linux-kernel - Re: computing drop-able caches

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Message-ID: <56ABEAA7.1020706@redhat.com>
Date:	Fri, 29 Jan 2016 17:41:43 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>,
	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>
Subject: Re: computing drop-able caches

On 01/28/2016 08:55 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?

No, not quite.  The page cache calculation spans two lines:

        pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
        pagecache -= min(pagecache / 2, wmark_low);

The assumption is that ALL of active & inactive file LRUs are
freeable, except for the minimum of the low watermark, or
half the page cache.

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