Linux Server Has Free Memory but Is Swapping: Why?

Linux swap usage can sometimes look confusing, especially when a server still has several gigabytes of available memory. At first glance, this may seem like a sign of memory pressure or poor system performance.

In reality, Linux memory management is designed to use RAM efficiently rather than keep swap permanently at zero. The kernel may move inactive pages to swap even when physical memory is still available.

Understanding Linux swap usage requires looking beyond the amount of free memory. Swap activity, available RAM, page faults, workload behavior, and kernel settings provide a much clearer picture of what is actually happening.

Why Free Memory Does Not Mean Unused Memory

Linux tries to use RAM efficiently. Memory that is not required by active applications is frequently used for page cache, filesystem metadata, buffers, and other kernel structures.

For this reason, the free command can be misleading if you only look at the free column:

free -h

Example:

               total        used        free      shared  buff/cache   available
Mem:            31Gi        12Gi       2.5Gi       820Mi        16Gi        17Gi
Swap:          8.0Gi       1.4Gi       6.6Gi

The more important field is available. Here, Linux estimates that about 17 GiB can be used by new applications without heavy swapping.

The swap usage of 1.4 GiB therefore does not automatically indicate a current memory shortage.

Some pages may have been swapped out earlier and simply remained there because the kernel has had no reason to bring them back into RAM.

Swap Usage and Swap Activity Are Different

One of the most important distinctions during troubleshooting is the difference between swap usage and active swapping. A system can show swap space in use while performing no current swap I/O.

Check this with:

vmstat 1 10

The most useful columns are:

• si: swap in, usually measured in KiB or pages per second depending on the implementation.
• so: swap out.
• free: free memory.
• buff and cache: memory used by buffers and cache.

Example:

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 1  0 1452800 2600000 210000 9500000  0    0     2     8  700 1300  2  1 97  0

Here, about 1.4 GB of swap is allocated, but both si and so are zero. This means the server is not actively moving pages between RAM and swap at that moment.

💡 That is very different from seeing continuous values in si and so, which can indicate active memory pressure.

Why Linux May Swap Pages Before RAM Is Full

The Linux kernel does not wait until every last megabyte of RAM is consumed before considering swap. Linux balances active application memory, filesystem cache, reclaimable pages, and protection against sudden memory exhaustion.

If the kernel identifies anonymous pages that have not been touched for some time, it may move them to swap and use the released RAM for cache or more active workloads. A rarely accessed application page may provide less value in RAM than filesystem cache being read repeatedly.

This behavior is influenced partly by the vm.swappiness parameter. Check it with:

sysctl vm.swappiness

or:

cat /proc/sys/vm/swappiness

A common default is 60, although values vary by distribution and workload. Higher values make the kernel more willing to swap anonymous memory; lower values make it favor other reclaim paths first. A low value does not disable swap.

Swapped Pages Can Stay in Swap for a Long Time

Suppose a server experienced a memory spike overnight. During the event, some inactive pages were moved to swap. Later, the workload dropped and several gigabytes of RAM became available again.

Linux does not automatically read all those swapped pages back into RAM just because memory became free. Doing so would create unnecessary disk I/O. The pages are usually brought back only when a process accesses them again.

This explains a common situation:

MemAvailable:  12000000 kB
SwapTotal:      8388604 kB
SwapFree:       6500000 kB

The server may currently have plenty of available RAM while still showing almost 2 GB of historical swap usage. If swap-in and swap-out activity is near zero, this is often harmless.

Check Current Memory Pressure with /proc/meminfo

For a more detailed view, inspect:

cat /proc/meminfo

Important fields include:

MemTotal
MemFree
MemAvailable
Buffers
Cached
SwapCached
SwapTotal
SwapFree
Active
Inactive
Active(anon)
Inactive(anon)
Active(file)
Inactive(file)

MemAvailable is generally more useful than MemFree. Inactive(anon) can also help because anonymous memory is the main type of memory that can be moved to swap. In this case, you can use the following:

egrep -i "MemAvailable|MemFree|Inactive|Swap" /proc/meminfo

Example:

Use sar to Check Historical Swap Activity

A server may look healthy when you log in even though it experienced memory pressure or active swapping earlier.

If sysstat is installed and historical data collection is enabled, you can review previous swap activity with:

sar -W

For a real-time sample that does not depend on previously collected data, use:

sar -W 1 10

The most useful fields are:

• pswpin/s: pages being read from swap back into RAM per second.
• pswpout/s: pages being written from RAM to swap per second.

Memory statistics can also be sampled in real time with:

sar -r 1 10

If running sar -W or sar -r without an interval returns an error such as:

Cannot open /var/log/sa/sa10: No such file or directory
Please check if data collecting is enabled

the system does not yet have a historical sysstat data file for that day. In that situation, either use real-time sampling or enable the sysstat collection service and timers for future analysis.

Historical data is especially useful because current swap usage alone cannot tell you when those pages were moved to swap. A spike during a backup, batch job, application restart, or database maintenance window may explain why swap remains allocated even though the server currently has plenty of available memory.

Enable sysstat Historical Data Collection

If sar returns an error such as:

Cannot open /var/log/sa/sa10: No such file or directory
Please check if data collecting is enabled

the sysstat package may be installed, but periodic data collection is not yet enabled.

On RHEL-based distributions such as RHEL, Rocky Linux, and AlmaLinux, start by enabling the main sysstat service:

systemctl enable --now sysstat

Then check whether the collection timers are active:

systemctl status sysstat-collect.timer
systemctl status sysstat-summary.timer

If they are disabled, enable them with:

systemctl enable --now sysstat-collect.timer
systemctl enable --now sysstat-summary.timer

You can confirm that the timers are scheduled with:

systemctl list-timers | grep sysstat

After the collector runs, daily statistics files should begin to appear under:

ls -lh /var/log/sa/

Typical files include entries such as:

sa10
sar10

The saXX file stores the binary performance data for that day, while the corresponding sarXX file may contain the daily summary.

Once data collection is active, commands such as:

sar -W
sar -r

can read historical swap and memory statistics instead of returning the missing-file error:

Important: Keep in mind that enabling sysstat does not create past data retroactively. Historical information becomes available only after the collector has started running. For immediate troubleshooting, you can still use real-time sampling:

sar -W 1 10
sar -r 1 10

Find Which Processes Are Using Swap

To determine whether one application is responsible for most swap usage, you can inspect /proc.

One practical command is:

for pid in /proc/[0-9]*; do
    awk '/VmSwap/{if ($2 > 0) print FILENAME, $2 " kB"}' "$pid/status" 2>/dev/null
done | sort -k2 -n

Another useful option is smem, which can report memory usage per process and help identify applications with pages stored in swap.

If it is already installed, run:

smem -rs swap

This sorts processes by swap usage, making it easier to identify applications that have accumulated significant swapped memory.

On RHEL-based systems, however, smem may not be installed by default. If the command returns:

-bash: smem: command not found

first check whether the package is available:

dnf search smem

On RHEL, Rocky Linux, and AlmaLinux, smem is commonly provided through the EPEL repository.

If EPEL is already enabled, install it with:

dnf install smem

If the package is not found, you may need to enable EPEL first. On RHEL 8, for example:

dnf install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm

Then install smem:

dnf install smem

After installation, run:

smem -rs swap

For example:

A system with several processes using swap does not necessarily have a current memory problem. The output should still be correlated with tools such as vmstat, especially the si and so columns, to determine whether active swapping is occurring.

💡 Important: A process using swap is not necessarily unhealthy. The important question is whether accessing that process causes latency or repeated swap-in activity.

Check for Major Page Faults

When a process accesses a page that is no longer in physical memory, a page fault occurs. Minor page faults are normal and do not require disk access. Major page faults are more expensive because the page must be retrieved from storage.

You can observe process-level faults with:

pidstat -r 1

Useful columns include memory usage and major faults per second.

A system with free memory but frequent major faults may be repeatedly retrieving previously swapped pages. That can create application pauses even though free -h looks comfortable.

Storage Performance Still Matters

Swap is much slower than RAM, so the performance impact depends heavily on the underlying storage.

Check block device latency with:

iostat -xz 1 10

Pay attention to fields such as:

await
r_await
w_await
%util

⚠️ If active swapping coincides with high storage latency, the effect can be significant. For this reason, memory and storage metrics should be evaluated together.

Verify Whether the System Is Under Real Pressure

A healthy server with some swap usage often has this pattern:

MemAvailable: high
swap used: nonzero
vmstat si: 0
vmstat so: 0
major faults: low
disk latency: normal

A server under active memory pressure looks different:

MemAvailable: low or falling
swap used: increasing
vmstat si/so: continuous
major faults: increasing
disk latency: elevated
application response time: degraded

The second pattern deserves investigation for memory leaks, application heap growth, unexpected workloads, or insufficient RAM.

Be Careful with swapoff

Administrators sometimes try to “fix” swap usage by running:

swapoff -a
swapon -a

This forces swapped pages back into RAM.

It can make swap usage return to zero, but it does not solve the reason those pages were swapped in the first place.

More importantly, swapoff requires enough memory to hold the pages being restored. On a busy server, forcing large amounts of swapped memory back into RAM can increase memory pressure and even trigger the Out-Of-Memory (OOM) killer.

Use it only when you understand the memory headroom and have a valid operational reason.

Should You Lower vm.swappiness?

The vm.swappiness parameter influences how the Linux kernel balances reclaiming file-backed memory, such as page cache, against moving anonymous memory pages to swap.

Check the current value with:

sysctl vm.swappiness

A typical RHEL system may return:

vm.swappiness = 60

It is important to understand that a value of 60 does not mean Linux will start swapping when RAM reaches 60% utilization. Swappiness is a relative preference used by the kernel during memory reclaim.

In practical terms, a higher value makes the kernel more willing to reclaim anonymous memory by moving inactive pages to swap, potentially preserving more filesystem cache in RAM. A lower value makes the kernel less willing to swap anonymous pages and generally favors reclaiming file-backed cache first.

For example:

sysctl -w vm.swappiness=10

changes the value immediately, but only until the next reboot.

To make the setting persistent, create a sysctl configuration file such as:

vi /etc/sysctl.d/99-swappiness.conf

and add:

vm.swappiness = 10

Then apply the configuration:

sysctl --system

A lower value such as 10 is sometimes appropriate for workloads where keeping application memory in RAM is particularly important, including some database, virtualization, and latency-sensitive environments.

However, lowering swappiness should not be treated as a general fix for swap usage.

For example, consider a server showing:

MemAvailable:  12 GB
SwapUsed:       2 GB
vmstat si:      0
vmstat so:      0

In this situation, changing vm.swappiness may provide little benefit. The 2 GB of swap could simply contain inactive pages moved there during an earlier period of memory pressure.

A different situation would be:

MemAvailable:  500 MB
SwapUsed:       increasing
vmstat si:      continuous activity
vmstat so:      continuous activity

Here, the system is experiencing real memory pressure. Reducing swappiness may change how aggressively the kernel uses swap, but it does not solve the underlying shortage of physical memory.

Be particularly careful with:

sysctl -w vm.swappiness=0

Setting swappiness to zero does not simply “disable unnecessary swapping.” It makes the kernel aggressively avoid swapping anonymous memory and can increase the risk of processes being terminated by the OOM killer when the system experiences heavy memory pressure.

For production systems, a better approach is to first determine whether active swapping is actually causing a performance problem. Check vmstat, sar, major page faults, memory availability, and application latency before modifying this parameter.

If tuning is justified, change the value gradually, monitor the workload, and follow application or vendor recommendations rather than selecting a low value simply because swap space is being used.

A Practical Troubleshooting Sequence

When you find a server with free memory and swap in use, start with:

free -h

Check available, not only free.

Then verify real-time swap activity:

vmstat 1 10

If si and so remain near zero, the swap usage may be historical.

Next, inspect:

cat /proc/meminfo

Look at MemAvailable, anonymous memory, and swap values.

Check which processes are using swap, then use:

pidstat -r 1

to identify major page faults.

If you suspect earlier pressure, review historical data with:

sar -W
sar -r

Finally, correlate memory behavior with storage latency using:

iostat -xz 1 10

This sequence helps separate harmless swap residency from active swapping that affects application performance.

Key Takeaways

A Linux server does not need to keep swap at zero to be healthy. What matters is whether the kernel is actively moving pages, whether applications are suffering major page faults, and whether storage latency is amplifying the effect.

In many cases, free memory and used swap can coexist for hours or even days without indicating a problem. Once you focus on swap activity instead of the swap total alone, the behavior becomes much easier to interpret and troubleshoot.