Memory consumption

Each request consumes resources, including RAM. Orders of magnitude:

• apache prefork : ~2MB - ~15MB (mod_php with some libraries)
• apache worker : ~300KB
• nginx : ~30KB

An apache with mod_php can therefore hardly accept more than 1000 permanent connections.

What can overload your web server

• KeepAlive HTTP
• Big files / slow connections
• Permanent connections (long polling, web sockets) for a chat, push...
• Slow requests (lack of CPU/IO or slow web-service/BDD/LDAP)
• System limits (max open files, TCP TimeWait/CloseWait)

KeepAlive HTTP

During KeepAlive, connections use a request. If you have a lot of clients, the recommended KeepAlive of 60s will block a prefork apache.

Simulation

Solutions:

• strongly reduce KeepAlive (1 second), or disable it
• use apache worker/event (in proxy mode) : the default KeepAlive is 5s
• use nginx : the low resource consumption allows a higher KeepAlive (its default value is 75s)

Slow clients

Slow connections use resources during download.

Simulation

Solutions :

• use FPM instead of apache mod_php. For files served by PHP for authorization reasons, you can use X-Sendfile or X-Accel-Redirect
• use a reverse proxy to buffer (nginx has proxy buffering by default) or cache files (difficult if many big files, impossible if authorized by a cookie)

Permanent connections (Tchat, Push)

It's hard to have a lot of permanent connections with PHP.

Slow queries

Queries can be slow or slow only in case of overload: lack of CPU/IO or slow web-service/BDD/LDAP.

Solutions

• specific optimizations
• check the opcode cache for PHP
• add load-balanced servers

System limits (max open files, TCP TimeWait/CloseWait)

Solutions

• for "time wait", PHP persistent connections

Manage the overloading of your web server

maxConnections/MaxRequestWorkers

In case of overload, apache / haproxy / tomcat / FPM / uwsgi use the backlog to keep pending connections.

nginx / nodejs do not use the backlog.

If worker_connections is exceeded, nginx:

• warns "worker_connections are not enough" in error.log
• returns an empty response to the client
• no entry in access log !!

So make sure you have a high worker_connections!

Simulation

Avoid saturating the backend

Using apache max option, you can limit the requests to the backend and keep the others waiting.

The equivalent in nginx (queue) is not available in Open Source nginx. Nginx limit_conn returns an error to the client.

Simulation with or without queue

Various

List of simulations

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