Warning: exec() [function.exec]: Unable to fork, problem with configuring git webhook

My server is Alibaba Cloud's Win server 2008
The git hosting warehouse is coding, and the hook.php content I configured is
exec("git pull", $array2);var_export($array2);
But the page returns
Warning : exec() [function.exec]: Unable to fork [git pull]
Why is this?
Help?

Reply content:

My server is Alibaba Cloud's Win server 2008
The git hosting warehouse is coding, and the hook.php content I configured is
exec("git pull", $array2);var_export($array2);
But the page returns
Warning : exec() [function.exec]: Unable to fork [git pull]
Why is this?
Help?

Maybe resources are exhausted

Forking means in this case the operation to create a new process using the fork() system call. fork() usually fails because necessary system resources are exhausted, it is likely that memory limits or the maximum number of open processes per system is reached.

From http://stackoverflow.com/questions/20732394/what-does-the-unable-to-fork-error-mean