Cannot allocate memory: fork: Unable to fork new process

I frequently got internal server error
I have this error from time to time in my server logs.
Can someone tell me what is this problem.
I have 1 gb of Ram and very low traffic at all.

Thank you in advance.


[Sun Jun 21 08:24:35 2015] [error] (12)Cannot allocate memory: fork: Unable to fork new process

[Sun Jun 21 08:57:37 2015] [error] [client 141.101.105.124] (12)Cannot allocate memory: couldn't create child process: /usr/local/sbin/suphp for /home/user/public_html/index.php