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I have a lighttpd server which easily handles about 800 concurrent connections. But there seems to be a problem:

When the number of users grows, the CPU utilization grows as well, until a limit is reached at about 90%. The problem is when the number of the users drop down again... Even if I get back to around 300 to 400 users, the CPU is still at its peak and it seems to slow again as soon as more people come in again, say 500 to 600.

For example, right now I'm holding at 200 users but the CPU is still at 90% as if there were 900 people, when the server peaked...

What could be the cause of the problem? Might it be some configuration issue? RAM flushing? fastcgi? And how might I solve this problem? I really don't know what do, any advice?

Simply restarting the server is not an option as I would lose all connected users, and that's no good.

free -m results:

             total       used       free     shared    buffers     cached
 Mem:          1658       1365        292          0        123       1106
-/+ buffers/cache:        135       1522
 Swap:          895          0        895
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i'm sorry i'm not that good with servers. what do you mean exactly? – enrico May 8 '12 at 22:52
there is it, i updated the question. and this is with little people now.... it could handle atleast double, before the cpu breaking maximum. – enrico May 8 '12 at 23:01
Thanks, it rules out what I was thinking about. But I'm sure others will appreciate the information you gave now. – 0xC0000022L May 8 '12 at 23:09
Can you provide more about your lighthttpd config file? – Mike Fiedler May 8 '12 at 23:16
i left all the fields as default really... as a fresh install of an ec2 instance. so it's all defaults. – enrico May 8 '12 at 23:18

Run a system call tracer (such as strace, if this is a Linux server) and see if that reveals what lighttpd is doing.

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oh, well... even by running i must say that i really don't understand anything of it. also many hours after the peak time, the cpu seems to finally decrese so i don't think it would show anything relevat anyway. do you think that switching to another server, like nginx, could be a solution? – enrico May 9 '12 at 11:28

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