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] 5x dom0 memory increase from Xen/Linux 3.4/2.6.18 to 4.1

To: Anthony Wright <anthony@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] 5x dom0 memory increase from Xen/Linux 3.4/2.6.18 to 4.1/3.0.0
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Wed, 22 Jun 2011 09:32:43 -0400
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Delivery-date: Wed, 22 Jun 2011 06:35:44 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4E01EAF7.9030900@xxxxxxxxxxxxxxx>
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: <20110616145611.GC6108@xxxxxxxxxxxx> <25543076.8.1308324671857.JavaMail.root@xxxxxxxxxxxxxxxxxxxxxx> <20110620124521.GC2973@xxxxxxxxxxxx> <4E01EAF7.9030900@xxxxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Jun 22, 2011 at 02:15:35PM +0100, Anthony Wright wrote:
> On 20/06/2011 13:45, Konrad Rzeszutek Wilk wrote:
> > On Fri, Jun 17, 2011 at 04:31:11PM +0100, Anthony Wright wrote:
> >> Lowering swiotlb helped, and got me down to 200M for dom0. What is the 
> >> effect of reducing this value?
> > Less amount of bounce buffer. But you don't need the bounce buffer for PCI 
> > devices b/c you don't
> > have more than 4GB of physical memory in the machine.
> Do I only need bounce buffers if I have > 4GB of physical memory? In

> this case should I allocate the 64M, or is it a sliding memory requirement?

It really is unknown. At some point the maintainer was thinking of adding
dynamic code so that it would increase as neccessary - but it never got
further than: "this would be neat". I would say try some small numbers
and when the machine is on full load it panics (or gives you a nasty
kernel message).

> >> I set CONFIG_XEN_MAX_DOMAIN_MEMORY down to 8, but that didn't seem to have 
> >> any effect on dom0's memory requirement. What is this value? Does it only 
> >> apply to a domU's memory usage?
> > It makes some internal datastructures (P2M) smaller. They are set up for 
> > 128GB or so machines initially.
> It sounds like this value applies to DomUs, does this config variables
> set the maximum amount of memory 128GB per DomU or across all DomUs,
> i.e. if I have 16 DomUs and a CONFIG_XEN_MAX_DOMAIN_MEMORY of 16, do
> they each have a maximum of 16GB, or do the get 1GB each?

I mistyped this. The internal datastructures just set the limits to what
they can go to. But they don't consume any real RAM - just virtual addresses
and during boot time that gets resolved (shrunk appropiately). So don't worry 
about it.

> >> I tried the memblock=debug options, and while I got lots of output, I 
> >> could see very little on the subject of memory usage.
> > The numbers are what amount of memory is reserved. You can find out which 
> > are are is eating the most
> > by computing the difference.

> Maybe I'm misreading the output, but I couldn't see any numbers that
> look like memory being assigned. I've attached the dmesg output. Do I
> need to enable a CONFIG variable to get the output I need or am I
> missing something.

The memblock=debug should give you some idea of what is Reserved. The
Reserved includes memory that is allocated by boot-time services (P2M,
pagetables, NUMA) and by real reservations (for example ACPI space).
Using the 'memblock=debug' can give you an idea of what services are
reserving the most. Then we can narrow down who or what is eating the gobs
of memory.

see the 'Memory: ".. numbers. Also you might want to eliminate
the balloon usage space algother by doing two things:

Xen command line: dom0_mem=max:512MB

Linux command line: mem=512MB

That will effectivly remove any balloon space (so your Dom0 will _never_
grow up).

Xen-devel mailing list