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Run cron job only when machine is inactive (linux)

How to run cron job (bash script) only when CPU is not working> 50%?

I can get cpu idle from TOP

top -b -d 00.10 -n 3 |grep ^Cpu Cpu(s): 0.3%us, 0.3%sy, 0.0%ni, 99.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st

My current setup:

 crontab 0,15,30,45 * * * * /usr/bin/php /home/user/batchprocess.php # I could use a bash script here to call PHP, if it is a good solution.

And I have a PHP script to idle the processor:

 batchprocess.php proc_nice(10); // wait for CPU idle do{ $cpu_stat = exec('top -b -d 00.10 -n 3 |grep ^Cpu'); $tmp = stristr($cpu_stat,'%id',TRUE); $cpuidle = trim(substr($tmp,strrpos($tmp,',')+1)); }while($cpuidle<$min_cpuidle); // do actual processing here

The problem with my current method is that it runs the program regardless of CPU usage. And the while loop working in TOP seems inefficient. I want it to start only when the CPU is idle> 50

Additional Information:

  • Centos 6.2, PHP5.3

  • I have several EC2 instances that never close, so I want to use their processing power in standby mode. But never load the server hard. (redundancy DB instance, development instance, NAT instance)

  • I know EC2 autoscaling, point instance. I just want to use the extra capacity.

  • background work - image compression (processor intensity, not a lot of input-output or network).

Any suggestion is welcome. thanks in advance!

Based on the introduction below, I realized that “good” is the best solution in my case. I have to redefine my goal to minimize the impact on the server, and not track CPU usage.

So, the new setting:

 crontab 0,15,30,45 * * * * nice -20 /usr/bin/php /home/user/batchprocess.php

And a PHP script:

 batchprocess.php if ($cpuidle < 50) exit(0); // do actual processing here

I will check it and send it back.

report: I put this code through DEV / PRD, it works very well. It does not solve the TOCTOU problem, but so far is good enough.

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2 answers

This is a typical TOCTOU case — you check to see if the system is idle and start your process — but after you check how or before starting your process, something else caused another process to start on the system, and you still load the system more than necessary.

The “right” way to do this is to give your process a low priority using the nice command. By the way, your CPU usage check cycle will use a 100% processor, so it probably won't work if the FIRST time you check it at idle.

You already have "proc_nice (10)", so it should do the job. I see no reason to spend effort on determining if the system is busy or not.

If you looked at the appropriate points in your code, you could do something like:

  if (check_cpu_usage() > 50%) sleep(1second);

But I'm not sure that if the system is busy, a “good” process will not get a lot of processor time and, therefore, will not compete with other processes that work with a higher priority.

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If you just want your script to be executed whenever the system boots, for example. 2.0 or less, you can use a shell script as follows:

 #!/bin/sh LOAD=`cat /proc/loadavg | cut -d" " -f1` THRESHOLD=2.0 if [ $(bc <<< "$LOAD <= $THRESHOLD") -eq 1 ]; then $@ fi

Save it, for example. /usr/local/bin/if-idle and insert if-idle before your command in the crontab file.

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