Section 2.6. uptime Command

2.6. uptime Command

The uptime command is a quick way to print the CPU load averages.^[1]

^[1] w -u prints the same line of output, perhaps not surprising since w is a hard link to uptime.

$ uptime  12:29am  up 274 day(s), 6 hr(s),  7 users,  load average: 2.00, 1.07, 0.46 

The numbers are the 1-, 5-, and 15-minute load averages. They represent both utilization and saturation of the CPUs. Put simply, a value equal to your CPU count usually means 100% utilization; less than your CPU count is proportionally less than 100% utilization; and greater than your CPU count is a measure of saturation. To compare a load average between servers, divide the load average by the CPU count for a consistent metric.

By providing the 1-, 5-, and 15-minute averages, recently increasing or decreasing CPU load can be identified. The previous uptime example demonstrates an increasing profile (2.00, 1.07, 0.46).

The calculation used for the load averages is often described as the average number of runnable and running threads, which is a reasonable description.^[2] As an example, if a single CPU server averaged one running thread on the CPU and two on the dispatcher queue, then the load average would be 3.0. A similar load for a 32-CPU server would involve an average of 32 running threads plus 64 on the dispatcher queues, resulting in a load average of 96.0.

^[2] This was the calculation, but now it has changed (see 2.12.2); the new way often produces values that resemble those of the old way, so the description still has some merit.

A consistent load average higher than your CPU count may cause degraded application performance. CPU saturation is something that Solaris handles very well, so it is possible that a server can run at some level of saturation without a noticeable effect on performance.

The system actually calculates the load averages by summing high-resolution user time, system time, and thread wait time, then processing this total to generate averages with exponential decay. Thread wait time measures CPU latency. The calculation no longer samples the length of the dispatcher queues, as it did with older Solaris. However, the effect of summing thread wait time provides an average that is usually (but not always) similar to averaging queue length anyway. For more details, see Section 2.12.2.

It is important not to become too obsessed with load averages: they condense a complex system into three numbers and should not be used for anything more than an initial approximation of CPU load.