Return-Path: <owner-linux-mm@kvack.org>
Subject: Re: computing drop-able caches
To: "Khalid Mughal (khalidm)" <khalidm@cisco.com>,
 Rik van Riel <riel@redhat.com>, "Daniel Walker (danielwa)"
 <danielwa@cisco.com>, Johannes Weiner <hannes@cmpxchg.org>
References: <56AAA77D.7090000@cisco.com> <20160128235815.GA5953@cmpxchg.org>
 <56AABA79.3030103@cisco.com> <56AAC085.9060509@cisco.com>
 <20160129015534.GA6401@cmpxchg.org> <56ABEAA7.1020706@redhat.com>
 <D2DE3289.2B1F3%khalidm@cisco.com> <56BB7BC7.4040403@cisco.com>
 <56BB7DDE.8080206@intel.com> <56BB8B5E.0@cisco.com>
 <1455228719.15821.18.camel@redhat.com> <D2E35753.2BB9D%khalidm@cisco.com>
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-fsdevel@vger.kernel.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "linux-mm@kvack.org" <linux-mm@kvack.org>,
 "xe-kernel@external.cisco.com" <xe-kernel@external.cisco.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <56BE52CE.5070403@intel.com>
Date: Fri, 12 Feb 2016 13:46:54 -0800
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On 02/12/2016 10:01 AM, Khalid Mughal (khalidm) wrote:
> If you look at the attached pdf, you will notice that OOM messages start
> to appear when memAvailable is showing 253MB (259228 KB) Free, memFree is
> 13.5MB (14008 KB) Free, and dropcache based calculation �Available memory�
> is showing 21MB (21720 KB) Free.
> 
> So, it appears that memAvailable is not as accurate, especially if data is
> used to warn user about system running low on memory.

Yep, that's true.

But, MemAvailable is calculated from some very cheap counters.  The
"dropcache-based-calculation" requires iterating over every 4k page
cache page in the system.

Do you have some ideas for doing cheap(er) MemAvailable calculations?

We track dirty and writebackw with counters, so we should theoretically
be able to pull those out of MemAvailable fairly cheaply.

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