Too many open files

We starting to have too many open files exception starting with 6.3.3.4 in our GemFire servers.

Bridge server: Unexpected IOException from accept

java.net.SocketException: Too many open files

when this happened, there are about 

118 REG, 2456 FIFO, 321 TCP, and 1227 anon file type. We didn't experience this with eariler version?

Does anyone have any ideas what could cause this?

Thanks.