interpreting memory usage

[ZIYAD A. M. AL-BATLY]

On Wed, 2005-06-08 at 11:52 +0100, R Kimber wrote:
> Last night, after a reboot and after opening a few standard apps, free
> gave
> 
>            total       used       free     shared    buffers cached
> Mem:2055816     482572    1573244   0 36560     179136
>  -/+ buffers/cache:   266876    1788940
> Swap:  2104464   0    2104464
> 
> i.e. it seemed to be saying that 76.5% was free.
> This morning, with the same apps remaining open, it gave
> 
>             total       used       free     shared    buffers cached
> Mem:2055816    1704252     351564          0 364044     248908
>  -/+buffers/ cache:    1091300     964516
> Swap:      2104464          0    2104464
> 
> i.e. 17.1% free.
> 
> How should this be interpreted?  It seems a big drop in "free"
> memory, whatever that means, given that no new apps were started and I
> did very little on the machine during the intervening period. The
> buffers and cached went up from about 10% to 30%
> 
> Are there system/housekeeping processes running in the background that
> aren't started at boot and that can take up such a large chunk of memory
> (presumably around half a GB)?  Or am I misinterpreting the
> figures?
No.  Subtract the “used +/-buffers/cache” and you will get the real
usage of your memory.  In the above, your real memory usage is (1704252
- 1091300)  which is 612952 out of 2055816 (about 70% free).

Linux tends to cache/buffer a lot to speed up the system as long as
there is memory to spear (i.e. not used by applications).

> 
> Subjectively I think that 5.04 uses more memory than its predecessor,
> the desktop seems quite expensive in memory terms, and to get slower
> after a while.
I didn't notice this myself.

> 
> I ask this not because I'm concerned about the problem but because I'd
> like to understand a little better (I normally use zero swap and I know
> that linux manages memory well, though the slowdown is an issue that I
> shall keep an eye on).  
> 
> - Richard.
> -- 
> Richard Kimber
> http://www.psr.keele.ac.uk/
> 

I hope this answered you question.
Ziyad.