Xen devel - Feb 2013 - Question about PoD and Transcendent Memory for HVM

Hi:
      I am working at the source code of Xen 4.1.3 and have two questions
that confused me.
      I remembered a mail of XCP-mail-list which said when a hvm is
created,xen allocate all memory it requests,which can guarantee the
stability and Qos or something.However,when I come into the part of memory
source code,I tind the existance of PoD,which can adjust memory
dynamically,so I want to make sure whether the mechanism of memory
management between XCP and Xen is differrent,Xen can adjust the memory size
of HVM dynamically instead of a fixed memory size,am I right?
      I know the Transcendent Memory is a better techniche of memory
reuse,is it used in xen for HVM now?
      Thank you for your answering!
Jeap


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At 10:30 +0800 on 21 Feb (1361442605), Kai Luo wrote:
> Hi:
>       I am working at the source code of Xen 4.1.3 and have two questions
> that confused me.
>       I remembered a mail of XCP-mail-list which said when a hvm is
> created,xen allocate all memory it requests,which can guarantee the
> stability and Qos or something.However,when I come into the part of memory
> source code,I tind the existance of PoD,which can adjust memory
> dynamically,so I want to make sure whether the mechanism of memory
> management between XCP and Xen is differrent,Xen can adjust the memory size
> of HVM dynamically instead of a fixed memory size,am I right?
If the OS running in the HVM VM is Xen-aware and has a balloon driver,
then it can adjust its memory size dynamically, yes.

The PoD feature is a stop-gap to allow the OS to boot and start its
balloon driver, if it''s booting with less than its full memory
allocation.  It''s not enough by itself: the guest needs to have a
balloon driver as well.
>       I know the Transcendent Memory is a better techniche of memory
> reuse,is it used in xen for HVM now?
I believe so, though again you need OS support in the VM.

Also you should note that using tmem is not recommended in production
environments, pending a security audit:

http://lists.xen.org/archives/html/xen-announce/2012-09/msg00006.html

Cheers,

Tim.

> From: Kai Luo [mailto:luokain@gmail.com]
> Subject: [Xen-devel] Question about PoD and Transcendent Memory for HVM
> 
> Hi:
>       I am working at the source code of Xen 4.1.3 and have two questions
that confused me.
>       I remembered a mail of XCP-mail-list which said when a hvm is
created,xen allocate all memory it
> requests,which can guarantee the stability and Qos or
something.However,when I come into the part of
> memory source code,I tind the existance of PoD,which can adjust memory
dynamically,so I want to make
> sure whether the mechanism of memory management between XCP and Xen is
differrent,Xen can adjust the
> memory size of HVM dynamically instead of a fixed memory size,am I right?
>       I know the Transcendent Memory is a better techniche of memory
reuse,is it used in xen for HVM
> now?
>       Thank you for your answering!
Transcendent Memory works fine for HVM domains.  It does require
a tmem-capable guest kernel.  This requires a very recent distro
(and means tmem does not help Windows guests... though it does
co-exist with them).

Dan