Cannot allocate memory: fork: Unable to fork new process?

We have our hosting in aws. Recently after moving our blog from wordpress to aws, we are experiencing noticeable delay in server response time. Mainly while accessing the blog. Below are the logs from the error_log file,

[Wed Feb 25 06:10:10 2015] [error] (12)Cannot allocate memory: fork: Unable to fork new process [Wed Feb 25 06:12:22 2015] [error] (12)Cannot allocate memory: fork: Unable to fork new process [Wed Feb 25 06:12:36 2015] [error] (12)Cannot allocate memory: fork: Unable to fork new process [Wed Feb 25 06:12:50 2015] [error] (12)Cannot allocate memory: fork: Unable to fork new process [Wed Feb 25 06:13:35 2015] [error] (12)Cannot allocate memory: fork: Unable to fork new process [error] (12)Cannot allocate memory: fork: Unable to fork new process [Wed Feb 25 06:27:14 2015] [error] (12)Cannot allocate memory: fork: Unable to fork new process

We increased the memory size from 256 to 512 mb in php.ini file. But, still the issue exist.

We also changed the KeepAlive as On. Still it doesn't resolve. Any suggestions / solutions would be of great help.