Floating Point Math in Bash, Part 2 (Wait for System Load)

If you run scripts that require a lot of execution time it's a good idea to try to avoid letting them overload your system. You can run them via nice, but if for example your script is sending a bunch of emails your email daemon isn't running via nice and it may itself get out of control. One way to deal with this is by using the values in /proc/loadavg to pause when your system load gets too high.

The contents of /proc/loadavg look something like this:

  $ cat /proc/loadavg
  0.05 0.14 0.09 1/243 12667

One slight problem that you'll notice is that the load average values are floating point values. This is where the floating point functions for bash that I wrote about come into play.

To wait for the load then we only need to extract the relevant value from /proc/loadavg then use float_cond described in the post I mentioned earlier to check if the load is above a certain point and pause if it is. We'll package this all up into its own file wait-for-load.sh so that it can be called from other scripts.

The command expects three command line arguments:

• The load average to pause at, as a floating point value
• The load average to restart at, as a floating point value
• The time, in seconds, to pause between load checks

The source for the script follows:

#!/bin/bash

source float.sh

if [[ $# -ne 3 ]]; then
    echo "Usage: $0 STOP-AT-LOAD RESTART-AT-LOAD PAUSE-SECONDS"
    exit 1
fi

stop_at=$1
restart_at=$2
pause_seconds=$3


##############################################################
function get_load()
{
    local loadavg=$(cat /proc/loadavg | cut -d ' ' -f 1)
    echo $loadavg
}


##############################################################
load=$(get_load)

if float_cond "$load >= $stop_at"; then
    echo -n "Load is $load, pausing"
    n=0
    while float_cond "$load >= $restart_at"
    do
        sleep $pause_seconds
        echo -n .
        load=$(get_load)
        let n++
        if [[ $n -eq 10 ]]; then
            echo -n $load
            n=0
        fi
    done
    echo
fi

# vim: tabstop=4: shiftwidth=4: noexpandtab:
# kate: tab-width 4; indent-width 4; replace-tabs false;

A sample run of the script follows:

  $ sh wait-for-load.sh 2.0 1.0 10
  Load is 5.06, pausing..........1.49...
  $