: parent process ID.

  • PRI: scheduling priority.

  • SZ: size of the core image of the process (kB).

  • WCHAN: if the process is not running, description of what it is waiting on.

  • RSS: total amount of resident memory is use by the process (kB).

  • TIME: measure of the amount of time that the process has spent running.

threads bash
  • If bash had multiple threads running this view would show them, so bash does not have multiple threads.

bash

process bash 2

$ ps # process information

  • I wish we could see a process with multiple threads…​

$ ps -Lf # thread information

ps threads

Created 2/17/2017
Updated 8/17/2017
Commit 4eceaab // History // View
Built 1/29/2016 @ 19:00 EDT