---
title: "How to Check the Memory Usage on Linux"
description: "Understanding Linux memory usage and how to check for used and free memory or RAM in Linux can be difficult. But once you learn about the Linux page cache and how to read the status of it, it's..."
url: https://www.inmotionhosting.com/support/server/linux/check-memory-usage/
date: 2014-02-07
modified: 2025-02-05
author: "Jesse Owens"
image: https://www.inmotionhosting.com/support/wp-content/uploads/2014/02/memory-usage-featured.png
categories: ["Linux"]
type: post
lang: en
---

# How to Check the Memory Usage on Linux

![featured image with text check memory usage diagnose memory issues in Linux](https://www.inmotionhosting.com/support/wp-content/uploads/2014/02/memory-usage-featured-1024x538.png)

Understanding Linux memory usage and how to check for used and free memory or RAM in Linux can be difficult. But once you learn about the [Linux page cache](#linux-page-cache) and how to read the status of it, it's really quite simple.

Linux very efficiently caches frequently used files from the hard drive, to very fast to access RAM on the server. Data is stored electronically in RAM, so its speed is not limited by physical magnetic heads or spindle motors.

In this guide I'll go over **Linux free memory**, and how to properly read **Linux memory info**.

## Linux free -m

The most common way you'll see on the web to check for free memory in Linux is by using the **free** command.

Using the **free -m** command to check your Linux memory usage, displays the values as **MB** instead of **KB**.

Most people will run this command and panic thinking they only have 1 MB of free memory on the server:

root@server [~]# **free -m**
total used free shared buffers cached Mem:
1024 1022 1 0 0 822
-/+ buffers/cache: 200 823
Swap: 0 0 0

The **free** column beside **-/+ buffers/cache** with 823 MB is the actual free memory available to Linux.

1024 MB is the total system memory available, which would be physical RAM.

1 MB and 823 MB both show **free** because an application has access to both for memory storage.

1 MB **free** plus the 822 MB **cached** gives the 823 MB of memory actually **free** to use if needed.

## Linux page cache

Linux uses something called a page cache in order to cache frequently hit files from the hard drive into memory.

There is an awesome helpful tool called [vmtouch](https://hoytech.com/vmtouch/) which allows you to directly interact with the Linux page cache. Below I'm using this to check if any parts of a **100 MB** log file called **access_log** are in the page cache already:

root@server [~]# **vmtouch -v /home/access_log**
/home/access_log [ ] 0/25642
Files: 1
Directories: 0
Resident Pages: 0/25642 0/100M 0%
Elapsed: 0.000568 seconds

You can see that 0/25642 of all **resident pages** that make up this file are in the page cache.

This is also represented as 0/100 MB worth of that file's data, or 0%.

Here I'm using the **time** command to show how long a **grep** search takes initially:

root@server [~]# **time grep wp-login.php access_log -c** 702754
real 0m30.056s
user 0m0.192s
sys 0m0.082s

It took 30.056 seconds to read that file from the hard drive, let's see if any of it made it to the page cache:

root@server [~]# **vmtouch -v /home/access_log**
/home/access_log 25642/25642
Files: 1
Directories: 0
Resident Pages: 25642/25642 100M/100M 100%
Elapsed: 0.003133 seconds

All 25642/25642 **resident pages** are in the page cache now, and all 100 MB worth of data, or 100%.

We know the whole **access_log** file is in memory, let's see what happens when we try to search in the file again:

root@server [~]# **time grep wp-admin access_log -c** 103161
real 0m0.168s
user 0m0.137s
sys 0m0.030s

It took 30.056 seconds reading the file from disk, down to just 0.168 seconds reading it from RAM.

That's over a **17,000%** speed increase! This is why I say that Linux uses memory very efficiently.

The same thing applies to your applications and scripts that run on the server. Linux will try to move them to the page cache for quicker access. If another application needs more memory, it can simply take it back.

This throws a lot of people off because they see Linux is **using** a lot of memory, while in reality it's using it to speed things along while the full memory available to the server isn't actively needed for running applications.

## Linux sar -r

If you wanted to take a look at your Linux memory usage historically then the [Linux sar command](/support/server/linux/using-the-linux-sar-command/) is a great tool which uses **System Activity Report** logs to pull historical usage data from your server:

root@server [~]# **sar -r**
10:06:01 PM kbmemfree kbmemused %memused kbbuffers kbcached kbcommit %commit
10:06:01 PM 688956 359620 34.30 0 142912 0 0.00
10:16:01 PM 688792 359784 34.31 0 143000 0 0.00
10:26:01 PM 678744 369832 35.27 0 152968 0 0.00
10:36:01 PM 678052 370524 35.34 0 153516 0 0.00
10:46:01 PM 677532 371044 35.39 0 154024 0 0.00
10:56:01 PM 676688 371888 35.47 0 154036 0 0.00
11:06:01 PM 676160 372416 35.52 0 154216 0 0.00
11:16:01 PM 3644 1044932 99.65 0 60588 0 0.00
Average: 682327 366249 34.93 0 113811 0 0.00

The kbmemfree column shows how much memory was **free**.

The kbmemused column shows how much memory was **used**.

The %memused column shows how much memory was **used** as a percentage of total memory available.

The kbcached column shows how much memory was **cached**.

The command below goes through your **sar** logs and shows anytime that your server's memory is spiking.

When kbmemused is over 917504 KB or 896 MB, and kbcached is under 131072 KB or 128MB this command will output the memory usage stats from **sar**.

Doing it this way we can ensure that we're only looking at times when our actual **free memory** on the server has dipped below **128MB**. That way we don't start panicking when the server says 99% of memory is being used, but in reality that memory is being used by the Linux page cache.

for log in `ls -1 /var/log/sa/sa[0-9]* | sed 's#@##'`; do echo $log; sar -r -f $log | egrep -v "Linux|kbmemfree" | awk '{ if ($4>=**917504** && $7<=**131072**) print $0}'; echo ""; done | sed 's#\(/var/log.*\)#\1\n--------------kbmemfree kbmemused %memused kbbuffers kbcached kbcommit %commit#' | less

Here is the output from the above command:

/var/log/sa/sa06
-------------- kbmemfree kbmemused %memused kbbuffers kbcached kbcommit %commit
12:16:01 AM 25208 1023368 97.60 0 37536 0 0.00
12:26:01 AM 23708 1024868 97.74 0 38792 0 0.00
12:36:01 AM 13532 1035044 98.71 0 41756 0 0.00

## /proc/meminfo

With Linux there is a **/proc** pseudo-file system that doesn't contain any real phyical files, but does allow you to take a peak at how the Linux kernel is controlling the system via various config files.

One of these is called **/proc/meminfo** and this is where commands like **free** get their data from. You can directly look at these values yourself by simply accessing this pseudo-file:

root@server [~]# **egrep "MemTotal|MemFree|Cached" /proc/meminfo**
MemTotal: 1048576 kB
MemFree: 842752 kB
Cached: 841728 kB

The VPS I'm looking at has 1048576 kB of total system memory, which is 1024 MB, or 1 GB of RAM.

Of that total available memory on the VPS, 842752 kB or 823 MB of that RAM is not in use, or free to use.

There is 841728 kB or 822 MB of memory currently being used by the Linux page cache to speed things up.

I know from the earlier testing I did, that 100 MB worth of that page cache usage is from my 100 MB log file that got placed in the page cache for quick access.