freebsd stable - Jul 2006 - Memory management

Hello there,

  I have a computer running FreeBSD 6.1.

  As time passing by, the memory fills up. When the machine starts,
memory is occupied to 30 %, and after two or three weeks memory is
occupied to 100 % and it begins to use swap.

  It is "inactive" pages that fills up the memory.

  I tried to restart every process, but memory usage does not
decrease. Only a reboot can fix that. And I'm not able to see which
process leaks.

  I was not able to find a correct definition of what "inactive"
memory is. First, I would like to know what are these kind of pages :
wired, active, inactive, cache and free.

  Is that normal that inactive memory usage grows ? What should I do ?

  Do you have any tools to monitor memory usage of processes ?

  Many thanks, regards,

On Wed, 2006-Jul-26 11:26:34 +0200, Stephane Dupille
wrote:
>  As time passing by, the memory fills up. When the machine starts,
>memory is occupied to 30 %, and after two or three weeks memory is
>occupied to 100 % and it begins to use swap.
How are you monitoring memory usage?  Do you mean 'swap' or
'page'?
A level of page-in's is normal because text and data areas for
processes are loaded by paging them in.
>  I was not able to find a correct definition of what "inactive"
>memory is. First, I would like to know what are these kind of pages :
>wired, active, inactive, cache and free.
Wired pages are pages that the kernel has wired to RAM so they cannot
be paged out.  Active pages are being mapped by virtual memory and
in use by running processes.  Inactive pages are not currently mapped
but the kernel knows their contents and can re-map them without
needing to retrieve them from disk - they may be dirty.  Cache pages
are similar to active pages but aren't dirty and are higher-priority
candidates for being freed.  Free pages have no useful content and
will be used to fulfil page-in requests.
>  Is that normal that inactive memory usage grows ?
Yes.  'Free' memory is basically wasted and so the kernel tries to limit
it, subject to having sufficient free memory to meet page-faults.  Most
of your RAM should be wired, active or inactive.  Inactive memory will
start at 0 and grow as active pages are released.
> What should I do ?
Nothing.  Why do you think you have a problem?
>  Do you have any tools to monitor memory usage of processes ?
ps(1)

-- 
Peter Jeremy
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On Wed, 26 Jul 2006, Stephane Dupille wrote:
>  I have a computer running FreeBSD 6.1.
>
>  As time passing by, the memory fills up. When the machine starts, memory
is
> occupied to 30 %, and after two or three weeks memory is occupied to 100 % 
> and it begins to use swap.
>
>  It is "inactive" pages that fills up the memory.
>
>  I tried to restart every process, but memory usage does not decrease. Only
> a reboot can fix that. And I'm not able to see which process leaks.
>
>  I was not able to find a correct definition of what "inactive"
memory is.
> First, I would like to know what are these kind of pages : wired, active, 
> inactive, cache and free.
>
>  Is that normal that inactive memory usage grows ? What should I do ?
>
>  Do you have any tools to monitor memory usage of processes ?
You can find an article discussing some of the FreeBSD VM system design here:

   http://www.freebsd.org/doc/en_US.ISO8859-1/articles/vm-design/article.html

This document is gradually aging, and doesn't cover certain elements of the 
design, including some recent optimizations, SMP behavior, etc, but still 
makes quite a good read.

It could be that you have applications leaking memory, but it's more likely 
that the VM system is simply being efficient.  Free memory is, in effect, 
wasted memory, since it's not being used.  FreeBSD will agressively cache
file
system data, and either drop or page out unused pages (depending on whether 
they are dirty) in order to maximize the amount of memory available for 
actively used data.  This means it will swap out dirty pages allocated by a 
process if the process isn't using them, rather than keep them in memory 
preventing that memory from being used by processes that need it.  The result 
is that at any given moment, an active system should have almost no free 
memory, and instead should be providing as much memory as possible to active 
pages, and the largest possible file system cache.  This may seem 
counter-intuitive compared to some other systems where a premium is placed on 
free pages as representing the resources available to run additional 
applications.  We consider memory floating around "unused" to be a
waste of
memory that could be used to improve system performance.

"systat -vmstat 1" is a good tool to monitor the VM system, as it will
let you
monitor memory use, in particular, the vnode and swap paging rates.  You can 
use ps(1) with various parameters to inspect process memory use.  A popular 
combination is "aux", which views all processes and displays, among
other
things, their virtual and resident sizes.  The resident size is the figure you 
want to look at, as it represents the number of pages actually in memory, 
rather than pages that could be paged in.

Robert N M Watson
Computer Laboratory
University of Cambridge