High memory usage on the server - can't determine the process

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http://serverfault.com – I've noticed high memory usage on the server. Details: OS: CentOS 6.3 - x86_64 Web server: Nginx with PHP-FPM The server is generating PDF documents so the traffic is minimum. top: # top -b -n 1 -a top - 10:04:51 up 21 days, 18:57, 1 user, load average: 0.00, 0.00, 0.00 Tasks: 92 total, 1 running, 91 sleeping, 0 stopped, 0 zombie Cpu(s): 0.3%us, 0.2%sy, 0.0%ni, 99.6%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 3923092k total, 3720380k used, 202712k free, 133904k buffers Swap: 4194296k total, 12k used, 4194284k free, 147404k cached PID USER PR NI VIRT (HowTos)