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!