Calculating Memory Utilization on Linux

Below is the way to calculate the "actual" memory utilization in the past using "sar" file. Below is the way to do the same.

Method 1:

tch memory utilization data from sar files using -r switch.

sar -r -f

It will display data in this format

04:40:01 PM kbmemfree kbmemused %memused kbbuffers kbcached kbswpfree kbswpused %swpused kbswpcad

04:50:01 PM 258788 16372260 98.44 12696 13912224 12484624 94144 0.75 420

Information being displayed here, is somewhat misleading. According to it, at 04:50PM, 98.44% memory was utilized (As its merely calculated by formula kbmemused/sum(kbmemused, kbmemfree)).

But in actual that was not the case. In order to get actual memory utilization, subtract kbbuffers and kbcached from kbmemused, and then used the above formula. In this case,

Memory utilization = (16372260-12696-13912224)/(258788+16372260) = 14.71%

The reason behind this is Linux treats unused memory as a wasted resource and so uses as much RAM as it can to cache process/kernel information.

Here Buffers= amount of physical memory used as buffers for disk writes

Cached = amount of physical memory used as cache for disk reads

Method 2:
I prefer this way

free -m command

# free -m
                total       used       free     shared    buffers     cached
Mem:         12011       9825       2186          0        243       5829
-/+ buffers/cache:       3752       8259
Swap:        16378        313      16065      

Real Free memory  = ((Buffers+Cached)  + free
                              = ((243 + 5829)  + 2186
                              = 8259 Free memory