每隔几天就会遇到一些问题，postgres 崩溃并进入恢复模式。postgres 的日志看起来像这样
... Lots of this for 5-10minutes
2015-09-24 10:07:27 GMT LOG: could not fork autovacuum worker process: Cannot allocate memory
2015-09-24 10:07:28 GMT LOG: could not fork autovacuum worker process: Cannot allocate memory
2015-09-24 10:07:29 GMT LOG: could not fork autovacuum worker process: Cannot allocate memory
2015-09-24 10:07:30 GMT LOG: could not fork autovacuum worker process: Cannot allocate memory
2015-09-24 10:07:32 GMT LOG: server process (PID 16244) was terminated by signal 9: Killed
2015-09-24 10:07:32 GMT DETAIL: Failed …