Diagnosing Swap Usage with smem on Linux

May 22, 2025 by , in Blog Linux

If you’ve ever looked at your server and wondered why it’s using swap even though plenty of RAM is available, you’re not alone. This is a common scenario on Linux systems, and it can impact performance if not addressed. One of the most useful tools for digging into this issue is smem, a memory reporting tool that shows proportional memory usage per process, including swap.

Why Swap Gets Used

Linux is designed to be efficient with memory, often using RAM for things like file caching. However, when the kernel decides that some memory pages haven’t been accessed in a while, it may move them to swap space, even if there’s still free RAM available. This behavior is influenced by settings like vm.swappiness, but before tweaking kernel parameters, it’s crucial to know what is actually using swap.

Also read: Linux Performance: Almost Always Add Swap Space.

Installing smem

smem isn’t usually installed by default, but it’s available in most package repositories.

On RHEL/CentOS/Alma/Fedora:

dnf install smem

On Debian/Ubuntu:

apt install smem

Basic Usage

Once installed, you can sort processes by swap usage like this:

smem -rs swap

This will output a table of processes, sorted by the amount of swap used, with additional columns for USS (Unique Set Size), PSS (Proportional Set Size), and RSS (Resident Set Size).

Example Output (Truncated):

   PID User    Command                          Swap      USS       PSS      RSS
28986 mysql    /usr/sbin/mysqld --daemoniz    476372 10963864  10963932 10965112
29152  root    /usr/sbin/rsyslogd -n          371424     3956     17475    43352
31423  root    /opt/fluent-bit/bin/fluent-     22508    26612     26739    29100

From this, we can clearly see that mysqld and rsyslogd are consuming significant swap.

What to Do Next

You should tune memory-heavy services like MySQL, rsyslog. Take a look at the MySQL Monitor script. Once you’ve addressed swap-heavy processes, consider reducing swappiness:

sysctl vm.swappiness=1
echo 'vm.swappiness=1' >> /etc/sysctl.conf

Also read: Linux server needs a RAM upgrade? Check with top, free, vmstat, sar.

With smem you can even generate a pie chart! For example, on my desktop: smem --pie name -s rss:

Conclusion

smem is a lightweight but powerful tool that gives you visibility into real memory and swap usage per process. It’s especially useful when trying to understand why a seemingly idle server is dipping into swap. Pair it with tuning vm.swappiness, vfs_cache_pressure, and careful service profiling to keep your systems running smoothly.

Tags: , , , ,

Comments from Our Members

  1. So here are a few other tools, similar to smem, that I make use of often. They all report per-process memory usage with varying levels of detail, and some also help identify swap-heavy processes:

    procps-ng tools (top, htop)

    • htop shows RES, VIRT, and SWAP columns interactively.
    • You can sort by swap in htop by enabling the SWAP column (F2 > Columns > Add SWAP).

    atop

    • Like top but logs system resource usage over time.
    • Press m to see memory metrics per process.
    • Tracks SWAP, RES, and virtual memory over time.
    • Great for historical memory bottleneck analysis.
    dnf install atop  # or apt install atop

    glances

    • Cross-platform, real-time system monitoring tool.
    • Shows per-process memory and swap usage, similar to smem.
    • Can run in server mode and be queried remotely.
    dnf install glances  # or apt install glances

    Let me know if you’re interested in GUI tools or logging-capable daemons for long-term memory profiling. Ironically one of the best alternatives to top is bottom. :laughing:

    There’s also…



Top ↑