Load (computing)

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.

All Unix and Unix-like systems generate a metric of three "load average" numbers in the kernel. Users can easily query the current result from a Unix shell by running the uptime command:

The w and top commands show the same three load average numbers, as do a range of graphical user interface utilities. In GNU/Linux, they can also be accessed by reading the /proc/loadavg file.

An idle computer has a load number of 0. Each process using or waiting for CPU (the ready queue or run queue) increments the load number by 1. Most UNIX systems count only processes in the running (on CPU) or runnable (waiting for CPU) states. However, Linux also includes processes in uninterruptible sleep states (usually waiting for disk activity), which can lead to markedly different results if many processes remain blocked in I/O due to a busy or stalled I/O system. This, for example, includes processes blocking due to an server failure or too slow media (e.g., 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 on how long users have to wait).

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