Why doesn't Siege run a test with more than 500 concurrent requests?

Question

ubuntu

Installed Siege for site load testing. I run a test with 500 concurrent requests:
/usr/bin/siege -c 500 -r 40 -f /etc/siege/urls.txt - easy (can be done without pauses),
and if you do -c 600 or more: / usr/bin/siege -c 700 -r 40 -f /etc/siege/urls.txt - Crash occurs:
ProblemType: Crash
Architecture: amd64
DistroRelease: Ubuntu 14.04
ExecutablePath: /usr/bin/siege
ProcCmdline: /usr/bin/ siege -c 700 -r 40 -f /etc/siege/urls.txt
ProcCwd: /etc/security
======= Backtrace: =========
/lib/x86_64-linux-gnu/ libc.so.6(+0x73e0f)[0x7ffe9a2d6e0f]
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x5c)[0x7ffe9a36e71c]
/lib/x86_64-linux-gnu/libc.so.6(+0x10a5e0)[0x7ffe9a36d5e0]
/lib/x86_64-linux-gnu/libc.so.6(+0x10b667)[0x7ffe9a36e667]
/usr/bin/siege(+ 0x141ea)[0x7ffe9b0b81ea]
/usr/bin/siege(+0x8598)[0x7ffe9b0ac598]
/usr/bin/siege(+0x87fd)[0x7ffe9b0ac7fd]
/usr/bin/siege(+0x99d3)[0x7ffe9b0ad9d3]
/usr/bin/siege (+0xae6e)[0x7ffe9b0aee6e]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x8182)[0x7ffe9ac69182]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7ffe9a35defd]
======= Memory map: ========
Some advise to increase open files, didn't help, now on the server:
ulimit -a
open files (-n) 100000
What to do?

Answer 1

And in dmesg, by any chance, there are no records about OOM Killer who came to visit you?