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Re: [Xen-users] Xen and distributing memory

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Subject: Re: [Xen-users] Xen and distributing memory
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 27 Feb 2007 02:27:06 +0000
Cc: Marduk <xen@xxxxxxxxxxxxxxxxxxxxxx>
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> I'm trying to understand how Xen's memory balloon driver works.  IIRC
> VMware's is "dynamic" so if my host has, say, 2 GB memory to spare then
> I can still have 3 guests with 1GB of memory assigned each and so long
> as all 3 aren't actually *using* all the memory then VMware will
> dynamically shuffle the hosts memory around between guests (actually I
> think VMware even preserves host swap space for guest RAM so
> theoretically they could all could be using 1GB).


> >From my understanding with Xen, this is also possible for the most part
> but it has to be done manually with 'xm mem-set'.  Is this true?

Yep.  VMware has an additional feature over Xen in that it's able to identify 
common memory contents within and across virtual machines and then share the 
underlying page frames transparently.  This looks the same to the virtual 
machines but frees up memory when multiple processes / operating systems have 
the same data in memory.

> The reason I am asking is because I have a few domUs that pretty much
> run idle except for certain parts of the day.  For example, my backup
> server is on a domU.  When it's idle it's only using about 33MB of it's
> assigned ram, but once a day when backups are run it uses up to 233MB.
> When it's idle I'd like it so the unused RAM is available to other
> guests.  Do I have to manually do a 'xm mem-set' and guesstimate how
> much memory is going to be used for each guest at certain times of the
> day or is there a way that Xen can do this dynamically (a la VMware)?

It can't be done automatically at the moment.  A nice feature would be to 
figure out a way of passing load metrics from the guests to dom0 (or deriving 
them from Xen directly) and then perform the ballooning automatically based on 
these metrics and an administrator-configured policy.  Maybe this will happen 
at some point...


Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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