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

To: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] [PATCH] PoD: Handle operations properly when domain is dying
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 11 Nov 2009 07:26:06 -0800 (PST)
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Hi George --

A possibly related issue came up in a team discussion
yesterday.  It is not uncommon for management tools to
check available memory to decide if there is sufficient
space to provision a new domain, for example when choosing
a machine on which to start a new domain.  But there is
no "lock" so if the answer is yes and the machine
is chosen, but moments later some memory becomes used
(by PoD or ballooning or ???), domain creation might
fail due to insufficient memory.

Tmem has a "freeze" feature to avoid this problem, but
tmem has the advantage that all domains will continue
to function even if tmem is frozen... I'm not sure
that's true for PoD.

Clearly management tools can be very conservative
and assume every domain on a machine is using its
maxmem, but that defeats the purpose of much of the
overcommitment work we are doing.  I know Citrix
is working on hypervisor swap, but that has a pretty
horrible performance penalty, as well as some
interesting functionality limitations/challenges.

Any clever ideas on how to deal with this problem
in the future?  For example, assume someone wants
to launch a sequence of PoD domains that will be
idle when launched but eventually will utilize all
(or even most) of maxmem.


> -----Original Message-----
> From: George Dunlap [mailto:George.Dunlap@xxxxxxxxxxxxx]
> Sent: Wednesday, November 11, 2009 8:08 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] [PATCH] PoD: Handle operations properly 
> when domain
> is dying
> Handle PoD operations properly when a domain is dying.
> No populate-on-demand activities should happen when a domain is dying.
>  Especially, it is a bug for memory to be added to the PoD cache when
> d->is_dying is non-zero, since if this happens after the cache has
> been emptied, these pages will never be freed. This may cause "zombie
> domains" to linger.
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

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