uptime
是在阅读linuxjournal Hack and / - Linux Troubleshooting, Part I: High Load时，发现的这个命令。
uptime(1) - Linux man page
System load averages is the average number of processes that are either in a runnable or uninterruptable state.
A process in a runnable state is either using the CPU or waiting to use the CPU.
A process in uninterruptable state is waiting for some I/O access, eg waiting for disk.
The averages are taken over the three time intervals.
Load averages are not normalized for the number of CPUs in a system, so a load average of 1 means a single CPU system is loaded all the time while on a 4 CPU system it means it was idle 75% of the time.
NOTE: 理解load average是关键所在，而理解load average的关键在于理解上面这段话中的**normalized**，它的意思是：uptime输出中的load average指的是process个数，它并没有除以 CPU 个数，这就是 normalized 的含义所在。关于此，在 Hack and / - Linux Troubleshooting, Part I: High Load 中也对它进行了详细介绍，作者将它作为查看系统 load average 的第一命令。