linux-kernel - Re: [PATCH v2 5/6] mm: workingset: size shadow nodes lru basing on file cache size

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Message-ID: <20160209231412.GA32427@cmpxchg.org>
Date:	Tue, 9 Feb 2016 18:14:12 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] mm: workingset: size shadow nodes lru basing on
 file cache size

On Tue, Feb 09, 2016 at 04:55:53PM +0300, Vladimir Davydov wrote:
> A page is activated on refault if the refault distance stored in the
> corresponding shadow entry is less than the number of active file pages.
> Since active file pages can't occupy more than half memory, we assume
> that the maximal effective refault distance can't be greater than half
> the number of present pages and size the shadow nodes lru list
> appropriately. Generally speaking, this assumption is correct, but it
> can result in wasting a considerable chunk of memory on stale shadow
> nodes in case the portion of file pages is small, e.g. if a workload
> mostly uses anonymous memory.
> 
> To sort this out, we need to compute the size of shadow nodes lru basing
> not on the maximal possible, but the current size of file cache. We
> could take the size of active file lru for the maximal refault distance,
> but active lru is pretty unstable - it can shrink dramatically at
> runtime possibly disrupting workingset detection logic.
> 
> Instead we assume that the maximal refault distance equals half the
> total number of file cache pages. This will protect us against active
> file lru size fluctuations while still being correct, because size of
> active lru is normally maintained lower than size of inactive lru.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>

Acked-by: Johannes Weiner <hannes@...xchg.org>

Begrudgingly, because I don't think it matters that much and I like
the dumber version. But it's a reasonable change nonetheless.