top 中的 load average 對 performance 的真正影響為何?

通常聽到的說法是, 如果 CPU 有 4 顆, load average 超過 8 (CPU 個數的兩倍) 的時候, 代表 CPU 的 loading 很重, 但往往此時還有 idle, 也許是 10% ~ 20%, 雖然有 idle 但真的覺得慢

1. 如果load average/CPU數量 < 3, performance is good
2. 如果load average/CPU數量 > 5, performance is very bad

The first CPU-related information is present on the very first line: the load average. The load average is a number that corresponds to the average number of runnable processes on the system. The load average is often listed as three sets of numbers (as seen here), which represent the load average for the past 1, 5, and 15 minutes, indicating that the system in this example was not very busy.

http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/admin-primer/s1-bandwidth-rhlspec.html

在console端，查看系统负载的运行状况，可以key入指令top
显示 load average，此处有三个值显示，明细如下：
load average: 0.00, 0.00, 0.00
这表示系统前5 分钟，前15分钟，前30分钟，系统负截的情况，一般此处值不超6~7，为正常