RE: [Xen-users] Memory allocation

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Badyk, Benjamin
> Sent: 02 October 2006 16:56
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] Memory allocation
> 
> I¹m sure this has been asked a bazillion times on this list, 
> but I couldn¹t
> find a definitive answer:
> 
> 4GB ram,  3 domU¹s
> 
> I boot each domU up with 1GB ram, then mem-max and mem-set them to 2GB
> (which obviously exceeds physical memory limit). They all 
> grow to 2048. It
> seems that you won¹t see the memory hit 2gb on a domU until 
> it has eaten up
> more than what it initially booted with, which is fine. The 
> question I have
> is what would happen if all three domU¹s tried using 2GB?

If this actually works in the latest (unstable) release of Xen, I suggest you 
report a bug. 

Something will go horribly wrong if you have 3 domU's using 6GB of RAM on a 
machine with 4GB or actual memory - it should not be possible to set it like 
that. 

Xen doesn't support "swapping" or in other ways overcommitting memory within 
the domain memory allocation, so somewhere, you'll have allocated the same 
memory to more than one domain [assuming "xm list" shows all three domains to 
have 2GB of RAM each] - which would obviously not work right... But I seem to 
remember someone sending patches for "sanity checking mem-set", so it may be 
that it's fixed in the latest version. 

I can certainly guarantee that the one or more domains will crash and/or 
corrupt themselves if you succeed in setting 2GB each for 3 domU's on a 4GB 
machine... [Not to mention that dom0 needs some memory, as does Xen itself - 
even if it's only some small measures of memory...]

--
Mats
> 
> Thanks,
> 
> BJ Badyk
> 
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