linux-kernel - Re: computing drop-able caches

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Message-ID: <20160129015534.GA6401@cmpxchg.org>
Date:	Thu, 28 Jan 2016 20:55:34 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Daniel Walker <danielwa@...co.com>
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 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.