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I found out that an imagemagick/convert application on my VPS is causing troubles due to high load on my box. Now i added renice with priority 10 to all of these convert-processes for throttling each minute.

However, I am not sure if it really slows down the process and relieves my CPU (only 1-core).

How to really make sure that specific processes do not increase the load significantly or attack the stable system?

I have >500MB free RAM, hardly any swap in use and a low iowait at any time. So I guess, it's really because of the cpu with only 1 core. Most of the time, this is enough and no waiting times. But in case of some conversions are running, it's failing.

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Re-nicing processes won't decrease your load average -- the lower-priority processes are still in the run queue, after all. However, load average is a poor measure of responsiveness, anyway. You should be monitoring the business-relevant performance statistics of your service (eg is latency important? Measure that. Is throughput important? Measure that). Correlating configuration changes like lowering the priority on your convert jobs against the business-level metrics should tell you whether your changes have had an effect, and if so, how much of an effect and whether it is sufficient.

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Which output will give me all the relevant information at once? It's hard to catch the metrics in realtime when the high load "happens"? nagios is only telling me that the load is high and i manually login and check top, free, iostat. I worry about nagios not giving me really useful information. – ledy Jul 2 '12 at 6:46
The relevant information is specific to your application and business requirements. The command is similarly specific. – womble Jul 2 '12 at 6:50
Business requirements: My priority is "only" on the mysql and apache2 which serve my website. It has normal traffic, no peaks. RAM is enough and I/O is average and maybe the 1-core cpu is a bottleneck. Usually it is enough, but sometimes there are some of the conversion processes which are NOT priority tasks. That's why i tried to renice them down to prio of 10. This is nonsense if renice does not affect anything strictly. – ledy Jul 2 '12 at 7:07
So monitor mysql and apache2; if their operation improves, then you have a win. nice does most assuredly affect the operation of the niced program, in precisely the manner it says on the tin; that it doesn't do what you expect is a limitation of your expectation, not the program. – womble Jul 2 '12 at 8:50

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