What do the changes in `free` output from 14.04 to 16.04 mean?

Question

I noticed that the free command reporting has changed somewhere between Trusty and Xenial. Here is what `free -m' shows on one of my Trusty computers:

$ free -m
             total       used       free     shared    buffers     cached
Mem:          7916       7645        271         99        455       1764
-/+ buffers/cache:       5426       2490
Swap:        24999        805      24194

Here is the equivalent on (a different) Xenial system:

$ free -m
              total        used        free      shared  buff/cache   available
Mem:           3553        1192         857          16        1504        2277
Swap:          3689           0        3689

The +/- buffers/cache line, which I mainly used to look at, is gone. How should I interpret the new numbers?

• Does Mem used/free include buffers and cache or not?
• Which numbers are the equivalent of the used and free numbers on the "+/- buffers/cache" line of the earlier version?

Answer 1

Please consider the sample output I got from the free command in my Ubuntu 12.04:

           total       used       free     shared    buffers     cached
Mem:       8074640    6187480    1887160     377056     365128    2113156
-/+ buffers/cache:    3709196    4365444
Swap:     15998972      82120   15916852

The Mem used(kb_main_used) field value is now calculated like this:

used = total - free - cached - buffers

Previously, it used to be:

used = total - free

This change was introduced in the following commit https://gitlab.com/procps-ng/procps/commit/6cb75efef85f735b72e6c96f197f358f511f8ed9

An intermediate value:

buffers_plus_cached = buffers (kb_main_buffers) + cached (kb_main_cached) = 365128 + 2113156 = 2478284

+/- buffers/cache value is calculated like this:

buffers = kb_main_used - buffers_plus_cached = 6187480 - 2478284 = 3709196
/
cache = kb_main_free + buffers_plus_cached = 1887160 + 2478284 = 4365444

The new buff/cache value is calculates like this:

buff/cache = kb_main_buffers+kb_main_cached = 365128 + 2113156 = 2478284

This is the same as the buffers_plus_cached, used in previous versions, the difference is that previously it was used internally, and now its displayed directly, and the further calculated line, -/+ buffers/cache has been removed

For more info, please check these commits, where these changes were introduced: https://gitlab.com/procps-ng/procps/commit/f47001c9e91a1e9b12db4497051a212cf49a87b1 https://gitlab.com/procps-ng/procps/commit/c9908b59712d1afd6b9bf7971ba1d8900ae5adb8

As of the new available field, for Linux kernels older than 2.6.27, its value is the same as the free value, but for the later versions of the Kernel, its a bit different:

Estimation of how much memory  is  available  for  starting  new
applications,  without swapping. Unlike the data provided by the
cache or free fields, this field takes into account  page  cache
and also that not all reclaimable memory slabs will be reclaimed
due to  items  being  in  use  (MemAvailable  in  /proc/meminfo,
available   on   kernels  3.14,  emulated  on  kernels  2.6.27+,
otherwise the same as free)

Courtesy: http://manpages.ubuntu.com/manpages/xenial/en/man1/free.1.html

So, the specific answer to your questions would be:

• The new version of free includes buffers/cache in the calculations of Mem used/free values.
• The +/- buffers/cache value that used to be there in previous versions of free is now available as:
  • -/+ buffers/cache used = Current Mem used column (Its calculation is detailed above)
  • -/+ buffers/cache free is available as the more accurate value in the current new column available

N.B: The kb_* variable names are the internal names used in the source code.