Keep-Alive Connections Flushed

A web site that can service 75 requests per second without keep-alive connections might be able to do 200-300 requests per second when keep-alive is enabled. Therefore, as a client requests various items from a single page, it is important that keep-alive connections are used effectively. If the KeepAliveCount shown in perfdump (Total Number of Connections Added, as displayed in the Admin Console) exceeds the keep-alive maximum connections, subsequent keep-alive connections are closed, or “flushed,” instead of being honored and kept alive.

Check the KeepAliveFlushes and KeepAliveHits values using statistics from perfdump or the Number of Connections Flushed and Number of Connections Processed under Keep Alive Statistics on the Monitoring Statistics page. For more information, see Keep-Alive Information.

On a site where keep-alive connections are running well, the ratio of KeepAliveFlushes to KeepAliveHits is very low. If the ratio is high (greater than 1:1), your site is probably not utilizing keep-alive connections as well as it can.

To reduce keep-alive flushes, increase the keep-alive maximum connections. You can do this in the configuration's Performance Tab ⇒ HTTP sub tab or using the wadm set-keep-ailve props command. The default is based on the number of available file descriptors in the system. By raising the keep-alive maximum connections value, you keep more waiting keep-alive connections open.

On UNIX/Linux systems, if the keep-alive maximum connections value is too high, the server can run out of open file descriptors. Typically 1024 is the limit for open files on UNIX/Linux, so increasing this value above 500 is not recommended.