Wednesday Night Load Averages
Science | Wed, Dec 3, 2008 at 6:33:33 pm PST
With the new web server installed and configured, at peak traffic time today with more 3,000 visitors online, the load average never got above 0.87. Typical load average for that kind of activity with our previous, somewhat underpowered web server was in the neighborhood of 4 to 6.
Big win. That’s the kind of worth-the-work-and-expense percentage we like.
In case you’re not already confused enough, here’s a link explaining the Linux ‘top’ command’s load average statistics: Examining Load Average.
The three load-average values in the first line of top output are the 1-minute, 5-minute and 15-minute average. (These values also are displayed by other commands, such as uptime, not only top.) That means, reading from left to right, one can examine the aging trend and/or duration of the particular system state. The state in question is CPU load—not to be confused with CPU percentage. In fact, it is precisely the CPU load that is measured, because load averages do not include any processes or threads waiting on I/O, networking, databases or anything else not demanding the CPU. It narrowly focuses on what is actively demanding CPU time. This differs greatly from the CPU percentage. The CPU percentage is the amount of a time interval (that is, the sampling interval) that the system’s processes were found to be active on the CPU. If top reports that your program is taking 45% CPU, 45% of the samples taken by top found your process active on the CPU. The rest of the time your application was in a wait. (It is important to remember that a CPU is a discrete state machine. It really can be at only 100%, executing an instruction, or at 0%, waiting for something to do. There is no such thing as using 45% of a CPU. The CPU percentage is a function of time.) However, it is likely that your application’s rest periods include waiting to be dispatched on a CPU and not on external devices. That part of the wait percentage is then very relevant to understanding your overall CPU usage pattern.
The load averages differ from CPU percentage in two significant ways: 1) load averages measure the trend in CPU utilization not only an instantaneous snapshot, as does percentage, and 2) load averages include all demand for the CPU not only how much was active at the time of measurement.
