linux-kernel - Re: computing drop-able caches

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Date:	Fri, 29 Jan 2016 13:21:47 -0800
From:	Daniel Walker <danielwa@...co.com>
To:	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-kernel@...r.kernel.org,
	linux-mm@...ck.org, "Khalid Mughal (khalidm)" <khalidm@...co.com>,
	"xe-kernel@...ernal.cisco.com" <xe-kernel@...ernal.cisco.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: computing drop-able caches

On 01/28/2016 05:55 PM, Johannes Weiner wrote:
> On Thu, Jan 28, 2016 at 05:29:41PM -0800, Daniel Walker wrote:
>> On 01/28/2016 05:03 PM, Daniel Walker wrote:
>> [regarding MemAvaiable]
>>
>> This new metric purportedly helps usrespace assess available memory. But,
>> its again based on heuristic, it takes 1/2 of page cache as reclaimable..
> No, it takes the smaller value of cache/2 and the low watermark, which
> is a fraction of memory. Actually, that does look a little weird. Rik?
>
> We don't age cache without memory pressure, you don't know how much is
> used until you start taking some away. Heuristics is all we can offer.

With a simple busybox root system I get this,

MemTotal:          16273996 kB
MemFree:          16137920 kB
MemAvailable:   16046132 kB

shouldn't MemAvailable be at least the same as MemFree ? I changed the 
code somewhat so it subtracted the wmark_low only, or the pagecache/2 
only, both are still under MemFree. This system has very little 
drop-able caches.

Daniel