How to Fix High Memory Usage on a Linux Server

Linux server running out of memory? Learn how to diagnose and fix high memory usage with real commands — before it takes down your app. Your app starts slowing down, the OOM killer fires, or your monitoring page turns red — and the culprit is memory. High memory usage on a Linux server is one of the most common production crises for small teams, and it's easy to misread. Linux intentionally uses most of your RAM for caching, so a server showing 95% memory used isn't necessarily in trouble. But one that's exhausting real working memory and swapping is. Here's how to tell the difference and actually fix it. Step 1: Get a Clear Picture of What's Using Memory Start with the basics. Run 'free -h' to see total, used, free, and available memory. Focus on the 'available' column — that's the real number. It accounts for reclaimable cache and is far more useful than 'free'. free -h — quick overview of RAM and swap usage vmstat 1 5 — five one-second snapshots; watch the 'si' and 'so' columns for swap-in and swap-out activity cat /proc/meminfo — full breakdown including Slab, PageTables, and AnonPages If swap is actively being used (si/so values above zero consistently), your server is genuinely memory-constrained. That's different from swap space existing but sitting idle. Step 2: Find the Processes Eating Your RAM Once you know memory is tight, you need to know what's consuming it. Run 'ps aux --sort=-%mem | head -20' to list the top 20 processes by memory percentage. For more detail on actual RSS (resident set size) in human-readable form: ps -eo pid,ppid,cmd,%mem,rss --sort=-%mem | head -20 RSS is the memory a process actually holds in RAM — not virtual memory, which is often misleadingly large. Another useful tool is 'smem', which calculates PSS (proportional set size) and gives a fairer view when processes share memory libraries. Install it with 'apt install smem' or 'yum install smem', then run 'smem -r -k | head -20'. Look for processes with unexpectedly high RSS. A Nod