Linux: How Much Memory is Really Free?
If you want to know quickly just how much memory is really free, type
free -m. Sample output:
◆ free -m total used free shared buffers cached Mem: 5798 2068 3730 0 157 1053 -/+ buffers/cache: 857 4941 Swap: 5885 0 5885
the 4941 is memory free for applications, in mega bytes.
Alternatively, you can see the same figure if you call “htop”. 〔➤ Linux: Monitor Processes, “htop” Tutorial〕
Understanding Memory from 「top」 and 「free」
First, know that the outputs from
free are basically the same.
Here's output from
Mem: 5937800k total, 2174256k used, 3763544k free, 163796k buffers Swap: 6027260k total, 0k used, 6027260k free, 1131896k cached
Here's output from
total used free shared buffers cached Mem: 5937800 2173968 3763832 0 163792 1131508 -/+ buffers/cache: 878668 5059132 Swap: 6027260 0 6027260
So, we can now just focus on studying the output from
First, understand this:
① Linux uses perm storage device (⁖ hard-disk ) as virtual memory. The virtual memory on hard-disk is called swap space. Virtual memory means the perm storage is used as temp space for RAM. So, when your RAM is full, the OS can off-load parts of it currently not used data to disk, therefore free up memory for application that needs it.
② Also, Linux uses RAM as cache for file data (from hard-disk). (Because RAM's IO speed is a thousand times faster than hard disk, so OS will load disk data to RAM as cache)
Now, let's look at the output of
free -m (“-m” means mega bytes).
total used free shared buffers cached Mem: 5798 2145 3653 0 160 1121 -/+ buffers/cache: 862 4935 Swap: 5885 0 5885
in the first row Mem, the total 5798 is the total RAM.
used + free = total. Check: 2145 + 3653 = 5798.
“used” includes used for disk cache purposes.
“cached” is amount of disk data sitting in RAM for fast access.
So, real free should be “free + buffers + cached”, and in this example it is 3653 + 160 + 1121 = 4934.
and actually used should be “used - buffers - cached” (2145 - 160 - 1121 = 864).
And if we add 4934 and 864 we get the “total”.
free command pretty much did this calculation for us, in the line:
used free -/+ buffers/cache: 862 4935