39.7 Checking System Load: uptime

The BSD command uptime, also available under System V Release 4, AIX, and some System V Release 3 implementations, will give you a rough estimate of the system load:

% uptime
3:24pm up 2 days, 2:41, 16 users, load average: 1.90, 1.43, 1.33

uptime reports the current time, the amount of time the system has been up, and three load average figures. The load average is a rough measure of CPU use. These three figures report the average number of processes active during the last minute, the last five minutes, and the last 15 minutes. High load averages usually mean that the system is being used heavily and the response time is correspondingly slow. Note that the system's load average does not take into account the priorities and niceness (39.9) of the processes that are running.

What's high? As usual, that depends on your system. Ideally, you'd like a load average under, say, 3, but that's not always possible given what some systems are required to do. Higher load averages are usually more tolerable on machines with more than one processor. Ultimately, "high" means high enough so that you don't need uptime to tell you that the system is overloaded - you can tell from its response time.

Furthermore, different systems will behave differently under the same load average. For example, on some workstations, running a single CPU-bound background job at the same time as the X Window System (1.31) will bring response to a crawl even though the load average remains quite "low." In the end, load averages are significant only when they differ from whatever is "normal" on your system.