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

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Flavio wrote:

>  > Now, for a couple of reasons this isn't ideal. The main reason is that
> >  certain buffers and structures used to keep track of "stuff" are sized at
> >  boot time based on the amount of memory available. So in your case, you
> >  start with 8G of RAM but potentially balloon down to as little as 1/4G
> >  (256M) - just 1/32 of the size. So you now have a system running in a small
> >  amount of RAM but with buffers and structures sized for a lot of RAM (about
> >  32 times what it actually has available in this case).
> This still confuses me a little bit. I don't know if the memory issue
> I've reported
> in my previous message makes things hard to be understood by me, but what
> I can see when I use dom0_mem=1024M, is that the dom0 has 1024MB of RAM
> for itself and the 'free' command shows that I only have 600MB of total RAM.
> If I'm not wrong, I should see about 7GB of free RAM, since 1GB is assigned to
> dom0.
That sounds about right. Dom0 will only "see" the RAM that's assigned 
to it, you need to use the Xen tools (eg xentop) to see how much 
memory is assigned to each domain and how much is free.
What you are probably missing is that Dom0 is itself a guest virtual 
machine running under the Xen hypervisor - it differs from a DomU PV 
guest mainly in having privileged  access to communicate with the 
hypervisor and hardware. But since it is itself a guest, it only has 
access to resources (in this discussion, RAM) given to it by the 
hypervisor. The dom0 memory allocation statements are instructions to 
the hypervisor, not Dom0.
> I would ask you if you know some book where all this memory issue is 
> explained.
Sorry, like a lot of FOSS, documentation tends to lag development - 
writing documentation (or at least documentation that's any good) is 
a) really hard, and b) a different skill to writing code. There tend 
to be less volunteers for this task.
I don't personally know of any good books - but then books are a 
tricky thing in this sort of area as they have a limited shelf life 
before the software moves on.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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