pmap Java

Introduction

In Java programming, memory management is crucial for optimizing the performance of the application. The pmap command is a useful tool that allows us to analyze the memory usage of a Java process. In this article, we will explore the pmap command and its usage in Java applications.

What is pmap?

pmap is a command-line utility available in Unix-like operating systems, including Linux, that displays the memory usage map of a process. It provides detailed information about the memory regions allocated to a process, such as the address range, permissions, and the type of memory (e.g., stack, heap, shared libraries).

Using pmap with Java

To utilize pmap with a Java process, we need to determine the process ID (PID) of the Java application. We can use the jps command-line tool to obtain the PID. Let's consider a simple Java program that creates an array of integers:

public class MemoryExample {
    public static void main(String[] args) {
        int[] array = new int[1000000];
        System.out.println("Array created!");
    }
}

To compile and run this program, we can use the following commands:

$ javac MemoryExample.java
$ java MemoryExample

Now, let's find the PID of our Java process using jps:

$ jps
12345 MemoryExample

In this example, the PID of our process is 12345. We can then use the pmap command to analyze the memory usage of this process:

$ pmap 12345

The pmap command will output a detailed map of the memory regions allocated to the Java process:

Pie Chart

Interpreting the Output

The output of pmap provides valuable information about memory usage. Let's examine the important parts of the output:

  1. Address Range: The address range represents the virtual memory addresses covered by a memory region. It consists of a starting address and an ending address.

  2. Permissions: The permissions field indicates the access rights for a memory region. Common permissions include r (readable), w (writable), and x (executable).

  3. Offset: The offset specifies the offset of the memory region in the corresponding file or device. For Java processes, this field is usually empty.

  4. Device: The device field shows the device or file associated with the memory region. Again, for Java processes, this field is typically empty.

  5. Inode: The inode field represents the inode number of the memory region's associated file. Similar to the device field, this field is not relevant for Java processes.

  6. Size: The size field specifies the size of the memory region in kilobytes (KB).

  7. Type: The type field categorizes the memory region. In the context of Java processes, we are mainly interested in the Heap and Stack types.

Memory Analysis Example

Let's analyze the memory usage of our simple Java program using pmap. After running the pmap command on the PID of our Java process, we obtain the following output:

$ pmap 12345
0000000000400000    4K r-x-- MemoryExample
0000000000600000    4K r---- MemoryExample
0000000000601000    4K rw--- MemoryExample
00007f987d3d3000  132K r-x-- libc-2.31.so
00007f987d3f3000  204K ----- libc-2.31.so
00007f987d43000    4K r---- libc-2.31.so
00007f987d44000    4K rw--- libc-2.31.so
00007f987d45000   20K rw---   [ anon ]
00007f987d49c000  136K r-x-- libm-2.31.so
00007f987d4bb000  204K ----- libm-2.31.so
00007f987d4ee000    4K r---- libm-2.31.so
00007f987d4ef000    4K rw--- libm-2.31.so
00007f987d4f0000   48K r-x-- ld-2.31.so
00007f987d4f7000   16K rw---   [ anon ]
00007f987d513000    4K r---- ld-2.31.so
00007f987d514000    4K rw--- ld-2.31.so
00007f987d515000    4K rw---   [ anon ]
00007fff4b9fd000  132K rw---   [ stack ]
00007fff4ba7d000    8K r----   [ anon ]
00007fff4ba7f000    4K r-x--   [ anon ]
ffffffffff600000    4K r-x--