ulimit - user limit set in linux

1. check your limitation
ulimit -a
ulimit -n (number of open file)

2. set new limitation temporarily
ulimit -n newvalue

3. set new limitation permenant

3.1 vi /etc/security/limits.conf
add
username - nofile 2048

3.2. sysctl -p


4. set limitation system-wide
4.1 vi /etc/sysctl.conf
4.2 sysctl -p


reference:
http://cherry.world.edoors.com/CPZKoGkpxfbQ
http://www.linuxquestions.org/questions/slackware-14/increasing-file-descriptors-228004/
http://www.xenoclast.org/doc/benchmark/HTTP-benchmarking-HOWTO/node7.html