Load (computing)

The system load is a UNIX computing term that describes the amount of work that a computer system is doing. The load average is the system load over a period of time. It is conventionally given as three numbers that represent the system load during the last one, five, and fifteen minute periods.

Load calculation under Unix(-like) systems

An idle computer has a load number of 0. Each process that is using CPU, waiting for CPU or is in uninterruptible sleep (usually waiting for disk activity) adds 1 to the load number.

The load average is calculated as the exponentially damped moving average of the load number. The three values of load average refer to the past one, five, and fifteen minutes of system operation.

For single CPU systems that are CPU-bound (where there are on average no processes in uninterruptible sleep), one can think of load average as a percentage of system utilization during the respective time period. For systems with multiple CPUs, the number needs to be divided by the number of CPUs in order to get a percentage.

For example, a load average of "3.73 7.98 0.50" on a single CPU system can be interpreted as:

the CPU was overloaded by 373% during the last minute.
it was only busy half of the time for the last fifteen minutes

Rather, this means that this CPU could have handled all of the work scheduled for the last minute if it were 3.73 times as fast (or if there were 3.73 times as many more CPUs), but over the last fifteen minutes it was twice as fast as it needed to be to keep up.

Conversely, in a system with four CPUs, a load average of 3.73 would indicate that there were on average 3.73 processes running or queued, that is, that the system is not lacking in this respect because 3.73 is still lower than 4.

Load calculation under Windows systems

On Microsoft Windows PC systems, the system load is given as an instantaneous percentage of CPU utilization.

Important things to note

Note that the load average is not a measure solely of CPU utilization, it is also a measure of disk I/O and, sometimes, network performance. It is only one factor in overall system performance (and is often the least significant).

Other meanings

Load (program linking and loading)

When loading compiled programs into computer memory, they are linked to the relevant program resources, and then the fully resolved codes were loaded into computer memory, for execution. This type of program is often called a linking loader.

Load (database loading)

When loading data into a database management system, a program designed to read input data, and then place it into database tables, is called a loader.

External links

UNIX® Load Average by Dr. Neil Gunther (This includes Linux)

Operating system technology

Load | Load average | Load average

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