Bash: wait until CPU usage gets below a threshold

Question

In a bash script I need to wait until CPU usage gets below a threshold.

In other words, I'd need a command wait_until_cpu_low which I would use like this:

# Trigger some background CPU-heavy command
wait_until_cpu_low 40
# Some other commands executed when CPU usage is below 40%

How could I do that?

Edit:

• target OS is: Red Hat Enterprise Linux Server release 6.5
• I'm considering the average CPU usage (across all cores)

Answer 1

A much more efficient version just calls mpstat and awk once each, and keeps them both running until done; no need to explicitly sleep and restart both processes every second (which, on an embedded platform, could add up to measurable overhead):

wait_until_cpu_low() {
    awk -v target="$1" '
    $13 ~ /^[0-9.]+$/ {
      current = 100 - $13
      if(current <= target) { exit(0); }
    }' < <(LC_ALL=C mpstat 1)
}

I'm using $13 here because that's where idle % is for my version of mpstat; substitute appropriately if yours differs.

This has the extra advantage of doing floating point math correctly, rather than needing to round to integers for shell-native math.

Answer 2

wait_for_cpu_usage()
{
    current=$(mpstat 1 1 | awk '$12 ~ /[0-9.]+/ { print int(100 - $12 + 0.5) }')
    while [[ "$current" -ge "$1" ]]; do
        current=$(mpstat 1 1 | awk '$12 ~ /[0-9.]+/ { print int(100 - $12 + 0.5) }')
        sleep 1
    done
}

Notice it requires sysstat package installed.

Answer 3

You might use a function based on the top utility. But note, that doing so is not very reliable because the CPU utilization might - rapidly - change at any time. Meaning that just because the check succeeded, it is not guaranteed that the CPU utilization will stay low as long the following code runs. You have been warned.

The function:

function wait_for_cpu_usage {
    threshold=$1
    while true ; do
        # Get the current CPU usage
        usage=$(top -n1 | awk 'NR==3{print $2}' | tr ',' '.')

        # Compared the current usage against the threshold
        result=$(bc -l <<< "$usage <= $threshold")
        [ $result == "1" ] && break

        # Feel free to sleep less than a second. (with GNU sleep)
        sleep 1
    done
    return 0
}

# Example call
wait_for_cpu_usage 25

Note that I'm using bc -l for the comparison since top prints the CPU utilization as a float value.

Answer 4

As noted by "Mr. Llama" in a comment above, I've used uptime to write my simple function:

function wait_cpu_low() {
  threshold=$1
  while true; do
    current=$(uptime | awk '{ gsub(/,/, ""); print $10 * 100; }')
    if [ $current -lt $threshold ]; then
      break;
    else
      sleep 5
    fi
  done
}

In awk expression:

• $10 is to get average CPU usage in last minute
• $11 is to get average CPU usage in last 5 minutes
• $12 is to get average CPU usage in last 15 minutes

And here is an usage example:

wait_cpu_low 20

It waits one minute average CPU usage is below 20% of one core of CPU.