List of open files grows steadily during load test through mod_cluster

I was able to configure wildfly 10 modcluster to work with Apache mod_cluster (1.3.1). However, when I was doing a load test, I found out that the test through web server eventually caused error in wildfly instance and I also saw error log in Apache web server

The obvious error in wildfly instance is the so-called "java.net.SocketException: Too many files open". When I used the command lsop -u | grep TCP | wc -l, I can see the number grew steadily until the wildfly instance reported the error. This was when I sent requests through web server.

However, when I sent the requests through wildfly instance (app server) directly, the number did not grow, and the app server can take a lot heavier load without this issue.

The issue did not happen until many rounds of load tests were executed through web server. If I restart the web server, everything is working fine until I execute many rounds of load tests again