Once you have Tomcat up and running, you will likely want to do some performance tuning so that it serves requests more efficiently on your computer. In this chapter, we give you some ideas on performance tuning the underlying Java runtime and the Tomcat server itself.

The art of tuning a server is a complex one. It consists of measuring, understanding, changing, and measuring again. The following are the basic steps in tuning:

1. Decide what needs to be measured.
   
2. Decide how to measure.
   
3. Measure.
   
4. Understand the implications of what you learned.
   
5. Modify the configuration in ways that are expected to improve the measurements.
   
6. Measure and compare with previous measurements.
   
7. Go back to step 4.

Note that, as shown, there is no “exit from loop” clause—perhaps a representative of real life. In practice, you will need to set a threshold below which minor changes are insignificant enough that you can get on with the rest of your life. You can stop adjusting and measuring when you believe you’re close enough to the response times that satisfy your requirements.

To decide what to tune for better performance, you should do something like the following.

Set up your Tomcat on a test computer as it will be in your production environment. Try to use the same hardware, the same OS, the same database, etc. The more similar it is to your production environment, the closer you’ll be to finding the bottlenecks that you’ll have in your production setup.

On a separate machine, install and configure your load generator and the response tester software that you will use for load testing. If you run it on the same machine that Tomcat runs on, you will skew your test results, sometimes badly. Ideally, you should run Tomcat on one computer and the software that tests it on another. If you do not have enough computers to do that, then you have little choice but to run all of the software on one test computer, and testing it that way will still be better than not testing it at all. But, running the load test client and Tomcat on the same computer means that you will see lower response times that are less consistent when you repeat the same test.

Isolate the communication between your load tester computer and the computer you’re running Tomcat on. If you run high-traffic tests, you don’t want to skew the test data by involving network traffic that doesn’t belong in your tests. Also, you don’t want to busy computers that are uninvolved with your tests due to the heavy network traffic that the test will produce. Use a switching hub between your tester machine and your mock production server, or use a hub that has only these two computers connected.

Run some load tests that simulate various types of high-traffic situations that you expect your production server to have. Additionally, you should probably run some tests with higher traffic than you expect your production server to have so that you’ll be better prepared for future expansion.

Look for any unusually slow response times and try to determine which hardware and/or software components are causing the slowness. Usually it’s software, which is good news because you can alleviate some of the slowness by reconfiguring or rewriting software. In extreme cases, however, you may need more hardware, or newer, faster, and more expensive hardware. Watch the load average of your server machine, and watch the Tomcat logfiles for error messages.

In this chapter, we show you some of the common Tomcat things to tune, including web server performance, Tomcat request thread pools, JVM performance, DNS lookup configuration, and JSP precompilation. We end the chapter with a word on capacity planning.