On Mon, Feb 15, 2016 at 11:14:13PM -0800, Nag Avadhanam wrote:

On Mon, 15 Feb 2016, Dave Chinner wrote:

quoted

On Tue, Feb 16, 2016 at 02:58:04AM +0000, Nag Avadhanam (nag) wrote:

quoted

Its the calculation of the # of bytes of non-reclaimable file system cache
pages that has been troubling us. We do not want to count inactive file
pages (of programs/binaries) that were once mapped by any process in the
system as reclaimable because that might lead to thrashing under memory
pressure (we want to alert admins before system starts dropping text
pages).

The code presented does not match your requirements. It only counts
pages that are currently mapped into ptes. hence it will tell you
that once-used and now unmapped binary pages are reclaimable, and
drop caches will reclaim them. hence they'll need to be fetched from
disk again if they are faulted in again after a drop_caches run.

Will the inactive binary pages be automatically unmapped even if the process
into whose address space they are mapped is still around? I thought they
are left mapped until such time there is memory pressure.

Right, page reclaim via memory pressure can unmap mapped pages in
order to reclaim them. Drop caches will skip them.

We only care for binary pages (active and inactive) mapped into the
address spaces of live processes. Its okay to aggressively reclaim
inactive
pages once mapped into processes that are no longer around.

Ok, if you're only concerned about live processes then drop caches
should behave as you want.

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com

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