Monday, December 7, 2015

Using machine monitoring for post-mortem kernel debugging.


My kid is a fan of The IT Crowd, so it didn't surprise me when I received the following bug report:
Hey Dad.  My computer was acting funny.  So, I just turned it off for awhile.  Just letting you know.
 My kid's desktop runs the following servers to collect and export machine measurements:
On another machine, I run Prometheus to collect, store, and analyze my measurements.  I wondered if I can use Prometheus debug what happened.

Up or Down

I would like to learn when the machine got turned off.  Prometheus uses HTTP to collect metrics from the target machine.  If Prometheus can finish this scrape, it stores a 1 in a time series named UP -- a failed scrape will get a value of 0. I learned two things:

  • The computer was turned off at 02:00.
  • The computer failed a scrape two hours earlier! This means Prometheus was unable to use http to collect metrics from the Collectd Exporter target.  Very odd.

up metric


I customized the existing prometheus console templates to work with collectd's plugins.  This makes it easier to view the common machine metrics in one spot.  I discovered the following from that machine's dashboard:

  • At one point, all the CPU and RAM were in use -- this was when the Prometheus HTTP scrape failed.
  • The machine went idle for two hours.
  • There was a spike before the machine was powered off.  I am guessing my kid used the soft power off button and didn't unplug the computer from the power outlet. 




At this point, I used the expression browser to hunt for another collectd metric.  I discovered that the whole swap partition was in use:

It looks like my kid used all the RAM and swap.  Afterwards the system didn't recover.  Not sure if that was a kernel issue or just regular userland ENOMEM kinda bugs.  I asked him what he was doing -- he said "changing settings to make a game go faster".

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