Mapped more than Cached in /proc/meminfo

[Prateek Sharma]

On Wed, Aug 10, 2011 at 12:56 PM, Mulyadi Santosa <mulyadi.santosa at gmail.com
> wrote:

> Hi///
>
> On 09/08/2011, =/_00/\/\ <z0032oom at gmail.com> wrote:
> > Hi,
> >
> > Thanks for replying
> > Here is one observation worth mentioning.
> > On boot system it shows Mapped < Cached (In fact much less)
>
> quite predictable.... during booting phase, your system read() much by
> doesn't mmap() that much....
>
> > After using system for some time following commands are executed.
> > # sync and echo 3 > /proc/sys/vm/drop_cache
> >
> > Whenever above command is executed I can see Mapped > Cached.
>
> by echoing "3" to drop_cache, you flush the content of page cache as
> much as possible ...
>
>
Here is my understanding of what drop_page_cache does:
All page-cache pages are 'dropped' except the following:
1. Dirty pages. (they are *not* synced)
2. Mapped pages (pages 'in use' , mapped by rmap )
(There are a few more exceptions i dont recall now.)

Hence even after dropping caches, free will still show some cached pages.
These are dirty+mapped.
Doing a sync before dropping will only show the mapped pages.
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