1. Use the free command to view the overall memory usage. The -h parameter makes the output more intuitive. Available is the key indicator of available memory; 2. Use top or htop to view the memory occupancy process in real time, and quickly locate high-occupancy programs according to memory order; 3. Check /proc/meminfo to obtain detailed memory data, pay attention to key items such as MemTotal, MemFree, Slab, Cached, etc.; 4. Use vmstat or sar to monitor memory trends. vmstat can view changes in real time, and sar can provide historical data. These methods can be used in combination to fully grasp the memory status and troubleshoot performance issues.

It is not difficult to view memory usage in Linux systems, but many users only stay at the level of knowing a few commands and do not really understand the meaning of the output. This article will talk about several common and practical methods, so that you can not only "see numbers", but also understand what they represent.

1. Use the free command to quickly view overall memory usage

free is one of the simplest and straightforward commands, suitable for quick viewing of overall system memory usage.

The operation mode is very simple, enter:

 free -h

The -h parameter means displaying units (such as MB, GB) in a "human-readable" manner, which makes it look more intuitive.

The output is roughly as follows:

 total used free shared buff/cache available
Mem: 15Gi 3.2Gi 10Gi 400Mi 2.1Gi 12Gi
Swap: 2.0Gi 0B 2.0Gi

Key understanding:

• It's not necessarily a bad thing to use used looks high, because Linux uses free memory as cache ( buff/cache ).
• available is the memory that can be used in new programs, which is more accurate than free .
• If available is low, it is a real memory tight signal.

2. Use top or htop to view memory usage in real time

If you want to know which processes are taking up memory, you can use top or more friendly htop (which may require installation).

run:

 top

In the top interface, memory information is usually displayed on the top lines, such as:

 KiB Mem : 16123456 total, 1023456 free, 3456789 used, ...

By Shift M , you can sort by memory usage, which is convenient for finding out the process that "eats memory".

Tips:

• htop is more intuitive than top , supports mouse operation and color highlighting, and is recommended to install and use.
• Pay attention to the usage of Mem and Swap . Swap is used extensively, indicating that there is insufficient physical memory.

3. Check /proc/meminfo to get detailed memory information

For scenarios that require more detailed information, such as calling in scripts or troubleshooting, you can view the /proc/meminfo file:

 cat /proc/meminfo

This file lists very comprehensive information, such as:

 MemTotal: 16123456 kB
MemFree: 1023456 kB
Buffers: 123456 kB
Cached: 234567 kB
Slab: 345678 kB

Pay attention to several key items:

• MemTotal : Total memory.
• MemFree : Really free memory.
• Slab : The memory used by the kernel, such as file system metadata cache.
• Cached and Buffers : are recyclable memory and are not considered to be real occupancy.

4. Use vmstat or sar to monitor memory changes

If you want to know how memory usage changes over time, you can use vmstat or sar :

 vmstat -s

Or update every 5 seconds:

 vmstat 5

In the information output by vmstat , you can see various indicators such as memory, swap, IO, etc.

If you have installed the sysstat package, you can also use sar to view historical data:

 sar -r 1 5

This outputs memory usage once a second, with a total of 5 outputs.

Basically that's it. These methods can be used in conjunction with each other to cover the daily needs of checking memory usage and troubleshooting performance issues. The key is to understand the meaning of each command output, rather than just looking at the surface numbers.

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