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Understanding C1 and C2 Compilers in Java

Understanding C1 and C2 Compilers in Java

Understanding C1 and C2 Compilers in Java

In Java, the Just-In-Time (JIT) compiler is a part of the Java Virtual Machine (JVM) that improves the performance of Java applications by compiling bytecode into native machine code at runtime. The JIT compiler includes two different compilers, known as the C1 and C2 compilers, each with distinct optimization strategies and purposes.

C1 Compiler (Client Compiler)

The C1 compiler, also known as the client compiler, is designed for fast startup times and lower memory consumption. It performs lighter and quicker optimizations, which makes it suitable for applications that require quick startup and responsiveness.

Key characteristics of the C1 compiler include:

  • Quick Compilation: Prioritizes fast compilation times over deep optimizations.
  • Low Overhead: Consumes less memory and resources during compilation.
  • Profile-Guided Optimization: Can use profiling data to guide optimizations but keeps them relatively simple.

C2 Compiler (Server Compiler)

The C2 compiler, also known as the server compiler, is designed for long-running server applications where peak performance is critical. It performs more aggressive and complex optimizations, which can lead to better performance at the cost of longer compilation times and higher memory consumption.

Key characteristics of the C2 compiler include:

  • Deep Optimization: Applies more advanced and resource-intensive optimizations to the code.
  • Higher Overhead: Takes more time and memory to compile, aiming for better runtime performance.
  • Profile-Guided Optimization: Uses extensive profiling data to apply sophisticated optimizations.

Tiered Compilation

Modern JVMs use a feature called tiered compilation, which combines both C1 and C2 compilers to balance the trade-offs between quick startup times and long-term performance.

In tiered compilation:

  • The JVM starts by using the C1 compiler for methods, benefiting from its fast compilation speed.
  • As the application runs and the JVM gathers profiling information, it identifies "hot" methods (methods that are frequently executed).
  • These hot methods are then recompiled using the C2 compiler, benefiting from its deeper optimizations.

Tiered compilation aims to provide a good balance, ensuring quick startup times while still optimizing critical parts of the application for peak performance.

How to Control Compilers

You can control the use of C1 and C2 compilers through JVM options:

  • Disable C2 Compiler: -XX:TieredStopAtLevel=1
  • Disable C1 Compiler: -XX:TieredStopAtLevel=4
  • Disable Tiered Compilation: -XX:-TieredCompilation

By default, tiered compilation is enabled in most modern JVMs, as it provides a good balance between startup performance and peak performance.

Improving Code Cache Size

The code cache is a region of memory where the JIT-compiled code is stored. If your application is large or has many hot methods, you might need to increase the code cache size to prevent frequent code cache flushing, which can degrade performance.

You can improve code cache size using the following JVM options:

  • Initial Code Cache Size: -XX:InitialCodeCacheSize= (default: 160K)
  • Reserved Code Cache Size: -XX:ReservedCodeCacheSize= (default: 240M in JDK 8)
  • Code Cache Expansion: -XX:CodeCacheExpansionSize= (size by which the code cache is expanded)

Example:

# Increase the initial and reserved code cache sizes
java -XX:InitialCodeCacheSize=32m -XX:ReservedCodeCacheSize=512m -jar myapp.jar

Example Usage

# Disable tiered compilation to use only C1 compiler
java -XX:-TieredCompilation -jar myapp.jar

# Use only C2 compiler
java -XX:TieredStopAtLevel=4 -jar myapp.jar

# Increase the initial and reserved code cache sizes
java -XX:InitialCodeCacheSize=32m -XX:ReservedCodeCacheSize=512m -jar myapp.jar

Summary

  • C1 Compiler (Client Compiler): Quick, less memory-intensive, good for fast startup and responsiveness.
  • C2 Compiler (Server Compiler): Slower, more memory-intensive, performs deep optimizations for peak performance.
  • Tiered Compilation: Uses both C1 and C2 compilers to balance startup speed and long-term performance.
  • Improving Code Cache Size: Adjust InitialCodeCacheSize, ReservedCodeCacheSize, and CodeCacheExpansionSize to optimize performance for large applications.

Understanding the roles of C1 and C2 compilers, how to configure them, and how to optimize code cache size can help you improve the performance of your Java applications based on their specific needs.

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