Centos – why centos7 free command output available value less than free value

The first line of the free output lists:

• total Your total, physical (assuming no virtualization) memory
• used How much of that is currently used (by anything)
• free How much of that is completely free (not used at all)
• shared Memory used (mostly) by tmpfs (for Linux, kernel >= 2.6.32)
• buffers Memory used by kernel buffers
• cached Memory used for cache

The last two items, cache and buffers, is memory that is not allocated to specific user processes. It is memory reserved by the kernel to improve performance overall, but is not "application" memory. These areas will grow or shrink depending on kernel policies with respect to caching, memory pressure, application I/O patterns, etc.

Since these two columns are not user-allocated memory, and the zones can shrink (practically to zero) if user allocations require it, they are in a sense "free" - there's RAM there that can be freed up by the kernel if your apps actively need it.

That's what the second line tells you. It removes the buffer and cache memory from the used column (that's what the - means), and adds (+) them to the free column. (Rounding issue will happen.)

(The last line shows the state of your swap space.)

free is provided by procps-ng; Debian 8 has version 3.3.9, which uses the old style with a separate line for buffers/cache, while Gentoo and presumably RHEL 7.x have version 3.3.10 or later which uses the new style. You can see the reasoning behind the change in the corresponding commit message.

If you really want the old-style output you can run an older version of procps, but you'll find that distributions will migrate to the newer style by default. The newer style also gives the amount of available memory which is a really useful piece of information (see How can I get the amount of available memory portably across distributions? for details).

Somewhat confusingly, version 3.3.9 refers to the format without the buffers/cache line as "old format", and you can see it in that version with free -o. So all told:

• versions 3.3.9 and earlier show by default

               total       used       free     shared    buffers     cached
  Mem:           31G        30G       539M       1.1G       2.2G        15G
  -/+ buffers/cache:        13G        18G
  Swap:          31G       180M        31G
  

• versions 3.3.9 and earlier, with -o, show

               total       used       free     shared    buffers     cached
  Mem:           31G        30G       549M       1.1G       2.2G        15G
  Swap:          31G       180M        31G
  

• versions 3.3.10 and later only show

                total        used        free      shared  buff/cache   available
  Mem:            31G        7.8G        525M        1.1G         23G         22G
  Swap:           31G        180M         31G
  

• versions 3.3.10 and later also have a wide output mode, -w, which shows

                total        used        free      shared     buffers       cache   available
  Mem:            31G        7.8G        531M        1.1G        2.2G         20G         22G
  Swap:           31G        180M         31G
  

(This is all on the same system; note how the accounting is more accurate with the later versions.)