Section 2.4. CPU Saturation

2.4. CPU Saturation

The ktHR:r metric from vmstat is useful as a measure for CPU saturation. However, since this is the total across all the CPU run queues, divide ktHR:r by the CPU count for a value that can be compared with other servers.

Any sustained non-zero value is likely to degrade performance. The performance degradation is gradual (unlike the case with memory saturation, where it is rapid).

Interval time is still quite important. It is possible to see CPU saturation (kthr:r) while a CPU is idle (cpu:idl). To understand how this is possible, either examine the %runocc from sar -q or measure the run queues more accurately by using DTrace. You may find that the run queue is quite long for a short period of time, followed by idle time. Averaging over the interval gives both a non-zero run queue length and idle time.