These days I get a very funny warning in the syslog.

syslog-ng[4484]: Number of allowed concurrent connections exceeded; num='10', max='10'

I have googled it for a while, it seems is the default values of syslog-ng limited the concurrent connections of any stream device to 10.

It can simply sloved by increase the limit.

in syslog-ng.conf

unix-stream("/dev/log" max-connections(20));