[[File:Big-load.png|thumb|[[htop]] displaying a significant computing load (top right: ''Load average:'')]] In [[UNIX]] [[computing]], the system '''load''' is a measure of the amount of computational work that a computer system performs. The '''load average''' represents the average system load over a period of time. It conventionally appears in the form of three numbers which represent the system load during the last one-, five-, and fifteen-minute periods.
== Unix-style load calculation ==
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The [[W (Unix)|<code>w</code>]] and [[Top (Unix)|<code>top</code>]] commands show the same three load average numbers, as do a range of [[graphical user interface]] utilities. In [[Linux]], they can also be accessed by reading the [[procfs|<code>/proc/loadavg</code>]] file.
An idle computer has a load number of 0 (the idle process isn'tis not counted). Each [[process (computing)|process]] using or waiting for [[Central processing unit|CPU]] (the ''ready queue'' or [[run queue]]) increments the load number by 1. Each process that terminates decrements it by 1. Most UNIX systems count only processes in the ''running'' (on CPU) or ''runnable'' (waiting for CPU) [[Process states|states]]. However, Linux also includes processes in [[uninterruptible sleep]] states (usually waiting for [[Hard disk|disk]] activity), which can lead to markedly different results if many processes remain blocked in [[Input/output|I/O]] due to a busy or stalled I/O system.<ref>{{Cite web|url=http://linuxtechsupport.blogspot.com/2008/10/what-exactly-is-load-average.html|title=Linux Tech Support: What exactly is a load average?|date=23 October 2008}}</ref> This, for example, includes processes blocking due to an [[Network File System (protocol)|NFS]] server failure or too slow [[Data storage device|media]] (e.g., [[Universal Serial Bus|USB]] 1.x storage devices). Such circumstances can result in an elevated load average which does not reflect an actual increase in CPU use (but still gives an idea of how long users have to wait).
Systems calculate the load ''average'' as the [[Moving average#Exponential moving average|exponentially damped/weighted moving average]] of the load ''number''. The three values of load average refer to the past one, five, and fifteen minutes of system operation.<ref name="drdobbs">{{cite web |url=http://www.linuxjournal.com/article/9001 |title=Examining Load Average |first=Ray |last=Walker |date=1 December 2006 |work=Linux Journal |access-date=13 March 2012 }}</ref>
