[CentOS] Find reason for heavy load

Noob Centos Admin centos.admin at gmail.com
Wed Dec 30 04:44:17 UTC 2009


My Centos 5 server has seen the average load jumped through the roof
recently despite having no major additional clients placed on it.
Previously, I was looking at an average of less than 0.6 load, I had a
monitoring script that sends an email warning me if the current load stayed
above 0.6 for more than 2 minutes. This script used to trigger perhaps once
an hour during peak periods. Even so, I seldom see numbers higher than 1.x

On 4th Dec, somebody from an Indian IP range started hammering my SMTP
service, attempting to use it as an open relay. Naturally that didn't work
and only end up budging my typical 400KB daily log report into 2MB~4MB
affairs.

After observing a few days to determine the IP range, I started blocking the
Indian subnet with apf. Initially I had problems with getting apf to wok
properly but after a couple of days managed to get the block working and my
daily log went back down to expected size when all those connection attempts
disappear from exim's log.

Now this is when my server load started to shoot through the roof with
figures like 8.64 5.90 3.62 being reported by my monitoring script,
triggering so often. I had to raise my threshold to 1.6 to keep my own
script from spamming myself.

I've tried changing several things on the server, since initially it seems
like the high load may be due to I/O wait. So I turning off non-essential
services like OpenNMS to see if that had any effect. I also turned off apf
and inserted rules manually into iptables to reduce the number of iptable
rules the system has to process.

All that doesn't seem to help much, I'm still getting consistent server
loads in the 2.x to 3.x range almost all the time.

The problem is using top, none of my processes are showing abnormal CPU%,
most are well under 5%, manually adding them up doesn't equate the 200% to
300% the load figures of 2.x and 3.x are indicating.

Even top's own summary says CPU % is in the 20~30% range, what's worrying is
the System% is also in the same range. I have no idea what is "system" doing
since it appears that anything running inside the kernel is lumped under
"system". Or why even totalling both % up, I would expect 50~60% to
translate to the expected load of 0.5~0.6 yet system load stats is 5x what's
expected.

I've installed utilities like dstat to try to see if I can figure out which
process is making the system calls that is clogging up the server but either
I don't understand it or it's not the right tool.

So I'll appreciate some advice on how/what should I do next to identify the
cause. Thanks in advance!
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