Ubuntu 20.04 sudden crash while processing, how to know why?

[Bo Berglund]

I have a HP 8440w workstation running Ubuntu 20.04 LTS since an upgrade from
18.04 about a month ago. It is headless (no monitor attached) and when I need
the GUI I use VNC from Windows 10.

It runs scheduled at tasks to download hourly from a video stream using ffmpeg
and this has worked fine for several weeks.

The download task captures the stream for one hour into ts type files.

Now while the at command was running I started a command line execution of
another ffmpeg command to convert the format of an already downloaded ts file
into mp4 and with a smaller video size (from 1280x720 to 852x480).
So it rescales the video and converts into mp4 format.

This command is contained inside a script which was started using the time
function like this:
time convertvideosize 480 input.ts output.mp4

About a minute into the conversion the computer fan started up at higher speed
and then suddenly the computer crashed.

I started it again and looked at the /var/log/syslog file to see what happened
but I did not find anything I could use.
These are the 3 log lines before the crash:

Mar 13 07:10:01 polaris-ubuntu systemd-resolved[554]: message repeated 14 times:
[ Server returned error NXDOMAIN, mitigating potential DNS violation
DVE-2018-0001, retrying transaction with reduced feature level UDP.]
Mar 13 07:10:01 polaris-ubuntu CRON[104176]: (bosse) CMD
(/home/bosse/bin/logactivity)
Mar 13 07:10:01 polaris-ubuntu systemd-resolved[554]: Server returned error
NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction
with reduced feature level UDP.

Then follows a line with about 600 NUL chars (0x00)

Next is this presumably from the restart I did:

Mar 13 07:16:51 polaris-ubuntu systemd-modules-load[361]: Inserted module 'lp'
Mar 13 07:16:51 polaris-ubuntu systemd-modules-load[361]: Inserted module
'ppdev'
Mar 13 07:16:51 polaris-ubuntu kernel: [    0.000000] microcode: microcode
updated early to revision 0xa, date = 2018-05-08

The last modified time of the output.mp4 file is 2021-03-13 07:15:02, so that is
probably when the crash occurred.
This shows that the syslog has not been written to close to the crash event.

QUESTIONS:
Is there any way I can find out what happened that caused the crash?

And is there any Ubuntu tool to monitor the computer activity like Task Manager
on Windows, for example to see the CPU load and temperature?


-- 
Bo Berglund
Developer in Sweden