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

  • To: xen-users <xen-users@xxxxxxxxxxxxx>
  • From: Simon Hobson <linux@xxxxxxxxxxxxxxxx>
  • Date: Sun, 15 Apr 2012 22:56:19 +0100
  • Delivery-date: Sun, 15 Apr 2012 21:57:42 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

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

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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