Re: 5x dom0 memory increase from Xen/Linux 3.4/2.6.18 to 4.1/3.0.0

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

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.
> 
> 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.
> 
> 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.
> 
> I'm at 4x memory usage, which is better, but I'd really like to understand where all the memory's going.

Me too.
> 
> thanks,
> 
> Anthony.
> 
> 
> ----- Original Message -----
> From: "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>
> To: "Anthony Wright" <anthony@overnetdata.com>
> Cc: xen-devel@lists.xensource.com
> Sent: Thursday, 16 June, 2011 3:56:11 PM
> Subject: Re: [Xen-devel] 5x dom0 memory increase from Xen/Linux 3.4/2.6.18 to	4.1/3.0.0
> 
> On Thu, Jun 16, 2011 at 01:58:25PM +0100, Anthony Wright wrote:
> > I have a fairly old machine with 512M of RAM that I use for testing. Under Xen 3.4 with the 2.6.18 linux kernel it would happily work with dom0_mem=50M, leaving 450M for VMs. I am in the process of upgrading to Xen 4.1 with a stock 3.0.0-rc3 linux kernel (with konrad's vga-support patch). This combination however requires dom0_mem=250M to work leaving only 250M for the VMs (if I try to set dom0_mem any lower I get a kernel panic early on in the kernel boot phase).
> 
> > 
> > This is an increase of 5x between the two versions, and I wondered if there's an explanation why. Is it possible to reduce the memory requirements, or do I have to learn to live with it.
> 
> The SWIOTLB is taking a 64MB chunk out of it. You could lower that by 'swiotlb=2048' or so.
> Also make sure to compile the CONFIG_XEN_MAX_DOMAIN_MEMORY from 128GB down to say 1.
> 
> Besides that .. run with 'memblock=debug loglevel=10 initcall_debug debug' and see what other
> regions memblock reserves that could be shrunk.
> > 
> > thanks,
> > 
> > Anthony Wright
> > 
> > 
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