VM: Buffer vs. Cache

VM: Buffer vs. Cache

[Martin.Knoblauch]

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Hi,

 just being curious. Since 2.4.4, I am watching my systems memory
behaviour a bit:-) Just recently I realized the following: in the
evening I leave my 128MB system at about 20 MB, 2 MB Buffered and 100 MB
Cached (plus som 40 MB unneccesary swap :-)). When I come back in the
morning, Used is still at 20 MB (a bit down maybe) but Buffered is 50 MB
and Cached is 55 MB. For a few minutes the system is definitely more
sluggish than in the evening. Something I can excuse before my first cup
of coffe anyway...

 So, what actually is the difference between Buffered and Cached.
Apparently quite a lot of the pages that are Cached in the evening are
Buffered 9 houres later.

Thanks
Martin
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Re: VM: Buffer vs. Cache

[Jonathan Morton]

> just being curious. Since 2.4.4, I am watching my systems memory
>behaviour a bit:-) Just recently I realized the following: in the
>evening I leave my 128MB system at about 20 MB, 2 MB Buffered and 100 MB
>Cached (plus som 40 MB unneccesary swap :-)). When I come back in the
>morning, Used is still at 20 MB (a bit down maybe) but Buffered is 50 MB
>and Cached is 55 MB. For a few minutes the system is definitely more
>sluggish than in the evening. Something I can excuse before my first cup
>of coffe anyway...
>
> So, what actually is the difference between Buffered and Cached.
>Apparently quite a lot of the pages that are Cached in the evening are
>Buffered 9 houres later.

Think about what happens in the meantime.  Most distros install maintenance
scripts which run late at night (usually at midnight and/or 4am), which
perform heavy disk activity as they update databases and scan for
file-permissions security holes.  Heavy disk activity usually means an
increase in buffer utilisation.  Since most files are only "touched" once,
the cache is shrunk as it aren't being used very much.

--------------------------------------------------------------
from:     Jonathan "Chromatix" Morton
mail:     chromi@cyberspace.org  (not for attachments)

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Re: VM: Buffer vs. Cache

[Martin.Knoblauch]

Jonathan Morton wrote:
> 
> >
> > So, what actually is the difference between Buffered and Cached.
> >Apparently quite a lot of the pages that are Cached in the evening are
> >Buffered 9 houres later.
> 
> Think about what happens in the meantime.  Most distros install maintenance
> scripts which run late at night (usually at midnight and/or 4am), which
> perform heavy disk activity as they update databases and scan for
> file-permissions security holes.  Heavy disk activity usually means an
> increase in buffer utilisation.  Since most files are only "touched" once,
> the cache is shrunk as it aren't being used very much.
> 

 thanks to all who set me straight. Should have thought of the nightly
stuff myself.

Martin
-- 
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Martin Knoblauch         |    email:  Martin.Knoblauch@TeraPort.de
TeraPort GmbH            |    Phone:  +49-89-510857-309
C+ITS                    |    Fax:    +49-89-510857-111
http://www.teraport.de   |    Mobile: +49-170-4904759