[TriLUG] Understanding output of "free"

[Andrew Perrin]

The buffers are reclaimable by new memory requests -- they're stored in
buffers rather than cleared because:

- Clearing memory takes processing time; and
- The kernel makes an assumption that a recently-used program is more
likely to be asked for again than is a non-recently-used program.

Therefore it makes sense to keep the information in RAM, but not reserved,
until the memory is needed (or the same information is "reloaded").

Corrections welcome.

ap

----------------------------------------------------------------------
Andrew J Perrin - http://www.unc.edu/~aperrin
Assistant Professor of Sociology, U of North Carolina, Chapel Hill
clists at perrin.socsci.unc.edu * andrew_perrin (at) unc.edu


On Fri, 3 May 2002, Jeremy P wrote:

> 
> Can anyone explain what the kernel buffers/cache are used for, as
> displayed in the output of "free"?
> 
> For example, one of my servers shows this:
> $ free -m
>              total       used       free     shared    buffers     cached
> Mem:           885        872         12        501        371        156
> -/+ buffers/cache:        344        540
> Swap:          133          7        125
> 
> (Yes, I know I need more swap.)
> 
> At first glance, it looks like there's only 12 MB free memory, but if you
> subtract the buffers/cache amount, there's 540 MB free (344 used).  What's
> all that buffer memory being used for?
> 
> Suppose an application requests more than 125 MB of memory, the total
> amount shown free. Will it "steal" extra memory from the buffers?  Or will
> it get out-of-memory errors?  
> 
> Also, why does it only show 885 MB physical RAM even though the computer
> has a 1 GB DIMM installed?  The BIOS reports something like 992MB at the
> POST.  That's a difference of 107MB -- that's a lot!
> 
> I'm trying to figure out whether this box really needs more physical RAM
> or not.  The extremely tight budget is becoming even tighter...
> 
> Thanks for any tips!
> 
> --Jeremy
> 
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