- Posted on
Introduction
A computer doing more than one thing at a time is using processes, these require resources, CPU time, memory and access to other devices like CD/DVD/USB drives, etc. Each process is allocated an amount of system resources to perform its function which is controlled by the operating system whose job it is to facilitate these processes. Signals have an important part to play on the interaction of the processes, usually these send exit signals and other information to each other, or to itself.
Programs, Processes, and Threads
A program is a set of instructions to be carried out which may local data such as information for output to the terminal via read or external data which may come from a database. Common programs include ls, cat and rm which would reside outside of the user operating system thus they have their own operating program on disk.
A process is a program which is executing which has associated resources, furthermore. These might include environments, open files or signal handlers etc. At the same time, two or more tasks or processes may share system resources meaning they are multi-threaded processes.
In other operating systems there may be a distinction between heavy-weight and light-weight processes; furthermore, heavy-processes may contain a number of lightweight processes. However, in Linux, each process is considered heavy or light by the amount of shared resources they consume, for faster context switching. Again, unlike other operating systems, Linux is much faster at switching between processes, creating and destroying them too. This means the model for multi-threaded processes is similar to that of simultaneous processes as they are treated as one. All the same, Linux respects POSIX and other standards for multi-threaded processes, where each thread returns the same process ID plus returning a thread ID.
Processes
Processes are running programs in execution, either running or sleeping. Every process has a process ID (pid), parent process ID (ppid) and a parent group ID (ppgid). In addition every process has program code, data, variables, file descriptions and an environment.
In Linux, init is the first process that is run, thus becoming the ancestor to all other programs executed on the system; unless they are started directly from the Linux Kernel, for which they are grouped with brackets ([]) in the ps listing. Other commonalities are the processes pid numbering, if a parent process ends before the child, the process will become adopted by init, setting its process ID (pid) to 1. If the system is more recent, this will result in the pid being set to 2 inline with systemd’s kernel thread kthreadd. In the circumstance where a child dies before its parent it enters a zombie state, using no resources and retaining only the exit code. It is the job of init to allow these processes to die gracefully and is often referred to as being the zombie killer or child reaper. Finally, the processes ID can not go above 16bit definition hence 32768 is the largest pid you will find on a Linux system; to alter this value see /proc/sys/kernel/pid_max; once this limit is reached they are restarted at 300.
Process Attributes
- All processes have certain attributes, as shown below:
- Program
- Context
- Permissions
- Resources
The program is the process itself, maybe it is a set of commands in a script or a loop which never ends checking against external data and performing an action when necessary.
The context is the state which the program is in at any point in time, thus a snapshot of CPU registers, what is in RAM and other information. Furthermore, processes can be scheduled in and out when sharing CPU time or put to sleep when waiting for user input, etc. Being able to swap out and put back the process context is known as context switching.
Permissions are inherited from the user who executed the program however programs themselves can have an s property assigned to them In order to define their effective user ID, rather than their real ID and are referred to as setuid programs. These setuid programs run with the permissions of the creator of the program, not the user who ran it. A commonly found setuid program is passwd.
Process Resource Isolation
This is the practice of isolating the process from other processes upon execution, promoting security and stability. Furthermore, processes do not have direct access to hardware devices, etc. Hardware is managed by the kernel meaning system calls must be made in order to interact with said devices.
Controlling Processes (ulimit)
This program, umlimit, reports and resets a number of resource limits associated with processes running under its shell. See below for a typical output of ulimit -a.
Here you see a screenshot of ulimit -a which contains values important for a system administrator to be aware of because it identifies the allocation of resources. You may want to restrict or expand resource allocation depending on the requirements of the processes and / or file access limits.
Hard and soft resource allocation terms come into play here, with hard limits imposed by the administrator and soft limited by those restrictions but allowing for user-level limits to be imposed. Run ulimit -H -n to see hard limits and ulimit -S -n for soft limits. File descriptors are probably the most common thing that may need changing, to allow more resources, typically this is set to 1024 which may make execution of an application virtually impossible so you can change this with ulimit -n 1600 which would change it to 1600. In order to make more permanent changes you would need to edit /etc/security/limits.conf then perform a reboot.
Process States
Processes can take on many different states, which is managed by the scheduler. See below for the main process states: - Running - Sleeping (waiting) - Stopped - Zombie
A running process is either using the CPU or will be waiting for its next CPU time execution slice, usually cited in the run queue - resuming when the scheduler deems it satisfactory to re-enter CPU execution.
A sleeping process is one that is waiting on its request that can not be processed until completed. Upon completion, the kernel will wake the process and reposition it into the run queue.
A stopped process is one that has been suspended and is used normally by developers to take a look at resource usage at any given point. This can be activated with CTRL-Z or by using a debugger.
A zombie process is one that has entered terminated state and no other process has inquired about it, namely “reaped it” - this can often be called a “defunct process”. The process releases all of its resources except its exit state.