Please suggest performance tweak and the right siege options for load test

2 nginx 1.4.6 web servers - ocfs cluster, web root inside mounted LUN
from SAN storage
2 MariaDB 5.5 servers - galera cluster, different network segment than
nginx web servers

nginx servers each two sockets quad core xeon, 128 gb ram
Load balanced via F5 load balancer (round-robin, http performance)

Based on my setup above, what options that I should use with siege to
perform load term to at least 5000 concurrent users?

There is a time when thousands of student storms university’s web

Below is result for 300 concurrent users.

siege -c 300 -q -t 1m

siege aborted due to excessive socket failure; you
can change the failure threshold in $HOME/.siegerc

Transactions: 370 hits
Availability: 25.38 %
Elapsed time: 47.06 secs
Data transferred: 4.84 MB
Response time: 20.09 secs
Transaction rate: 7.86 trans/sec
Throughput: 0.10 MB/sec
Concurrency: 157.98
Successful transactions: 370
Failed transactions: 1088
Longest transaction: 30.06
Shortest transaction: 0.00

Posted at Nginx Forum:

Have you checked the socket level, and checking kernel log on all 3
servers (nginx and load balancer) meanwhile doing the test?
It could be that for some reason you reach a limit really fast (We had
an issue that we reached the nf_conntrack limit at 600 concurrent users
because we had like 170 requests per page load)

I tried siege a lot, but could never get it to really use all cores on
server, I found the tool wrk much more useful for load testing.