Skip to main content

Understanding -XX:+PrintCompilation Output in Java

Understanding -XX:+PrintCompilation Output in Java

Understanding -XX:+PrintCompilation Output in Java

The -XX:+PrintCompilation flag in the Java Virtual Machine (JVM) prints information about the methods being compiled by the Just-In-Time (JIT) compiler. When you enable this flag, the JVM will output a log of compilation events to the standard output. Each line of the output provides information about a specific method being compiled. Here, I'll explain the meaning of the different columns and markers, specifically focusing on the n, s, and % markers as seen in your example.

Explanation of Output Columns and Markers

Here's a breakdown of what each column and marker means:

  • Timestamp: The time (in milliseconds) since the JVM started when the compilation event occurred.
  • Compilation ID: A unique identifier for each compilation task within the JVM's lifecycle.
  • Optimization Level: The level of optimization applied (e.g., 1, 3, 4).
  • Method Name and Details: The fully qualified name of the method being compiled, along with the size of the method in bytes.
  • Marker: Indicates special conditions or additional information about the compilation.

Common Markers

  • n: Indicates that the method is a native method.
  • s: Indicates that the method is synchronized.
  • %: Indicates that the method has been compiled at a higher optimization level.

Detailed Breakdown of Example Output

Let's go through a portion of your example output line by line:

44    1       3       java.lang.String::charAt (29 bytes)
46    2     n 0       java.lang.System::arraycopy (native)   (static)
48    3       3       java.lang.String::hashCode (55 bytes)
48    4       3       java.lang.String::indexOf (70 bytes)
49    5       3       java.lang.String::equals (81 bytes)
49    7       3       java.lang.Object:: (1 bytes)
49    6       3       java.lang.String::length (6 bytes)
51    8       3       java.lang.AbstractStringBuilder::ensureCapacityInternal (27 bytes)
52    9       3       java.lang.CharacterData::of (120 bytes)
53   14       3       java.lang.AbstractStringBuilder::append (29 bytes)
53   10       3       java.lang.CharacterDataLatin1::getProperties (11 bytes)
55   19       4       java.lang.String::charAt (29 bytes)
57   15       3       java.lang.StringBuilder::append (8 bytes)
58    1       3       java.lang.String::charAt (29 bytes)   made not entrant
59   26       1       java.lang.Object:: (1 bytes)
59    7       3       java.lang.Object:: (1 bytes)   made not entrant
59   22       3       PrimeNumbers::isPrime (35 bytes)
60   25       3       java.lang.Integer:: (10 bytes)
60   29 %     4       PrimeNumbers::isPrime @ 2 (35 bytes)

Explanation of Specific Lines

Line 1:

44    1       3       java.lang.String::charAt (29 bytes)
  • 44: Timestamp in milliseconds since JVM start.
  • 1: Compilation ID.
  • 3: Optimization level.
  • java.lang.String::charAt (29 bytes): Method name and size.

Line 2:

46    2     n 0       java.lang.System::arraycopy (native)   (static)
  • 46: Timestamp in milliseconds since JVM start.
  • 2: Compilation ID.
  • n 0: Native method compilation.
  • java.lang.System::arraycopy (native): Method name, indicating it's a native method.
  • (static): Indicates that the method is static.

Line 3:

48    3       3       java.lang.String::hashCode (55 bytes)
  • 48: Timestamp in milliseconds since JVM start.
  • 3: Compilation ID.
  • Similar to Line 1, but for the hashCode method.

Line 16:

60   29 %     4       PrimeNumbers::isPrime @ 2 (35 bytes)
  • 60: Timestamp in milliseconds since JVM start.
  • 29: Compilation ID.
  • % 4: Indicates high-level optimization.
  • PrimeNumbers::isPrime @ 2 (35 bytes): Method name and size, compiled at high optimization level.

Not Entrant Methods

Not Entrant: Indicates that the method is no longer considered "hot" and is de-optimized, potentially to be recompiled if it becomes hot again.

58    1       3       java.lang.String::charAt (29 bytes)   made not entrant
59    7       3       java.lang.Object:: (1 bytes)   made not entrant

Summary of Compilation Markers

  • n Marker: Indicates native methods.
  • s Marker: Indicates synchronized methods.
  • % Marker: Indicates methods compiled with high-level optimization.

This detailed breakdown should help you understand the output of the -XX:+PrintCompilation flag and the significance of the n, s, and % markers in the context of JIT compilation in the JVM.

Labels:

Comments

Post a Comment

Popular posts from this blog

Java Increment Operations: `n++` vs `n = n + 1` vs `n += 1`

In Java, incrementing a variable by one can be done in several ways: n++ , n = n + 1 , and n += 1 . While these expressions achieve the same end result, they differ slightly in syntax and use cases. Let's explore each one and discuss their performance. 1. n++ Post-Increment Operator : Increments the value of n by 1 after its current value has been used. Common Usage : Typically used in loops and other contexts where the current value needs to be used before incrementing. int n = 5; n++; // n is now 6 2. n = n + 1 Addition Assignment : Explicitly sets n to its current value plus 1. Readability : Straightforward and clear, though slightly more verbose. int n = 5; n = n + 1; // n is now 6 3. n += 1 Compound Assignment Operator : Equivalent to n = n + 1 , but more concise. Usage : Combines addition and assignment into one step. int n = 5; n += 1; // n is
Post a Comment
Read more

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: Ca
Post a Comment
Read more

When To Use Indexes In MySQL

When deciding when and how to create an index in your MySQL database, it's important to consider how the data is being used. Let's say you have a database of  students . We will create it like this: CREATE TABLE `students` ( `id` int ( 11 ) NOT NULL AUTO_INCREMENT , `first_name` varchar ( 255 ) DEFAULT NULL , `last_name` varchar ( 255 ) DEFAULT NULL , `class` varchar ( 255 ) DEFAULT NULL , PRIMARY KEY ( `id` ) ) ENGINE = InnoDB Indexes are best used on columns that are frequently used in where clauses, and in any kind of sorting, such as "order by". You should also pay attention to whether or not this information will change frequently, because it will slow down your updates and inserts. Since you wont frequently be adding students, you don't have to worry about the inserts Let's say that you will be looking up the students with a web interface and the end user will be typing in the students name to find them, since r
Post a Comment
Read more