Load AverageIf you’ve spent some time on a Unix or Unix-like machine (e.g., Linux, OS X, Solaris, etc.) then you’re probably at least vaguely familiar with the concept of a load average. A system’s load average can be easily determined from the Unix shell by running the uptime command:

mmalone@www:~$ uptime
 15:37:38 up 133 days,  3:37,  3 users,
    load average: 0.37, 0.37, 0.41

The load average is also displayed by the w and top commands, and by pretty much every system monitoring package on the planet. But what the heck is a load average, exactly?

To most people, a load average is some mysterious number that is somehow related to the amount of work that their computer is currently handling. But what is a good load average, and how high is too high? The answer is actually quite simple. But first you have to understand what the load average is actually measuring.

Without getting into the vagaries of every Unix-like operating system in existence, the load average more or less represents the average number of processes that are in the running (using the CPU) or runnable (waiting for the CPU) states. One notable exception exists: Linux includes processes in uninterruptible sleep states, typically waiting for some I/O activity to complete. This can markedly increase the load average on Linux systems.

The load average is calculated as an exponential moving average of the load number (the number of processes that are running or runnable). The three numbers returned as the system’s load average represent the one, five, and fifteen minute moving load average of the system.

So, for a single processor machine a load average of 1 means that, on average, there is always a process in the running or runnable state. Thus, the CPU is being utilized 100% of the time and is at capacity. If you tried to run another process, it would have to wait in the run queue before being executed. For multiprocessor systems, however, the system isn’t CPU bound until the load average equals the number of processors (or cores, for multi-core processors) in the machine. My database server, for example, has two dual core processors. Thus, the system isn’t fully utilized until the load average reaches 4.

In summary, the load average is a moving average of the number of processes in the running or runnable states. You shouldn’t be worried about your system’s load unless it is consistently higher than the number of processors (or cores) in your machine. In general, you can calculate a system’s CPU utilization by dividing the load average by the number of processors/cores in the system.