Question
Why do I keep getting error notifications about httpd being down?
The service “httpd” appears to be down.
Answer
The chksrvd service checks if Apache is available by sending a GET request over port 80. If chksrvd doesn't receive a timely response, it assumes that Apache is down. If Apache hits the MaxRequestWorker limit or the server is under high load, Apache will not respond to the chksrvd request in a timely manner, even though the httpd service is running.
Additional resources
PHP-FPM Limit Tuning Basics for Max Children