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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Re: [Qemu-commits] [COMMIT 3086844] Instead of
	writing a zero page, madvise it away
Date: Mon, 22 Jun 2009 20:38:00 +0100
References: <200906221549.n5MFn3Qd015389@d03av02.boulder.ibm.com>
	<4A3FB390.4060809@redhat.com> <4A3FB95D.3060404@us.ibm.com>
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To: qemu-devel@nongnu.org
Cc: Anthony Liguori <aliguori@us.ibm.com>, Avi Kivity <avi@redhat.com>

> >>> We have the same issue on the migration source node.  I don't see a
> >>> simple way to solve it, though.
> >>
> >> I don't follow.  In this case, the issue is:
> >>
> >> 1) Start a guest with 1024, balloon down to 128MB.  RSS size is now
> >> ~128MB
> >> 2) Live migrate to a different node
> >> 3) RSS on different node jumps to ~1GB
> >
> > 3.5) RSS on source node jumps to ~1GB, since reading the page
> > instantiates the pte
>
> Surely we can do better here...

Huh, I'm surprised we have to. I was expecting linux to populate the region 
with CoW mappings of a single zero page, but apparently not. I guess reading a 
zero page without writing to it is a fairly rare case, and it's usually better 
to instantiate a writable page on the first access.

Paul