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Unix Dweeb

How much memory is installed and being used on your Linux systems?

Feb 07, 20196 mins

Several commands report on how much memory is installed and being used on Linux systems. You can be deluged with details or get a quick and easy answer, depending on the command you use.

There are numerous ways to get information on the memory installed on Linux systems and view how much of that memory is being used. Some commands provide an overwhelming amount of detail, while others provide succinct, though not necessarily easy-to-digest, answers. In this post, we’ll look at some of the more useful tools for checking on memory and its usage.

Before we get into the details, however, let’s review a few basics. Physical memory and virtual memory are not the same. The latter includes disk space configured to be used as swap. Swap may include partitions set aside for this usage or files that are created to add to the available swap space when creating a new partition may not be practical. Some Linux commands provide information on both.

Swap expands memory by providing disk space that can be used to house inactive pages that are moved to disk when physical memory fills up.

One file that plays a role in memory management is /proc/kcore. This file looks like a normal (though extremely large) file, but it does not occupy disk space at all. Instead, it is a virtual file like all of the files in /proc.

$ ls -l /proc/kcore
-r--------. 1 root root 140737477881856 Jan 28 12:59 /proc/kcore

Interestingly, the two systems queried below do not have the same amount of memory installed, yet the size of /proc/kcore is the same on both. The first of these two systems has 4 GB of memory installed; the second has 6 GB.

system1$ ls -l /proc/kcore
-r--------. 1 root root 140737477881856 Jan 28 12:59 /proc/kcore
system2$ ls -l /proc/kcore
-r--------  1 root root 140737477881856 Feb  5 13:00 /proc/kcore

Explanations that claim the size of this file represents the amount of available virtual memory (maybe plus 4K) don’t hold much weight. This number would suggest that the virtual memory on these systems is 128 terabytes! That number seems to represent instead how much memory a 64-bit systems might be capable of addressing — not how much is available on the system. Calculations of what 128 terabytes and that number, plus 4K would look like are fairly easy to make on the command line:

$ expr 1024 * 1024 * 1024 * 1024 * 128
$ expr 1024 * 1024 * 1024 * 1024 * 128 + 4096

Another and more human-friendly command for examining memory is the free command. It gives you an easy-to-understand report on memory.

$ free
              total        used        free      shared  buff/cache   available
Mem:        6102476      812244     4090752       13112     1199480     4984140
Swap:       2097148           0     2097148

With the -g option, free reports the values in gigabytes.

$ free -g
              total        used        free      shared  buff/cache   available
Mem:              5           0           3           0           1           4
Swap:             1           0           1

With the -t option, free shows the same values as it does with no options (don’t confuse -t with terabytes!) but by adding a total line at the bottom of its output.

$ free -t
              total        used        free      shared  buff/cache   available
Mem:        6102476      812408     4090612       13112     1199456     4983984
Swap:       2097148           0     2097148
Total:      8199624      812408     6187760

And, of course, you can choose to use both options.

$ free -tg
              total        used        free      shared  buff/cache   available
Mem:              5           0           3           0           1           4
Swap:             1           0           1
Total:            7           0           5

You might be disappointed in this report if you’re trying to answer the question “How much RAM is installed on this system?” This is the same system shown in the example above that was described as having 6GB of RAM. That doesn’t mean this report is wrong, but that it’s the system’s view of the memory it has at its disposal.

The free command also provides an option to update the display every X seconds (10 in the example below).

$ free -s 10
              total        used        free      shared  buff/cache   available
Mem:        6102476      812280     4090704       13112     1199492     4984108
Swap:       2097148           0     2097148

              total        used        free      shared  buff/cache   available
Mem:        6102476      812260     4090712       13112     1199504     4984120
Swap:       2097148           0     2097148

With -l, the free command provides high and low memory usage.

$ free -l
              total        used        free      shared  buff/cache   available
Mem:        6102476      812376     4090588       13112     1199512     4984000
Low:        6102476     2011888     4090588
High:             0           0           0
Swap:       2097148           0     2097148

Another option for looking at memory is the /proc/meminfo file. Like /proc/kcore, this is a virtual file and one that gives a useful report showing how much memory is installed, free and available. Clearly, free and available do not represent the same thing. MemFree seems to represent unused RAM. MemAvailable is an estimate of how much memory is available for starting new applications.

$ head -3 /proc/meminfo
MemTotal:        6102476 kB
MemFree:         4090596 kB
MemAvailable:    4984040 kB

If you only want to see total memory, you can use one of these commands:

$ awk '/MemTotal/ {print $2}' /proc/meminfo
$ grep MemTotal /proc/meminfo
MemTotal:        6102476 kB

The DirectMap entries break information on memory into categories.

$ grep DirectMap /proc/meminfo
DirectMap4k:      213568 kB
DirectMap2M:     6076416 kB

DirectMap4k represents the amount of memory being mapped to standard 4k pages, while DirectMap2M shows the amount of memory being mapped to 2MB pages.

The getconf command is one that will provide quite a bit more information than most of us want to contemplate.

$ getconf -a | more
LINK_MAX                           65000
_POSIX_LINK_MAX                    65000
MAX_CANON                          255
_POSIX_MAX_CANON                   255
MAX_INPUT                          255
_POSIX_MAX_INPUT                   255
NAME_MAX                           255
_POSIX_NAME_MAX                    255
PATH_MAX                           4096
_POSIX_PATH_MAX                    4096
PIPE_BUF                           4096
_POSIX_PIPE_BUF                    4096
_POSIX_NO_TRUNC                    1
_POSIX_VDISABLE                    0
ARG_MAX                            2097152
ATEXIT_MAX                         2147483647
CHAR_BIT                           8
CHAR_MAX                           127

Pare that output down to something specific with a command like the one shown below, and you’ll get the same kind of information provided by some of the commands above.

$ getconf -a | grep PAGES | awk 'BEGIN {total = 1} {if (NR == 1 || NR == 3) total *=$NF} END {print total / 1024" kB"}'
6102476 kB

That command calculates memory by multiplying the values in the first and last lines of output like this:

PAGESIZE                           4096    

Calculating that independently, we can see how that value is derived.

$ expr 4096 * 1525619 / 1024

Clearly that's one of those commands that deserves to be turned into an alias!

Another command with very digestible output is top. In the first five lines of top's output, you'll see some numbers that show how memory is being used.

$ top
top - 15:36:38 up 8 days,  2:37,  2 users,  load average: 0.00, 0.00, 0.00
Tasks: 266 total,   1 running, 265 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.2 us,  0.4 sy,  0.0 ni, 99.4 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   3244.8 total,    377.9 free,   1826.2 used,   1040.7 buff/cache
MiB Swap:   3536.0 total,   3535.7 free,      0.3 used.   1126.1 avail Mem

And finally a command that will answer the question "So, how much RAM is installed on this system?" in a succinct fashion:

$ sudo dmidecode -t 17 | grep "Size.*MB" | awk '{s+=$2} END {print s / 1024 "GB"}'

Depending on how much detail you want to see, Linux systems provide a lot of options for seeing how much memory is installed on your systems and how much is used and available.

Unix Dweeb

Sandra Henry-Stocker has been administering Unix systems for more than 30 years. She describes herself as "USL" (Unix as a second language) but remembers enough English to write books and buy groceries. She lives in the mountains in Virginia where, when not working with or writing about Unix, she's chasing the bears away from her bird feeders.

The opinions expressed in this blog are those of Sandra Henry-Stocker and do not necessarily represent those of IDG Communications, Inc., its parent, subsidiary or affiliated companies.

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