Q : Nginx with libevent

I am trying to use nginx load balancing with an http server running on
top of libevent. Because nginx makes connection requests in round
robin fashion I am seeing a strange behavior when one of the http server
is running a long query against backend database. What I am seeing is
nginx picks this particular server and the request from client blocks on
the call until the server comes back and entertains the request. Is
there a way I can tell nginx to not use a connection when there’s
already a pending request ? I guess BIG IP does something similar where
you can pick LRU algorithm and it will skip over active connections
until we exhaust every server in the pool.