linux-kernel - Re: computing drop-able caches

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Date:	Thu, 11 Feb 2016 17:11:59 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Daniel Walker <danielwa@...co.com>,
	Dave Hansen <dave.hansen@...el.com>,
	"Khalid Mughal (khalidm)" <khalidm@...co.com>,
	Johannes Weiner <hannes@...xchg.org>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Michal Hocko <mhocko@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"xe-kernel@...ernal.cisco.com" <xe-kernel@...ernal.cisco.com>
Subject: Re: computing drop-able caches

On Wed, 2016-02-10 at 11:11 -0800, Daniel Walker wrote:
> On 02/10/2016 10:13 AM, Dave Hansen wrote:
> > On 02/10/2016 10:04 AM, Daniel Walker wrote:
> > > > [Linux_0:/]$ echo 3 > /proc/sys/vm/drop_caches
> > > > [Linux_0:/]$ cat /proc/meminfo
> > > > MemTotal:        3977836 kB
> > > > MemFree:         1095012 kB
> > > > MemAvailable:    1434148 kB
> > > I suspect MemAvailable takes into account more than just the
> > > droppable
> > > caches. For instance, reclaimable slab is included, but I don't
> > > think
> > > drop_caches drops that part.
> > There's a bit for page cache and a bit for slab, see:
> > 
> > 	https://kernel.org/doc/Documentation/sysctl/vm.txt
> > 
> > 
> 
> Ok, then this looks like a defect then. I would think MemAvailable
> would 
> always be smaller then MemFree (after echo 3 > 
> /proc/sys/vm/drop_caches).. Unless there is something else be
> accounted 
> for that we aren't aware of.

echo 3 > /proc/sys/vm/drop_caches will only
drop unmapped page cache, IIRC

The system may still have a number of page
cache pages left that are mapped in processes,
but will be reclaimable if the VM needs the
memory for something else.

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