Re: [Xen-users] Dom0 Memory?

[Flavio <fbcyborg@xxxxxxxxx>]

On 10 May 2012 12:38, Dario Faggioli <raistlin@xxxxxxxx> wrote:
> On Wed, 2012-05-09 at 17:52 +0200, Flavio wrote:
>> > What does 'xm info' say?
>> # xl info
>> total_memory      : 8107
>> free_memory      Â: 6968
>>
> So, 8GB of RAM is what Xen knows, ~7 of them being free... What's the
> problem?
Yes, but as far as I can see, even if xl info gives me such values,
the real amount
of free RAM is very less than it declares. I've reported also the free and the
cat /proc/meminfo output on the purpose and what I can see, practically, is that
I cannot run Chromium, or other "big" applications because of this lack of free
RAM. This is an issue that has been confirmed many months ago.

>
>> As you can see, my 8GB of RAM have disappeared.
>> What can you say about that?
>>
> Uh? :-O
If you look at xl info, you are right when you do this face, but
the practical issues that I am facing with, confirm that there is
something not working properly, when the hypervisor is running.

>
> Ok, this might be a bit too technical, but perhaps still of help:
> http://blog.xen.org/index.php/2012/04/30/do%EF%BB%BFm0-memory-where-it-has-not-gone/
Thanks, but this didn't help me to fix the problem.

>
> I'm also sure this has been mentioned in this or another thread related
> to this "issue":
> http://wiki.xen.org/wiki/Xen_Best_Practices
> http://wiki.xen.org/wiki/Xen_Best_Practices#Why_should_I_dedicate_fixed_amount_of_memory_for_Xen_dom0.3F
Already seen that a lot of times.

>
> Also trying to quickly summarize what's happening, no, your memory
> _has_not_ disappeared. Actually, it is right there for your VMs to use
> it (and xl invocation above shows right that).
OK, I was just only wandering why I was experiencing system slow downs, as
reported above. I don't know what would happen if several VMs would be running
at the same time.

>
> With dom0_mem=1G you told Xen that one of your VM (the one that it will
> automatically call 'Domain0' and that it will automatically create on
> boot) will use 1G of your 8G of RAM. If you now try to create another VM
> (say 'VM_1') with 2G of RAM, it will just do that, and you'll have 1G
> +2G=3G out of 8G of your RAM dedicated to these two VMs, while the other
> 5G are still free.
Great. This is perfectly clear now. But you would certainly understand my
perplexity when I see only 662416 kB of free memory in cat /proc/meminfo.
That is smaller than 5GB you mentioned above.

>
> Have you tried that? If you do, try running `xl info' on Dom0 again
> after that. Try also SSH-ing in 'VM_1' and asking (e.g., via `free -m')
> how many total and free memory it has. As one should expect, it will say
> it has 2GB of RAM in total... Does this make sense?
Sure it has. I don't have anything to say about that actually. If my domUs
have the right amount of RAM I decided to assign to each one, this is
certainly perfect, but I was worried about what is happening on the dom0, not
allowing me to see all the free RAM I should see.

>
> So, again, the reason why you're not seeing 7GB of RAM in Dom0 is
> because they're not there! Dom0 is a virtual machine to which you've
> given 1GB of RAM via the cmdline... Having only 1GB of RAM is just doing
> right what you asked for!
Uhm, OK. So all my doubts come from the fact that, when I was using
xen-sources (and not the vanilla (gentoo) sources), I've always seen all the
free RAM correctly. But if you say this, I can be more pacific. ;-)

>
> I can understand that it might look a bit tricky, but what you seem to
> be missing here is you should consider Dom0 pretty much like any other
> VM that Xen is managing as the hypervisor.
Yes, this is clear to me. Thank you for remarking this point anyway.

>
> Hope of being of some help.
For sure yes! I appreciate your interest.

Thank you so much.

Best Regards,

Flavio

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