linux-kernel - Re: [PATCH 0/3] Unmapped page cache control (v5)

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Date:	Fri,  1 Apr 2011 12:18:38 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Balbir Singh <balbir@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	akpm@...ux-foundation.org, npiggin@...nel.dk, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, cl@...ux.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 0/3] Unmapped page cache control (v5)

> On Wed, Mar 30, 2011 at 11:00:26AM +0530, Balbir Singh wrote:
> > 
> > The following series implements page cache control,
> > this is a split out version of patch 1 of version 3 of the
> > page cache optimization patches posted earlier at
> > Previous posting http://lwn.net/Articles/425851/ and analysis
> > at http://lwn.net/Articles/419713/
> > 
> > Detailed Description
> > ====================
> > This patch implements unmapped page cache control via preferred
> > page cache reclaim. The current patch hooks into kswapd and reclaims
> > page cache if the user has requested for unmapped page control.
> > This is useful in the following scenario
> > - In a virtualized environment with cache=writethrough, we see
> >   double caching - (one in the host and one in the guest). As
> >   we try to scale guests, cache usage across the system grows.
> >   The goal of this patch is to reclaim page cache when Linux is running
> >   as a guest and get the host to hold the page cache and manage it.
> >   There might be temporary duplication, but in the long run, memory
> >   in the guests would be used for mapped pages.
> 
> What does this do that "cache=none" for the VMs and using the page
> cache inside the guest doesn't acheive? That avoids double caching
> and doesn't require any new complexity inside the host OS to
> acheive...

Right. 

"cache=none" has no double caching issue and KSM already solved
cross gues cache sharing. So, I _guess_ this is not a core motivation 
of his patch. But I'm not him. I'm not sure.


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