Question

I have a Java web application that works a lot with file conventions.
I am using Tomcat 6 as my servlet container. When many requests are submitted, Tomcat becomes very memory hungry. I wonder how I can fine-tune tomcat to reduce the memory consumption.
I am also considering changing my servlet container.
What do you suggest?

Answer 1

You can limit the accepted/operational connection numbers in the conf/server.xml configuration.

Have

<Executor name="tomcatThreadPool" namePrefix="catalina-exec-" 
    maxThreads="16" minSpareThreads="1"/>

and

<Connector executor="tomcatThreadPool"
           port="8080" protocol="HTTP/1.1" 
           connectionTimeout="20000" 
           redirectPort="8443" 
           />

or

<Connector port="8080" protocol="HTTP/1.1" 
           connectionTimeout="20000" 
           redirectPort="8443" 
           maxThreads='16'/>

in the config file and this should brake you.

Edit: Based on your comment you could move the processing into dedicated thread pool sized according to your CPU count (Runtime.getRuntime().availableProcessors()) (see ExecutorService and Executors.) Then you could apply a bounded LinkedBlockingQueue to throttle the number of pending tasks (don’t forget to specify a RejectedExecutionHandler to do the blocking add when the queue gets full).

Edit 2: Added links to the classes. There you find some samples.

Edit 3: A sample method I used in a project.

/**
 * Creates a new thread pool based on some attributes
 * @param poolSize the number of worker threads in the thread pool
 * @param poolName the name of the thread pool (for debugging purposes)
 * @param priority the base priority of the worker threads
 * @param capacity the size of the task queue used
 * @return the ExecutorService object
 */
private ExecutorService newPool(int poolSize, 
String poolName, final int priority, int capacity) {
    int cpu = Runtime.getRuntime().availableProcessors();
    ExecutorService result = null;
    if (poolSize != 0) {
        if (poolSize == -1) {
            poolSize = cpu;
        }
        if (capacity <= 0) {
            capacity = Integer.MAX_VALUE;
        }
        result = new ThreadPoolExecutor(poolSize, poolSize, 
                120, TimeUnit.MINUTES, 
                new LinkedBlockingQueue<Runnable>(capacity), 
        new ThreadFactory() {
            @Override
            public Thread newThread(Runnable runnable) {
                Thread t = new Thread(runnable);
                t.setPriority(priority);
                return t;
            }
        }, new RejectedExecutionHandler() {
            @Override
            public void rejectedExecution(Runnable r,
                    ThreadPoolExecutor executor) {
                if (!executor.isShutdown()) {
                    try {
                        executor.getQueue().put(r);
                    } catch (InterruptedException ex) {
                        // give up
                    }
                }
            }
        });
    }
    return result;
}

And you could use it this way:

ExecutorService exec = newPool(-1, "converter pool", Thread.NORM_PRIORITY, 500);
servletContext.setAttribute("converter pool", exec);

And in your servlet

ExecutorService exec = (ExecutorService)servletContext
.getAttribute("converter pool");

exec.submit(new Runnable() {
    public void run() {
        // your code for transformation goes here
    }
}