Nginx or php-fpm error?

I found my vps nginx error log like this

2013/04/06 03:28:04 [alert] 8557#0: mmap(MAP_ANON|MAP_SHARED, 5242880000) failed (12: Cannot allocate memory)2013/04/06 04:05:30 [alert] 10299#0: mmap(MAP_ANON|MAP_SHARED, 5242880000) failed (12: Cannot allocate memory)2013/04/06 04:05:58 [alert] 10786#0: mmap(MAP_ANON|MAP_SHARED, 5242880000) failed (12: Cannot allocate memory)

What is the caused?ram about 16GB

Regards