Keep Alive

Both HTTP 1.0 and HTTP 1.1 support the ability to send multiple requests across a single HTTP session. A server can receive hundreds of new HTTP requests per second. If every request was allowed to keep the connection open indefinitely, the server could become overloaded with connections. On Unix/Linux systems, this could easily lead to a file table overflow.

The Application Server’s Keep Alive system addresses this problem. A waiting keep alive connection has completed processing the previous request, and is waiting for a new request to arrive on the same connection. The server maintains a counter for the maximum number of waiting keep-alive connections. If the server has more than the maximum waiting connections open when a new connection waits for a keep-alive request, the server closes the oldest connection. This algorithm limits the number of open waiting keep-alive connections.

If your system has extra CPU cycles, incrementally increase the keep alive settings and monitor performance after each increase. When performance saturates (stops improving), then stop increasing the settings.

The following HTTP keep alive settings affect performance:

• Thread Count

• Max Connections

• Time Out

• Keep Alive Query Mean Time

• Keep Alive Query Max Sleep Time

Thread Count

Thread Count determines the number of threads in the Keep Alive subsystem. Adjust this setting to be a small multiple of the number of processors on the system. For example, a two-CPU system can have two or four keep-alive threads.

The default is one. Do not change the default for a server with a small number of users and Max Connections.

Max Connections

Max Connections controls the maximum number of keep-alive connections the server maintains. The possible range is zero to 32768, and the default is 256.

Adjust this setting based on number of keep alive connections the server is expected to service and the server’s load, because it will add up to resource utilization and might increase latency.

The number of connections specified by Max Connections is divided equally among the keep alive threads. If Max Connections is not equally divisible by Thread Count, the server can allow slightly more than Max Connections simultaneous keep alive connections.

Time Out

Time Out determines the maximum time (in seconds) that the server holds open an HTTP keep alive connection. A client can keep a connection to the server open so that multiple requests to one server can be serviced by a single network connection. Since the number of open connections that the server can handle is limited, a high number of open connections will prevent new clients from connecting.

The default time out value is 30 seconds. Thus, by default, the server will close the connection if idle for more than 30 seconds. The maximum value for this parameter is 300 seconds (5 minutes).

The proper value for this parameter depends upon how much time is expected to elapse between requests from a given client. For example, if clients are expected to make requests frequently then, set the parameter to a high value; likewise, if clients are expected to make requests rarely, then set it to a low value.

Keep Alive Query Mean Time

Keep Alive Query Mean Time specifies the interval between polling keep alive connections. If this parameter has a value of n milliseconds, the response time seen by a client that has requested a keep alive connection will have an overhead between 0 and n milliseconds.

The default value of this parameter is one millisecond, which works well for an expected concurrent load of less than 300 keep alive connections. The default value can severely reduce the scalability with higher concurrent loads. For applications with higher connection loads, increase the default value.

Set this parameter with asadmin or in Admin Console HTTP Service page, by choosing Add Property and specifying:

• Name: keep-alive-query-mean-time

• Value: number of milliseconds

Keep Alive Query Max Sleep Time

Keep Alive Query Max Sleep Time specifies the maximum time (in milliseconds) to wait that after polling keep alive connections for further requests. If your system has extra CPU cycles, keep incrementally increasing this parameter and monitor performance after each incremental increase. When performance saturates (stops improving), then stop increasing the settings.

Set this parameter with asadmin or in the Admin Console HTTP Service page, by choosing Add Property and specifying:

• Name: keep-alive-query-max-sleep-time

• Value: number of milliseconds