Linux efficiently uses memory by caching files and buffers, so high "used" memory is normal if "available" memory is sufficient. 2. Swap helps prevent crashes on low-RAM systems or during spikes but should be limited on systems with ample RAM or SSDs to avoid performance degradation and wear. 3. Adjust swappiness (e.g., set to 10) to reduce swap usage and prioritize physical memory. 4. Manually clearing cache (via drop_caches) is rarely needed and can hurt performance temporarily. 5. Monitor memory with tools like htop, vmstat, and sar, and optimize by limiting runaway processes and tuning application memory use. 6. Focus on system responsiveness and available memory rather than used memory when deciding to optimize, as high usage with adequate available memory indicates efficient resource utilization, not a problem.

Linux memory management is often misunderstood, especially when interpreting tools like free, top, or htop. What looks like “l(fā)ow memory” on the surface might actually be a sign of efficient usage. Understanding how Linux manages memory—and how to optimize it—is key to maintaining system performance, especially on servers or resource-constrained environments.

How Linux Uses Memory Efficiently

Unlike some operating systems that leave RAM idle, Linux tries to use as much memory as possible to improve performance. This includes:

• Application memory (RSS): Memory actively used by running processes.
• Page cache: Frequently accessed files are cached in memory so that subsequent reads are faster.
• Buffers and shared memory: Temporary storage for disk I/O and inter-process communication.

When you run free -h, you might see something like this:

              total        used        free      shared  buff/cache   available
Mem:           8.0G        6.5G        200M        400M        1.3G        1.1G

Beginners often panic at the high “used” value. But the real metric to watch is available, which accounts for memory that’s currently used for cache but can be freed instantly if an application needs it.

Key insight: High “used” memory with plenty of “available” is normal and desirable.

Swap: When and How It Helps

Swap space is disk storage used as an extension of RAM. While much slower than physical memory, it prevents the system from crashing when RAM is fully utilized.

When swap is useful:

• On systems with limited RAM.
• During memory spikes (e.g., large file processing).
• To allow hibernation (suspend-to-disk).

When to limit swap usage:

• On systems with ample RAM, aggressive swapping can hurt performance.
• SSDs wear out faster with heavy swap usage.

You can control how aggressively the kernel uses swap via the swappiness parameter:

cat /proc/sys/vm/swappiness  # Default is usually 60

To reduce swapping (e.g., for a desktop or server with lots of RAM):

echo 'vm.swappiness=10' | sudo tee -a /etc/sysctl.conf
sudo sysctl -p

A value of 10 tells the kernel to avoid swap unless absolutely necessary.

Clearing Cache (Only When Necessary)

Linux automatically manages cache, but in rare cases—like before running a memory-intensive benchmark—you might want to manually clear it:

# Clear page cache, dentries, and inodes
echo 3 | sudo tee /proc/sys/vm/drop_caches

?? Warning: This is generally unnecessary and can temporarily degrade performance, as cached data must be reloaded from disk.

Monitoring and Optimization Tips

Use these tools and practices to keep memory usage in check:

• htop or btop: Visualize memory usage per process.
• vmstat 1: Monitor paging activity (watch for high si/so values indicating heavy swap usage).
• sar -r (from sysstat): Historical memory usage analysis.
• Limit runaway processes: Use ulimit to cap memory per user or process.
• Tune applications: Reduce memory footprint of services (e.g., JVM heap size, database buffers).

For example, if running a web server:

# Limit processes to 2GB of RAM
ulimit -v 2097152

Conclusion

Linux memory management is designed to be efficient, not minimal. High memory usage isn’t a problem if the system remains responsive and applications have room to grow. The key is understanding the difference between used and available memory, tuning swap behavior, and monitoring real performance, not just numbers on a screen.

Basically: don’t optimize just because memory looks “full.” Optimize when performance suffers—and then, look at the right metrics.

The above is the detailed content of Understanding Linux Memory Management and Optimization. For more information, please follow other related articles on the PHP Chinese website!