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Re: [Xen-devel] [PATCH] PoD: Handle operations properly when domain is

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] PoD: Handle operations properly when domain is dying
From: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Date: Wed, 11 Nov 2009 22:49:06 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
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On Wed, Nov 11, 2009 at 5:58 PM, Dan Magenheimer
<dan.magenheimer@xxxxxxxxxx> wrote:
> Thanks for the detailed explanation.  I think
> "Populate on Demand" connotes too much like
> "Copy on Write" and doesn't really describe
> the essence of what is going on: Booting pre-ballooned.
> Thus my confusion.

Well, "populate on demand" describes exactly what's actually
happening: the p2m table is being populated as it's used, as opposed
to being populated all at once at the beginning.  If it was allocating
memory on demand it would have been called "allocate on demand". :-)

> This is of course less than ideal as it ensures that
> administrators will choose some safe (probably fairly large)
> amount of memory, a significant portion of which will often
> be wasted.  But I suppose its better than the (much larger)
> alternative.

PoD is only meant to keep the domain going until the balloon driver
loads, which should be fairly early in boot.  How much is required
changes depending on the OS and the maximum amount of memory;  For
Windows XP, 32-bit, with 4GiB maximum (i.e., reported in the e820
map), 256 KiB is sufficient to start with (if I recall correctly).
For Windows 7, that's a lot higher.  I forget the exact number, but
it's 1-2GiB to boot with a 4GiB maximum.  Citrix is doing extensive
testing to find out minimums for a large number of configurations, so
that administrators using XenServer can confidently set the value to a
minimum knowing that they're neither wasting memory nor risking a
crash. :-)


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