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Apache KeepAlive – On or Off?

KeepAlive is to reuse the same connection to transfer multiple files between the web browser and the web server. Its a persistant connection.



KeepAlive [Advantage]:

  • Improves website speed: It reduces latency associated with HTTP transfers and delivers a better user experience.
  • Reduces CPU usage: It reduces CPU usage. If KeepAlive is disabled then separate connection gets created for each images, stylesheets, javascript files etc for a web page and it will increase CPU usage.

Keepalive [Disadvantage]:

  • Increases memory usage: Apache processes has to keep connections open waiting for new requests from established connections, thus occupying RAM and increases memory usage on the server.

KeepAlive [On or Off]: Its depends on many factors like

  • Server resources:
    Should be turned Off
    if RAM size is less/limited because having Apache processes waiting for more requests from persistent connections will be a waste of memory.
    Should be turned On if CPU power is limited because it reduces CPU load.
  • Types of sites:
    Should be turned On if Web page having a lot of images or other files linked to it because a single connection will be used to transfer multiple files and will improve the user experience significantly.
  • Traffic patterns:
    Should be turned On
    if web traffic is spread out evenly throughout a day.
    Should be turned off if Site has bursty traffic where a lot of concurrent users access during a short time period else will cause RAM usage to skyrocket.
  • Server under Load Balancer:
    Should be turned On
    if servers are under load balancer because for each request that a client makes through a load balancer, the load balancer maintains two connections and keeping KeepAlive On enables the load balancer to re-use connections to the back-end instances, which reduces the CPU utilisation.

So there is no one universal solution to tuning Apache. It all depends on the resources and the type of sites you have.


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