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what's the actual meaning of "load average"?

Posted on 1998-10-02
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Last Modified: 2008-03-06
Hi there,

I have a rough idea of what "load average" is. When more programs are running, the load average is higher.  But, how do this relate to things like the user, system and idle jiffies?

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Question by:oldjst
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mlev earned 30 total points
ID: 1638184
Load average is the average number of processes in the running queue, IOW, processes whose state is "RUNNING", IOW, processes between which the CPU time is shared (i.e.processes that are idle or waiting for I/O don't affect it)
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by:oldjst
ID: 1638185
More specifically, can we describe "load average" as user jiffies/ (system jiffies + idle jiffies) ? If not, is there any easy means to calculate that using the info available in the /proc file system. (Sine I need to calculate the load average for varying time length, the average data from say uptime is not so useful)

Thanks.
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by:mlev
ID: 1638186
You cannot calculate it based on jiffies. For example, if you have one heavy calculation job running, almost 100% jiffies will
be user jiffies, and the load average will be around 1.0.
And if you have 10 such jobs running simultaneously, almost
100% jiffies will still be user jiffies, but the load average
will be around 10.0.

That said, there is a file /proc/loadavg. It contains the following info:
The load average over the last 1, 5 and 15 minutes, updated every 5 seconds;
(*) The current number of running tasks and total number of tasks;
PID of the last process in the CPU (i.e. the process accessing the file).

You can sample (*) and compute the average of their ratio over an arbitrary period of time.
Hope this helps.
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