Problem

Confluence will become unresponsive during periods of high load. Given a little time, the instance may recover naturally and become usable again without having to restart. Profiling tools will show that the majority of tomcat HTTP threads are busy and CPU usage will be high. Thread dumps will contain a lot of stack traces with the following:

apache.coyote.http11.InternalInputBuffer.parseRequestLine

Cause

By default Confluence uses the Coyote Connector, along with a 20 second timeout.

This is the default connector in Confluence 5.5.4 and all following versions:

<Connector port="8090" connectionTimeout="20000" redirectPort="8443"
	maxThreads="200" minSpareThreads="10"
    enableLookups="false" acceptCount="10" debug="0" URIEncoding="UTF-8" />

This is the default connector from before Confluence 5.5.4:

<Connector className="org.apache.coyote.tomcat4.CoyoteConnector" port="8090" minProcessors="5"
	maxProcessors="75"
	enableLookups="false" redirectPort="8443" acceptCount="10" debug="0" connectionTimeout="20000"
	useURIValidationHack="false" URIEncoding="UTF-8"/>

When a request is made to the server, Confluence will provide that request a thread, meaning there will be one thread consumed per concurrent connection. If the connection is used just enough to not trigger the timeout (20 seconds by default), the thread will be continually held by this connection and wasted.

Under high load this can result in connections that are consuming server resources and preventing them from being used by other HTTP threads for long periods, even if they are doing very little.

Workaround

• Shut down Confluence
• Locate server.xml, in your <installation_directory>/conf/ folder; and open it in a text editor

• Locate your connector - it will be similar to the connector shown above

  In Confluence 5.5.4 and beyond, Tomcat will be configured to use the default http11protocol connector. However, you can add protocol="org.apache.coyote.http11.Http11NioProtocol" to the connector

  Here's the default connector, using the NIO Connector - notice the protocol attribute:

  <Connector port="8090" connectionTimeout="20000" redirectPort="8443"
  	maxThreads="200" minSpareThreads="10"
      enableLookups="false" acceptCount="10" debug="0" URIEncoding="UTF-8"
      protocol="org.apache.coyote.http11.Http11NioProtocol" />

• Save the file and restart Confluence

The NIO Connector is a non-blocking type of connector. It will poll connections to see which ones are actually being used (as opposed to waiting for them to timeout); and should result in the available threads being used more efficiently.

For more advanced tomcat configuration options please refer to Tomcat Official Documentation.