Resin Threads

Resin will automatically allocate and free threads as the load requires. Since the threads are pooled, Resin can reuse old threads without the performance penalty of creating and destroying the threads. When the load drops, Resin will slowly decrease the number of threads in the pool until is matches the load.

Most users can set thread-max to something large (200 or greater) and then forget about the threading. Some ISPs dedicate a JVM per user and have many JVMs on the same machine. In that case, it may make sense to reduce the thread-max to throttle the requests.

Since each servlet request gets its own thread, thread-max determines the maximum number of concurrent users. So if you have a peak of 100 users with slow modems downloading a large file, you'll need a thread-max of at least 100. The number of concurrent users is unrelated to the number of active sessions. Unless the user is actively downloading, he doesn't need a thread (except for "keepalives").

Keepalives

Keepalives make HTTP and srun requests more efficient. Connecting to a TCP server is relatively expensive. The client and server need to send several packets back and forth to establish the connection before the first data can go through. HTTP/1.1 introduced a protocol to keep the connection open for more requests. The srun protocol between Resin and the web server plugin also uses keepalives. By keeping the connection open for following requests, Resin can improve performance.

But keepalive connections tie up a Resin thread waiting for the next client request. So the number of keepalive threads needs to be lower than the maximum number of threads. In the resin.conf, thread-keepalive must be less than thread-max. It's a good idea to have a gap of at least 50 between the two. The following might be a typical configuration:

...
<thread-max>250</thread-max>
<thread-keepalive>150</thread-keepalive>
...

In the above case, at most 150 clients can have an open connection even if they're not doing anything. That still leaves 100 threads accepting new connections. Note, these keepalives also apply to the web server plugin using the srun protocol. So thread-keepalive will limit the number of Apache/mod_caucho connections that are left open.

Timeouts

Requests and keepalive connections can only be idle for a limited time before Resin closes them. Each connection has a read timeout, request-timeout. If the client doesn't send a request within the timeout, Resin will close the TCP socket. The timeout prevents idle clients from hogging Resin resources.

... <thread-max>250</thread-max> <thread-keepalive>150</thread-keepalive> <request-timeout>30</request-timeout> <live-time>20</live-time> ...

In general, the request-timeout and keepalives are less important for Resin standalone configurations than Apache/IIS/srun configurations. Very heavy traffic sites may want to reduce the timeout for Resin standalone.

Since request-timeout will close srun connections, its setting needs to take into consideration the live-time setting for mod_caucho or isapi_srun. live-time is the time the plugin will keep a connection open. request-timeout must always be larger than live-time, otherwise the plugin will try to reuse a closed socket.

Plugin keepalives (mod_caucho/isapi_srun)

The web server plugin, mod_caucho, needs configuration for its keepalive handling because requests are handled differently in the web server. Until the web server sends a request to Resin, it can't tell if Resin has closed the other end of the socket. If the JVM has restarted or if closed the socket because of request-timeout, mod_caucho will not know about the closed socket. So mod_caucho needs to know how long to consider a connection reusable before closing it. live-time tells the plugin how long it should consider a socket usable.

Because the plugin isn't signalled when Resin closes the socket, the socket will remain half-closed until the next web server request. A netstat will show that as a bunch of sockets in the FIN_WAIT_2 state. With Apache, there doesn't appear to be a good way around this. If these become a problem, you can increase request-timeout and live-time so the JVM won't close the keepalive connections as fast.

unix> netstat ... localhost.32823 localhost.6802 32768 0 32768 0 CLOSE_WAIT localhost.6802 localhost.32823 32768 0 32768 0 FIN_WAIT_2 localhost.32824 localhost.6802 32768 0 32768 0 CLOSE_WAIT localhost.6802 localhost.32824 32768 0 32768 0 FIN_WAIT_2 ...

TCP limits (TIME_WAIT)

A client and a server that open a large number of TCP connections can run into operating system/TCP limits. If mod_caucho isn't configured properly, it can use too many connections to Resin. When the limit is reached, mod_caucho will report "can't connect" errors until a timeout is reached. Load testing or benchmarking can run into the same limits, causing apparent connection failures even though the Resin process is running fine.

The TCP limit is the TIME_WAIT timeout. When the TCP socket closes, the side starting the close puts the socket into the TIME_WAIT state. A netstat will short the sockets in the TIME_WAIT state. The following shows an example of the TIME_WAIT sockets generated while benchmarking. Each client connection has a unique ephemeral port and the server always uses its public port:

unix> netstat
...
tcp   0   0 localhost:25033  localhost:8080  TIME_WAIT   
tcp   0   0 localhost:25032  localhost:8080  TIME_WAIT   
tcp   0   0 localhost:25031  localhost:8080  TIME_WAIT   
tcp   0   0 localhost:25030  localhost:8080  TIME_WAIT   
tcp   0   0 localhost:25029  localhost:8080  TIME_WAIT   
tcp   0   0 localhost:25028  localhost:8080  TIME_WAIT
...

The socket will remain in the TIME_WAIT state for a system-dependent time, generally 120 seconds, but usually configurable. Since there are less than 32k ephemeral socket available to the client, the client will eventually run out and start seeing connection failures. On some operating systems, including RedHat Linux, the default limit is only 4k sockets. The full 32k sockets with a 120 second timeout limits the number of connections to about 250 connections per second.

If mod_caucho or isapi_srun are misconfigured, they can use too many connections and run into the TIME_WAIT limits. Using keepalives effectively avoids this problem. Since keepalive connections are reused, they won't go into the TIME_WAIT state until they're finally closed. A site can maximize the keepalives by setting thread-keepalive large and setting live-time and request-timeout to large values. thread-keepalive limits the maximum number of keepalive connections. live-time and request-timeout will configure how long the connection will be reused.

...
<thread-max>250</thread-max>
<thread-keepalive>200</thread-keepalive>
<request-timeout>120</request-timeout>
<live-time>100</live-time>
...

thread-max must always be larger than thread-keepalive and request-timeout must always be larger than live-time. In addition, thread-keepalive should be larger than the maximum number of Apache processes.

Apache issues

Using Apache as a web server on Unix introduces a number of issues because Apache uses a process model instead of a threading model. The Apache processes don't share the keepalive srun connections. Each process has its own connection to Resin. In contrast, IIS uses a threaded model so it can share Resin connections between the threads. The Apache process model means Apache needs more connections to Resin than a threaded model would.

In other words, the keepalive and TIME_WAIT issues mentioned above are particularly important for Apache web servers. It's a good idea to use netstat to check that a loaded Apache web server isn't running out of keepalive connections and running into TIME_WAIT problems.

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