nginx core dump explodes (no replies)

nginx version: nginx/1.11.1
built with OpenSSL 1.0.2h 3 May 2016

I try to debug those alerts currently, which only appear after a reload:

2016/06/17 13:10:49 [alert] 14624#0: *15709 open socket #626 left in connection 628

I compiled nginx with --with-debug and set the flags CFLAGS="-g -O0" ./configure...

The following core dump settings are defined:

debug_points abort;
working_directory /var/tmp/cores/;
worker_rlimit_core 512M;
error_log /var/log/nginx/error.log debug;

I can see that it starts to generate the core file but it becomes very large (>500GB) and removes it at the end.

Any idea what I'm doing wrong here?

Thanks!