This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: [Xen-devel] Re: GSoC 2010 - Migration from memory ballooning to memo

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] Re: GSoC 2010 - Migration from memory ballooning to memory hotplug in Xen
From: Andi Kleen <andi@xxxxxxxxxxxxxx>
Date: Fri, 9 Jul 2010 02:34:09 +0200
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Daniel Kiper <dkiper@xxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx
Delivery-date: Thu, 08 Jul 2010 17:35:02 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4C36648F.6020006@xxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <20100708194553.GA30124@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> <871vbdr4ey.fsf@xxxxxxxxxxxxxxxxx> <4C36648F.6020006@xxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.20 (2009-06-14)
> Yes.  Another approach would be to fiddle with the E820 maps early at
> boot to add more RAM, but then early_reserve it and hand it over to the
> control of the balloon driver.  But it does mean you need to statically
> come up with the max ever at boot time.

You need to do that too for memory hotadd -- you need predeclared
hotadd regions. Linux mainly needs it to know in which node
to put the memory. Other OS use it for other things too.

> > The only advantage of using memory hotadd is that the mem_map doesn't
> > need to be pre-allocated, but that's only a few percent of the memory.
> >
> > So it would only help if you want to add gigantic amounts of memory
> > to a VM (like >20-30x of what it already has).
> >   
> That's not wildly unreasonable on the face of it; consider a domain
> which starts at 1GB but could go up to 32GB as demand requires.  But

The programs which need 32GB will probably not even start in 1GB :)

> > One trap is also that memory hotadd is a frequent source of regressions,
> > so you'll likely run into existing bugs.
> That could be painful, but I expect the main reason for regressions is
> that the code is fairly underused.  Adding new users should help.

Yes, and we fixed a lot of the bugs, but still a lot of them 
were tricky and frankly new ones might be too difficult for a SoC.


ak@xxxxxxxxxxxxxxx -- Speaking for myself only.

Xen-devel mailing list