linux-kernel - Re: [RFC][PATCH 2/7] RSS controller core

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Message-Id: <20070312220439.677b4787.akpm@linux-foundation.org>
Date:	Mon, 12 Mar 2007 22:04:39 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Herbert Poetzl <herbert@...hfloor.at>
Cc:	hansendc@...ibm.com, containers@...ts.osdl.org,
	linux-kernel@...r.kernel.org, menage@...gle.com, xemul@...ru
Subject: Re: [RFC][PATCH 2/7] RSS controller core

> On Mon, 12 Mar 2007 23:41:29 +0100 Herbert Poetzl <herbert@...hfloor.at> wrote:
> On Mon, Mar 12, 2007 at 11:42:59AM -0700, Dave Hansen wrote:
> > How about we drill down on these a bit more.
> > 
> > On Mon, 2007-03-12 at 02:00 +0100, Herbert Poetzl wrote:
> > >  - shared mappings of 'shared' files (binaries 
> > >    and libraries) to allow for reduced memory
> > >    footprint when N identical guests are running
> > 
> > So, it sounds like this can be phrased as a requirement like:
> > 
> > 	"Guests must be able to share pages."
> > 
> > Can you give us an idea why this is so? 
> 
> sure, one reason for this is that guests tend to
> be similar (or almost identical) which results
> in quite a lot of 'shared' libraries and executables
> which would otherwise get cached for each guest and
> would also be mapped for each guest separately

nooooooo.  What you're saying there amounts to text replication.  There is
no proposal here to create duplicated copies of pagecache pages: the VM
just doesn't support that (Nick has soe protopatches which do this as a
possible NUMA optimisation).

So these mmapped pages will contiue to be shared across all guests.  The
problem boils down to "which guest(s) get charged for each shared page".

A simple and obvious and easy-to-implement answer is "the guest which paged
it in".  I think we should firstly explain why that is insufficient.

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