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JVM performance optimization, Part 2: Compilers

Use the right Java compiler for your Java application

By Eva Andreasson, JavaWorld.com, 09/05/12

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Page 4 of 6

Server-side compilers

For long-running applications such as server-side enterprise Java applications, a client-side compiler might not be enough. A server-side compiler like C2 could be used instead. C2 is usually enabled by adding the JVM startup option -server to your startup command-line. Since most server-side programs are expected to run for a long time, enabling C2 means that you will be able to gather more profiling data than you would with a short-running light-weight client application. So you'll be able to apply more advanced optimization techniques and algorithms.

Tip: Warm up your server-side compiler

For server-side deployments it may take some time before the compiler has optimized the initial "hot" parts of the code, so server-side deployments often require a "warm up" phase. Before doing any kind of performance measurement on a server-side deployment, make sure that your application has reached the steady state! Allowing the compiler enough time to compile properly will work to your benefit! (See the JavaWorld article "Watch your HotSpot compiler go" for more about warming up your compiler and the mechanics of profiling.)

A server compiler accounts for more profiling data than a client-side compiler does, and allows more complex branch analysis, meaning that it will consider which optimization path would be more beneficial. Having more profiling data available yields better application results. Of course, doing more extensive profiling and analysis requires expending more resources on the compiler. A JVM with C2 enabled will use more threads and more CPU cycles, require a a larger code cache, and so on.

Tiered compilation

Tiered compilation combines client-side and server-side compilation. Azul first made tiered compilation available in its Zing JVM. More recently (as of Java SE 7) it has been adopted by Oracle Java Hotspot JVM. Tiered compilation takes advantage of both client and server compiler advantages in your JVM. The client compiler is most active during application startup and handles optimizations triggered by lower performance-counter thresholds. The client-side compiler also inserts performance counters and prepares instruction sets for more advanced optimizations, which will be addressed at a later stage by the server-side compiler. Tiered compilation is a very resource-efficient way of profiling because the compiler is able to collect data during low-impact compiler activity, which can be used for more advanced optimizations later. This approach also yields more information than you'll get from using interpreted code profile counters alone.

The chart schema in Figure 1 depicts the performance differences between pure interpretation, client-side, server-side, and tiered compilation. The X-axis shows execution time (time unit) and the Y-axis performance (ops/time unit).

Figure 1. Performance differences between compilers (click to enlarge)

 

Compiler performance comparisons

Compared to purely interpreted code, using a client-side compiler leads to approximately 5 to 10 times better execution performance (in ops/s), thus improving application performance. The variation in gain is of course dependent on how efficient the compiler is, what optimizations are enabled or implemented, and (to a lesser extent) how well-designed the application is with regard to the target platform of execution. The latter is really something a Java developer should never have to worry about, though.

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Resources
  • "JVM performance optimization, Part 1: A JVM technology primer" (Eva Andreasson, JavaWorld, August 2012) launches the JVM performance optimization series with an overview of how a classic Java virtual machine works, including Java's write-once, run-anywhere engine, garbage collection basics, and some common GC algorithms.
  • See "Watch your HotSpot compiler go" (Vladimir Roubtsov, JavaWorld.com, April 2003) for more about the mechanics of hotspot optimization and why it pays to warm up your compiler.
  • If you want to learn more about bytecode and the JVM, see "Bytecode basics" (Bill Venners, JavaWorld, 1996), which takes an initial look at the bytecode instruction set of the Java virtual machine, including primitive types operated upon by bytecodes, bytecodes that convert between types, and bytecodes that operate on the stack.
  • The Java compiler javac is fully discussed in the formal Java platform documentation.
  • Get more of the basics of JVM (JIT) compilers, see the IBM Research Java JIT compiler page.
  • Also see Oracle JRockit's "Understanding Just-In-Time Compilation and Optimization" (Oracle® JRockit Introduction Release R28).
  • Dr. Cliff Click gives a complete tutorial on tiered compilation in his Azul Systems blog (July 2010).
  • Learn more about using performance counters for JVM performance optimization: "Using Platform-Specific Performance Counters for Dynamic Compilation" (Florian Schneider and Thomas R. Gross; Proceedings of the 18th international conference on Languages and Compilers for Parallel Computing, published by ACM Digital Library).
  • Oracle JRockit: The Definitive Guide (Marcus Hirt, Marcus Lagergren; Packt Publishing, 2010): A complete guide to the JRockit JVM.