[Xen-users] Doubt on XEN memory management: please clarify

[Flavio <fbcyborg@xxxxxxxxx>]

Hello,

this is a question I wanted to ask you since a long time because there is
something not too much clear to me, as regard the memory management
in XEN. I don't want to go too much in deep with such argument, but It's just
to understand what is the best setup for my environment.

I know about memory ballooning
(http://wiki.xensource.com/xenwiki/XenBestPractices)
and the dom0_mem parameter, but I would like to know what such parameter
implies, in practise.

Let's go with my XEN setup now.
I have 8GB of RAM and I have seen that if I put dom0_mem=1024M option, the dom0
has 1GB of RAM for itself.
What does this really mean? This is confusing me, because I don't understand how
to decide the amount of RAM to assign to the dom0. For example, what if I assign
256M? What if 8192M?
For sure you can say to try, but it is not so clear to me what I am doing.

As far as I understand, this would be the situation:

dom0_mem=1024M implies that 'xl list' shows that the dom0 has got 1024MB of
RAM reserved for itself. Are the remaining 7GB available for the domUs and the
host operating system?

i.e.:
this could be the case study:
1024M for the dom0
1024M for domU1
2048M for domU2
2048M for domU3

Total: 6144MB "occupied"

So, the 2 remaining GB would be available for the System or other 1/2 domUs
with 1/2GB of RAM each.
Of course memory ballooning helps to not crash the dom0, so the total amount
of assigned memory would hopefully be never at saturation point, but what does
such dom0 parameter imply in practise?

This doubt has arisen to me even because of the memory management issue
reported here: http://www.gossamer-threads.com/lists/xen/users/226845

Regards,

-- 
Flavio

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