linux-kernel - Re: [PATCH 06/10] mm: remove unnecessary use-once cache bias from LRU balancing

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Message-ID: <20160607141124.GC9978@cmpxchg.org>
Date:	Tue, 7 Jun 2016 10:11:24 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michal Hocko <mhocko@...e.cz>,
	Tim Chen <tim.c.chen@...ux.intel.com>, kernel-team@...com
Subject: Re: [PATCH 06/10] mm: remove unnecessary use-once cache bias from
 LRU balancing

On Mon, Jun 06, 2016 at 10:20:31PM -0400, Rik van Riel wrote:
> On Mon, 2016-06-06 at 15:48 -0400, Johannes Weiner wrote:
> > When the splitlru patches divided page cache and swap-backed pages
> > into separate LRU lists, the pressure balance between the lists was
> > biased to account for the fact that streaming IO can cause memory
> > pressure with a flood of pages that are used only once. New page
> > cache
> > additions would tip the balance toward the file LRU, and repeat
> > access
> > would neutralize that bias again. This ensured that page reclaim
> > would
> > always go for used-once cache first.
> > 
> > Since e9868505987a ("mm,vmscan: only evict file pages when we have
> > plenty"), page reclaim generally skips over swap-backed memory
> > entirely as long as there is used-once cache present, and will apply
> > the LRU balancing when only repeatedly accessed cache pages are left
> > -
> > at which point the previous use-once bias will have been neutralized.
> > 
> > This makes the use-once cache balancing bias unnecessary. Remove it.
> > 
> 
> The code in get_scan_count() still seems to use the statistics
> of which you just removed the updating.
> 
> What am I overlooking?

As I mentioned in 5/10, page reclaim still does updates for each
scanned page and rotated page at this point in the series.

This merely removes the pre-reclaim bias for cache.