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Csaba Toth
Asked: 2016-12-21 09:16:59 +0800 CST

cron.daily hanging my system?

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It happens about once a week and the system completely freezes. Doesn't react to Ctrl+Alt+F1, Ctrl+Alt+Delete, I need to hard reset it. Despite it happens different time of the day, every time I check the last message is from CRON:

Dec 20 08:17:01 titan CRON[4106]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)

/etc/cron.hourly is empty, there's just a .placeholder in it. I executed every task in /etc/cron.daily manually, but neither of them caused a crash that time.

How can I debug this further? I don't see any detailed logs about cron. Should I place log writes in the cron jobs? Is there any other place where daily or hourly cron jobs can be picked up besides /etc/cron.hourly and /etc/cron.daily?

I'm running 16.04 Ubuntu Studio, with Xfce. I had trouble before with my wifi (Getting Killer Wireless-AC 1535 working for installing Ubuntu 16.04) and even with the install to btrfs based RAID 0 NVMe setup, I don't suspect these though this time.

16.04

1 Answers

  1. Best Answer

    Cron is not hanging your system. The log is misleading you.

    Every hour, the cron job runs and adds that line to your log, before running the commands in /etc/cron.hourly.

    Just because it is the last line in your log does not mean the program that wrote that line is the culprit of the system hang. This is actually the opposite: at the time cron ran (at 08:17:01), you can be quite sure your system was working fine.

    Since cron runs this task every hour, all you know from the log is that the hang happened some time between 08:17:01 and 09:17:01.

    Most programs run without creating a single line in the log. It actually takes some (programming) effort to write a line in the log; a log entry is a feature of the program that wrote it; it is not surveillance data written by the system to ensure programs are well behaved.

    Most programs cannot hang the machine either. Most machine hangs are caused by faulty hardware and kernel driver issues. This is what you should investigate, especially with your hardware (Wi-Fi that needs specific firmware, NVMe that is still getting patches in Ubuntu, etc.).

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