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Re: [Xen-devel] what happens when a PoD page is touched?

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] what happens when a PoD page is touched?
From: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Date: Mon, 16 May 2011 11:05:20 +0100
Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, Paul Durrant <Paul.Durrant@xxxxxxxxxx>, xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On Mon, May 16, 2011 at 10:22 AM, James Harper
<james.harper@xxxxxxxxxxxxxxxx> wrote:
> I've not seen any slowdown on boot with any version of Windows until I
> go to actually do the balloon down...

You mean, you don't see slowness on boot unless you boot with maxmem
!= memory (i.e,. in PoD mode)?

For one thing, after the balloon driver is finished with its initial
allocation, there shouldn't be any sweeping; if there is, then it may
be that you're not actually inflating the balloon as much as Xen
thinks you should.

Before the balloon driver is done, the emergency sweep could be an
issue.  We had a bunch of stuff in XenServer in our most recent
release to try to mitigate this, but it was all pretty hacky and
unsuitable for upstreaming.  I determined that it just needs to be
re-written in a way that actually made things Better; I haven't gotten
a chance to do that yet.

It looks like ATM the sweep is limited to doing 2MiB at a time; so if
you have a reasonably sized guest, every other superpage that gets hit
will cause another sweep -- potentially a big problem.

I have a patch that attempts to greedily re-grab pages after a sweep;
I'll check to see if it applies to tip, and if so, I'll send it to you
to see if that helps any.


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