Tuesday, 5 August 2014

How to calculate memory used/free in Linux

Formula for used% in linux

free -o
             total       used       free     shared    buffers     cached
Mem:       8167004    8115292      51712          0      87680    1444676
Swap:      8388584    3252996    5135588

((memUsed-mebBuffers-memCached)*100)/memTotal => totalMemoryUsed%

((8115292-87680-1444676)*100/8167004) => 81%

 Display free memory in Linux / Ubuntu
Short Answer:
free -m | awk 'NR==3 {print $4 " MB"}'

The command 'free' displays some formatted information from /proc/meminfo.
The throw '-m' displays these numbers in rounded megabytes.

# free -m
         total      used     free   shared     buffers     cached
Mem:      4049      3982       67        0          16       3530
-/+ buffers/cache:   435     3614
Swap:     6142        53     6088

It's strange, cause I always figured the "first row, third column" - right under 'FREE' - would be the amount of free memory on a Linux system. But that number was always low after I'd been running awhile.

And it turns out that it is NOT really what I was looking for. A more accurate representation of the memory being used by your applications and available for new processes is displayed in the SECOND line.

In addition to the memory that is actually being USED by the kernel and processes resident in memory - Linux also reserves memory to allocate to processes as 'buffers' AND uses pretty much any left over memory to hold "cached" files.

Looking only at the top line...
total = all memory in the system (4GB on this server)
used = all memory currently in use/reserved by running processes and the OS
free = total - used
shared = memory being shared by multiple processes (deprecated?)
buffers = memory reserved by the OS to alloc as buffers when process need them (aka the 'heap')
cached = recently used files being stored in ram (THANK YOU LINUX!)

Here's a simple example I found to show off the power of 'caching':

for i in 1 2 ; do free -o; time grep -r foo /usr/bin >/dev/null 2>/dev/null; done

So really the buffers would be allocated to a running process if it asked for them anyway, and the memory being used to cache copies of recently used files would be released immediately if it makes sense to allocate the RAM elsewhere. So all that memory is 'available'.

Using these definitions:

When thinking about 'how much memory is really being used' - I want to calculate:
'used' - ('buffers' + 'cached')

When thinking about 'how much memory is really free' - I want to calculate:
'free' + ('buffers' + 'cached')

With this in mind, the meaning of the second row header form the output of the Linux command "free" (-/+ buffers/cache:) makes more sense...

Free is doing some light lifting for us, using the formula's above to display:
"minus buffers and cache" for the used column
"plus buffers and cache" for the free colum

So when you run free on Linux - the amount of free memory is always displayed right there in the second row, third column. Hence the 'Short Answer'...

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