pmap

Understanding the Output of pmap

The core functionality of pmap is to display a process’s memory map. This map outlines the different sections of memory allocated to the process, including:

• Address: The virtual memory address range used by the process.
• Permissions: Indicates the access rights (read, write, execute) for each memory segment.
• Offset: The offset within the file (if applicable) from which the memory is mapped.
• Device: The device from which the memory is mapped (e.g., /dev/zero).
• inode: The inode number of the file (if applicable).
• pathname: The path to the file mapped into memory (e.g., shared libraries).

A typical pmap output might look like this:

Address           Kbytes     RSS   Swap  Path
00400000-0041b000     184     184    0     /usr/bin/gnome-terminal
0041b000-0041c000       4       4    0     /lib/x86_64-linux-gnu/libc-2.35.so
0041c000-00432000     120     120    0     /lib/x86_64-linux-gnu/libpcre2-8.so
... more lines ...

Here:

• Address shows the virtual address range.
• Kbytes shows the size of the memory region in kilobytes.
• RSS (Resident Set Size) is the amount of memory currently resident in RAM.
• Swap shows how much of the memory is swapped out to disk.
• Path displays the file or library the memory is mapped from.

Practical Examples with pmap

Let’s look at some practical scenarios using pmap:

1. Viewing the memory map of a specific process:

To view the memory map of a process with PID 1234, use:

pmap 1234

2. Identifying memory leaks:

By repeatedly running pmap on a process and observing the RSS values, you can potentially identify memory leaks. A constantly increasing RSS without a corresponding increase in functionality may indicate a leak.

pmap <PID>
sleep 60
pmap <PID>
sleep 60
pmap <PID>

3. Analyzing shared library usage:

pmap helps identify which shared libraries a process is using and how much memory each library consumes. This is helpful for debugging issues related to library conflicts or excessive library usage.

pmap <PID> | grep "libc"

This command filters the output to show only lines containing “libc”, revealing the memory usage of the C standard library.

4. Investigating memory mapping from specific files:

You can see the memory usage related to a particular file by searching in the output. For example, to check memory mapping from /path/to/my/file:

pmap <PID> | grep "/path/to/my/file"

5. Using -x option for extended information:

The -x option provides a more detailed and verbose output, including information about the mapping type, major and minor device numbers, and other attributes.

pmap -x <PID>

By mastering the pmap command, you gain a powerful tool for diagnosing memory-related issues and optimizing the performance of your Linux applications. Its ability to provide a granular view into process memory makes it an indispensable part of any Linux system administrator’s or developer’s toolkit.