apache high system CPU time

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http://serverfault.com – I have a web server: ubuntu apache+php app+postgresql and a stats server: ubuntu apache+php - piwik and munin2 installed. The communication for munin2 is made through ssh. In munin i see a lot of system cpu activity, that I assume it it because of apache (i see 5 or 6 apache instances using ~5% CPU on top) I was not having this system CPU activity before. Does anyone knows how can I see where that comes from? EDIT: some munin graphs (HowTos)