Message-ID: <441180DD.3020206@wpkg.org>
Date: Fri, 10 Mar 2006 14:36:29 +0100
From: Tomasz Chmielewski <mangoo@wpkg.org>
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Subject: can I bring Linux down by running "renice -20 cpu_intensive_process"?
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I have a Linux server (kernel 2.6.8.1 + Linux RAID1) which is a "backup" 
machine: it gets the files from other servers, compresses it, writes to 
the tape, checks md5sums etc.

It's been running for quite a bit, no problems with stability so far.

Yesterday, something happened though.

I was logged in remotely, and the system was running md5sum against a 30 
GB file.

I wanted the things to speed up a bit, and made "renice -20 <md5sum_pid>".

Few minutes after that I couldn't start any process, so I thought I made 
the system so busy with renice -20, that my SSH session probably 
disconnected.

In the morning, the system was still unavailable - I could ping it, I 
could telnet to any of the ports opened, but nothing more happened.

SSH was waiting forever after:

debug1: identity file /root/.ssh/identity type -1
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_dsa type -1


Nothing was displayed on the monitor (all black).

As I restarted the machine, I saw that the logging ends few minutes 
after I changed the priority of md5sum to -20.


So here is my question: is it possible to bring down the machine by 
simply doing "renice -20 cpu_intensive_process"?

As I said, this machine does heavy compression and md5sum calculations 
of big files every day, and was stable all the time - but stopped 
responding after I changed the priority of a CPU-intensive process to -20.

Coincidence and a hardware failure?

-- 
Tomasz Chmielewski
http://wpkg.org
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