Hi I was wondering if anyone can explain this to me. I have a system in sort of an idle state. I mean nothing is going on on it. It is not a server of any kind. Built brand new openSuse 11.2 i386. What I have been noticing is the change in free memory amount: it is gradually declines and the buff amount is increasing. Here are my monitoring results.

Sat Feb 20 02:00:02 EST 2010
procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
4 0 0 611880 27640 270432 0 0 8 1 19 10 0 0 99 0 0
Sat Feb 20 09:00:01 EST 2010
procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
1 0 0 601824 37536 270884 0 0 3 1 17 8 0 0 100 0 0
Sun Feb 21 02:00:01 EST 2010
procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
4 0 0 517748 72840 319172 0 0 1 1 19 12 0 0 100 0 0
Sun Feb 21 15:00:01 EST 2010
procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
3 0 0 498504 91016 319988 0 0 1 1 17 10 0 0 100 0 0

There's loads of qns here at LQ about this ; have a search. In short, Linux uses all the RAM to cache anything that's been used. No point having the RAM and not using it.
If the system needs some more RAM after it's full, it'll flush out the oldest/least used stuff. Don't worry, this is perfectly normal and Linux has a very good Virtual Memory driver.

Here's a couple to get you started:
- linux-memory-management
- linux-ate-my-ram

(the first one has a definition of buffer and cache)

Thanks. This is really helpful.

Think about this...

"Memory" is the only thing in the entire computer that "is (almost) as fast as the CPU." It's the one-and-only system resource that the CPU doesn't have to wait for (much...).

Therefore, the system is designed to "fill it up." Once something has been loaded into memory, even if that "something" is no longer being used, the system won't go out of its way to replace it or remove it. It'll allocate unused memory instead (if it's available), in hope that "the previously-loaded something" will again be needed very soon ... a quite-reasonable assumption.

"Laziness" is a virtue among operating systems.

Today, most computers have a plentiful supply of RAM. So, "laziness" is richly rewarded with excellent perceived performance. The computer will not make any attempt whatsoever to be anything but "lazy" until, and unless, actual memory-pressure materializes.

Then, it will begin "an organized retreat" from its lazy position... withdrawing first one use of the "extra" memory, then another, then another.

All modern operating systems follow this very successful strategy. This is why, if you want to make a computer "more zippy," you should always add memory (buy as much as it will hold...) before you contemplate buying a faster CPU.

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