What you're looking for is the output from "free":
$ free
total used free shared buffers cached
Mem: 775556 759456 16100 0 22132 592484
-/+ buffers/cache: 144840 630716
Swap: 500344 21972 478372
Here's a tour:
This is a box w/ 768MB of physical RAM and a 500344KB swap partition.
759456KB is "used" (second column, top row). Of this "used" memory, 23132KB is buffers (5th column, top row) and 592484KB (sixth column, top row) is cache, leaving 144840KB (2nd column, 2nd row) of physical memory that's being used by active processes.
When you consider that the memory used by buffers and cache could be used for processes, that leaves 630716KB (3rd column, 2nd row) of physical memory free.
The swap partition, as I said, is 500344KB (1st column, bottom row). 21972KB (2nd column, bottom row) of the swap file is in use, leaving 478372KB (3rd column, bottom row) free.
So, your definition of % free memory depends on whether you're counting buffers and cache or not, and whether you're counting swap or not.
That ought to give you enough to go on to calculate a percentage as you see fit.
Here's the output of top and free:
$ top -n1 | grep "used," ; free
Mem: 775556k total, 751472k used, 24084k free, 20776k buffers
Swap: 500344k total, 21972k used, 478372k free, 586648k cached
total used free shared buffers cached
Mem: 775556 751344 24212 0 20776 586648
-/+ buffers/cache: 143920 631636
Swap: 500344 21972 478372
You can see how top and free agree (albeit with a little difference-- this is a multi-user machine and the memory usage of the top and free programs are reflected in there).
Add a "-s" when you call vmstate, and you will see much the same output, just in a different form:
$ vmstat -s ; free
775556 total memory
759920 used memory
674680 active memory
18440 inactive memory
15636 free memory
21892 buffer memory
594372 swap cache
500344 total swap
21972 used swap
478372 free swap
... output truncated ...
total used free shared buffers cached
Mem: 775556 759920 15636 0 21892 594372
-/+ buffers/cache: 143656 631900
Swap: 500344 21972 478372
You can see that vmstat is showing all the same numbers as free.
The cache memory in Linux-based machines is often used to cache disk blocks into memory to prevent writing to the harddisk and speed up I/O operations. Also, a lot of free memory is cached and released when it is actually required.
When interpreting the output of free, you should look at the second row for the actual memory usage.
The above numbers can be interpreted as:
- 9534176 kilobytes cached memory
- 737752 actually used memory
- 15701956 actually free memory
For more information, take a look at http://www.faqs.org/docs/linux_admin/buffer-cache.html.
Best Answer
Were you getting out of memory errors with a particular application? Or do you have a performance issue? What specifically leads you to believe that your are out of memory (aside from the 'free -m' command).
The first thing to look for is a LOT of processes using a small amount of memory. If you have 100 processes only using 128mb of memory you will be short of memory for example.
It's very normal to see almost 100% of your memory used in Linux. However I don't know enough about how this works within your particular VPS provider to know if your usage is considered normal. With my own VPS provider I have 24mb / 512m free and it's purring along nicely. If required it will dump out the almost 400mb of cached data it has.