[CentOS] Where is cached memory going?

ABBAS KHAN bashukhan at gmail.com
Tue Aug 19 16:56:37 UTC 2008


An awesome reply.
Makes sense!

Thanks.




On Tue, Aug 19, 2008 at 9:42 AM, David Dyer-Bennet <dd-b at dd-b.net> wrote:

>
> On Tue, August 19, 2008 11:31, ABBAS KHAN wrote:
> > As by the time, I've learned that Linux works by caching apps by using a
> > lot
> > of RAM and then it reallocates the new stuff by cleaning the old cached
> > pages from memory as compared to other OSs. With 2 gigs of RAM often I
> see
> > the free memory only as 100-400MB. Using TOP or PS, it doesn't look like
> > any
> > program or process is using excessive memory (the highest process is seen
> > with 1-2% total memory). *So, my questions are:*
> >
> >  what programs are using that much of memory? (or cached memory)
>
> It's very likely being used as disk cache.  You can get some more numbers
> by running top, and looking at the last two lines of the headers.  I
> routinely see over 1GB of cache on a not very active 4GB system.  Your
> meminfo output is the same numbers, and looks completely normal to me.
>
> Free memory is *bad*; it means it's being wasted completely.  Memory used
> for disk caching is instantly available if it's suddenly needed for a
> program.
>
> > Is that really due to a lot of cache in the memory
> > *if yes, then, is there a way to parse the cache to findout what
> > applications are eating up the cache?*
>
> It's only indirectly tied to an application; it's cached disk blocks.  You
> could say the process that read that file last is responsible, but the
> *next* process to read those blocks is the one that would benefit.
>
> > *how to free the cached memory?*
>
> Why do you want to?  As I said, "free" memory is memory that's going to
> waste.  Unless you have severe real-time issues with a process becoming
> runnable where the difference between discarding a clean cached page, and
> just allocating a free page, will make a difference, there's no point.  If
> you DO have that level really extreme real-time performance issues, you
> need to understand the whole virtual memory system an order of magnitude
> better than you seem to.  That's off in a far corner of the Linux
> application space -- Linux can do some real-time stuff, but it's not the
> first choice for hard real-time environments last time I talked to any of
> those people.
>
> --
> David Dyer-Bennet, dd-b at dd-b.net; http://dd-b.net/
> Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/
> Photos: http://dd-b.net/photography/gallery/
> Dragaera: http://dragaera.info
>
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