mysql error: Can't create a new thread (errno 11)

I have a db server running Percona Xtradb server, and 5 slaves. I always get the error mysql error: Can't create a new thread (errno 11); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug Though i set ulimited ``` root@master:~# ulimit -a core file size (blocks, -c) 0 data seg size (kbytes, -d) unlimited scheduling priority (-e) 0 file size (blocks, -f) unlimited pending signals (-i) 2062915 max locked memory (kbytes, -l) 64 max memory size (kbytes, -m) unlimited open files (-n) 1000000 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) 8192 cpu time (seconds, -t) unlimited max user processes (-u) 1000000 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited root@master:~# cat /proc/`pidof mysqld`/limits | egrep "(processes|files)"Max processes 1000000 1000000 processes Max open files 1000000 1000000 files ``` I really dont know what's going on? Please help me.