pmap Command: Tutorial & Examples

Introduction to pmap

pmap is a command line utility that reports the memory map of a process in Linux. It is extensively used for debugging and performance tuning. The name pmap stands for Process MAP, a name that succinctly describes what this tool does.

What pmap Does

pmap displays detailed information about the memory usage of a process. It shows the address space for each process, including all memory mappings, the amount of memory consumed by each segment, and the permissions of each segment. This information can be very valuable when you're trying to understand the memory footprint of your applications and identify potential memory leaks.

How pmap Works

When you call pmap, it reads the memory information from the /proc directory, specifically from the /proc/<pid>/maps and /proc/<pid>/smaps files. Note that <pid> is the process ID of the process you're interested in. pmap then formats this information and prints it to the standard output.

How to Use pmap

To use pmap, you need to know the process ID (PID) of the process you want to inspect. You can find this by using commands like ps or top. Once you have the PID, you can call pmap like this:

pmap <pid>

Replace <pid> with the actual process ID. For example:

pmap 12345

This will output the memory map of the process with PID 12345.

Common pmap Parameters

pmap has a few command line parameters that you can use to modify its behavior. Here are some of the most common ones:

• -x: This shows the extended format, which includes the size, RSS (Resident Set Size), shared, and dirty pages of each mapping.
• -d: This shows the device format, which includes the major and minor device number for each mapping.
• -q: This makes pmap quiet. In this mode, it doesn't display some header and footer lines.

Typical pmap Output

A typical pmap output may look like this:

00400000   8364K r-x--  /usr/bin/python3.6
00c52000    308K r----  /usr/bin/python3.6
00c9f000     88K rw---  /usr/bin/python3.6
00cb6000     16K rw---    [ anon ]
023fe000   2548K rw---    [ heap ]

Each line represents a memory mapping. The columns display the address, size, permissions, and the file associated with the mapping.

Potential Problems and Pitfalls

While pmap is a very useful tool, it's important to be aware of its limitations and potential problems. Here are a few to consider:

• pmap requires the PID of the process you want to inspect. If you don't have this, you'll need to find it using another command like ps or top.
• The information provided by pmap is a snapshot at a specific point in time. If the process is active and changing, the memory map may change.
• pmap reports memory usage in kilobytes (K), which can be misleading for processes that use large amounts of memory. In such cases, it may be more useful to use a tool that reports memory usage in a larger unit, like megabytes (M) or gigabytes (G).

Despite these potential issues, pmap remains a valuable tool for anyone working with Linux processes and interested in understanding their memory footprint. It's a great starting point for troubleshooting memory issues, and a vital part of any Linux administrator's toolkit.

Further Reading

• Understanding the Linux Kernel by Daniel P. Bovet, Marco Cesati (partner link)
• Linux Kernel Programming by Kaiwan N. Billimoria (partner link)
• The Linux Programming Interface by Michael Kerrisk (partner link)
• Linux Kernel Development by Robert Love (partner link)
• Linux for System Administrators by Viorel Rudareanu, Daniil Baturin (partner link)

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