file descriptor exceeds limit

[Shawn Zhou]

Hello,
BIND on my resolvers reaches the max open file limit and I am getting lots of SERVFAILs
http://pastebin.com/SxRsHLff

After I increased the max-socks (-s 8192) to 8192, I no longer saw the file limit error from the log anymore; however, I am still many SERVFAILs.
Our resolvers were doing about 15k queries per seconds when this was happening and those were legit traffic. I am aware that I am setting recursive clients to a very high number. Those resolvers are running on 12-cores cpu and 24G RAM hardware. cpu utilization was at about 20% and plenty of RAM left.
I am wondering if I've reached the limit of BIND for the amount of recursive queries it can serve. Any other tunings I should try? By the way, the resolvers are running RHEL 6.x.
Thanks,Shawn
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