From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave McCracken <dcm@mccr.org>
Subject: Re: [PATCH] Add hypercall to mark superpages to improve
	performance
Date: Fri, 30 Apr 2010 16:43:01 -0500
Message-ID: <201004301643.01580.dcm@mccr.org>
References: <C8009B11.11991%keir.fraser@eu.citrix.com>
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To: Keir Fraser <keir.fraser@eu.citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Xen Developers List <xen-devel@lists.xensource.com>
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On Friday 30 April 2010, Keir Fraser wrote:
> One option would be an array of reference counts indexed by superpage
>  number (i.e, mfn>>9). So kind of a separate array to page_info, and a
>  non-zero superpage refcount would arrange to hold a reference on every
>  relevant page in page_info.
> 
> That could be implemented with no extra hypercalls, and I reckon it's
> probably easier to make this race-free too. Obviously it does have extra
> code complexity to construct this array (which I suppose needs to be
>  sparse, just like page_info array, in the face of very sparse memory
>  maps). The space overhead (about 8 bytes per 2MB, or 0.0004% of total
>  system memory) would be trivial. Compared with an extra reference count in
>  every page_info, which would have a much higher 0.2% overhead.

I like this idea.  I'll look into it.

Dave