linux-kernel - Re: [PATCH] kernel: fs: drop_caches: add dds drop_caches

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Date:	Tue, 16 Feb 2016 11:12:23 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Dave Chinner <david@...morbit.com>,
	Daniel Walker <danielwa@...co.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Khalid Mughal <khalidm@...co.com>,
	xe-kernel@...ernal.cisco.com, dave.hansen@...el.com,
	hannes@...xchg.org, Jonathan Corbet <corbet@....net>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	"Nag Avadhanam (nag)" <nag@...co.com>
Subject: Re: [PATCH] kernel: fs: drop_caches: add dds drop_caches_count

On Tue, 2016-02-16 at 16:28 +1100, Dave Chinner wrote:
> On Mon, Feb 15, 2016 at 03:52:31PM -0800, Daniel Walker wrote:
> > On 02/15/2016 03:05 PM, Dave Chinner wrote:
> > > 
> > > As for a replacement, looking at what pages you consider
> > > "droppable"
> > > is really only file pages that are not under dirty or under
> > > writeback. i.e. from /proc/meminfo:
> > > 
> > > Active(file):     220128 kB
> > > Inactive(file):    60232 kB
> > > Dirty:                 0 kB
> > > Writeback:             0 kB
> > > 
> > > i.e. reclaimable file cache = Active + inactive - dirty -
> > > writeback.
> .....
> 
> > As to his other suggestion of estimating the droppable cache, I
> > have considered it but found it unusable. The problem is the
> > inactive file pages count a whole lot pages more than the
> > droppable pages.
> 
> inactive file pages are supposed to be exactly that - inactive. i.e.
> the have not been referenced recently, and are unlikely to be dirty.
> They should be immediately reclaimable.

Inactive file pages can still be mapped by
processes.

The reason we do not unmap file pages when
moving them to the inactive list is that
some workloads fill essentially all of memory
with mmapped file pages.

Given that the inactive list is generally a
considerable fraction of file memory, unmapping
pages that get deactivated could create a lot
of churn and unnecessary page faults for that
kind of workload.

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