Process

A process is a program that has been loaded into memory and is executing. The operating system gives each process a process ID (PID), its own address space, and a share of CPU time.

Why it matters

Looking at processes explains most performance and outage problems: which program uses all the memory, which one is stuck, and whether a service is actually running. A single program can start several processes, and each process can contain multiple threads.

When a process uses too much memory, Linux may terminate it with the out-of-memory killer.

Example

$ ps aux --sort=-%mem | head -3
USER   PID %CPU %MEM COMMAND
mysql  941  2.1 38.4 /usr/sbin/mysqld

The database process uses about 38 percent of memory.

Common mistakes

• Killing a process with SIGKILL first, which gives it no chance to save data; try SIGTERM.
• Confusing a process with a thread.
• Leaving zombie or runaway processes running unnoticed.

Related

• Daemon (Background Service)
• Kernel
• Bash
• Operating System (OS)

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