The story of the 20°C cronjobThu 25 May 2017
For the last month or so, the lifespan of my beloved Thinkpad X1 Carbon battery had been getting down the drains, from 5-6 hours to less than 3. Following @padenot's advice, I installed
powertop and started investigating what was draining this good'ol battery of mine.
Looking at the
powertop output, I immediatly realized that something fishy was happening on this laptop:
The battery reports a discharge rate of 4.95 W The estimated remaining time is 2 hours, 6 minutes Summary: 1111.7 wakeups/second, 7.9 GPU ops/seconds, 0.0 VFS ops/sec and 23.0% CPU use Usage Events/s Category Description 264.4 ms/s 3656.7 Process /bin/bash /usr/sbin/sendmail -FCronDaemon -i -odi -oem -oi -t -f br 114.3 ms/s 626.2 Process /usr/lib64/firefox/firefox 20.7 ms/s 95.5 Process /opt/sublime_text_3/plugin_host 3272 ...
sendmail so busy, and why in the hell was it running anyway?
strace showed me that the process was indeed very busy, and
mailq showed that I had more than 15000 outgoing emails in the system mail queue!
$ mailq ... mail in dir /home/br/.esmtp_queue/TSRueRJD: From: "(Cron Daemon)" <br> To: br mail in dir /home/br/.esmtp_queue/ZI1LtzhT: From: "(Cron Daemon)" <br> To: br 15653 mails to deliver
Ok, so all these mails were being sent by
cron. My user crontab only had one job, and it was
* * * * * rm $HOME/crash_dump.erl. Indeed, I had been experimenting with Elixir recently, and when I crashed the Erlang VM, this file would pop-up in my home directory. At some point, I added this cronjob to make it go away and forgot about it. As the job's
stdout was not redirected to
/dev/null, each time the file was not found, the cron job would fail and a mail would be added to the queue.
After removing this job, purging the mail queue, and adding
MAILTO="" at the beginning of my crontab (to avoid repeating this investigation down the road),
sendmail went quiet, my battery life went back to ~6 hours, and the laptop average temperature went down 20°C.