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Subject: Re: computing drop-able caches
To: Johannes Weiner <hannes@cmpxchg.org>, Daniel Walker <danielwa@cisco.com>
References: <56AAA77D.7090000@cisco.com> <20160128235815.GA5953@cmpxchg.org>
 <56AABA79.3030103@cisco.com> <56AAC085.9060509@cisco.com>
 <20160129015534.GA6401@cmpxchg.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>, Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, "Khalid Mughal (khalidm)" <khalidm@cisco.com>,
        "xe-kernel@external.cisco.com" <xe-kernel@external.cisco.com>
From: Rik van Riel <riel@redhat.com>
Message-ID: <56ABEAA7.1020706@redhat.com>
Date: Fri, 29 Jan 2016 17:41:43 -0500
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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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