Will the Guru

32 bit vs 64 bit

• Differs per Operating System.
• 32bit
  • The size of memory pointer is small. The size of memory which is supported by OS is small as well.
    • Might be faster if heap is less than 3GB
  • Only C1 compiler(=Client Compiler) is supported.
  • Application like client application which ends right away after finishing its job is adequate for 32 bit
    • The startup time is important for these applications
• 64bit
  • Both Client Compiler(C1) and Server Compiler(C2) are supported.
  • Application like server application which runs for a long time is adequate for 64 bit
    • The startup time is less important for these applications

Mac doesn’t have 32 bit JVM. In this case, we can decide which compiler to use by providing a flag(-client). If we use client compiler(C1 compiler) only, the startup time can be improved.

• Some Operating system uses C2 compiler even though we provide the -client flag. However, using the -client flag would improve the startup time anyways because it performs less analysis of the code.

JVM complier flags

• -client: specifies to use only client compiler. This flag might be ignored from some operating systems.
• -server: selects 32 bit server compiler.
• -d64: selects 64 bit server compiler.
• -XX:-TieredCompilation: Ignore the tiered compilers and without the code cache, run the code only with the interpreters.

fyi – JVM decides when/which method to compile into native code cache by below factors.

• How many times the method has been called
• Whether loop exists inside the method

Tuning native compilation within the VM

There are 2 factors that usually affects the application compiling performance.

1. How many threads available to run this compiling process
   • For programs that require lot of compiling, the more threads we have, the faster compiling process it will be.
2. What’s the threshold for native compilation
   • The number of method invocation to get it compiled to native machine code.

Those two factors can be adjusted by providing flags.

1. How many threads available to run this compiling process
   • => -XX:CICompilerCount can adjust the thread count for compiling
   • ex) java -XX:CICompilerCount=6 -XX:+PrintCompilation <CLASS_NAME> <PARAMS>
2. What’s the threshold for native compilation
   • => -XX:CompileThreshold=n

Tips

Using -XX:PrintFlagsFinal , you can check how many compilers are being used for the process.

Also, you can check by using jinfo command, by retrieving the process id from jps.

• jps: List of the java processes. Shows the process ID
• jinfo: Java info.
  • ex) jinfo -flag CICompilerCount <PROCESS_ID>

Reference

• https://www.udemy.com/course/java-application-performance-and-memory-management/
• https://inspirit941.tistory.com/352