Re: [Xen-users] Increase Domain 0 memory

[Peter Milesson <miles@xxxxxxxx>]

Hi Mark,

For example see this:

http://fclose.com/b/linux/2258/managing-xen-dom0s-cpu-and-memory/

There's lots of other information recommending to avoid ballooning of 
dom0 memory. That it is possible, doesn't necessary imply that it's a 
good thing.
I notice performance improvements and "smoothness" in the Xen system, 
giving dom0 a fixed amount of memory and pinning a couple of cores for 
exclusive use by dom0.
Peter

On 14.2.2012 11:34, Mark van Dijk wrote:
> Hi,
>
> > The dom0_mem parameter locks the dom0 memory to the given amount
> > (which is good for performance). You cannot balloon it at runtime.
> > According to the documentation (and experiments) you should also pin
> > dom0 to use specific cpus and keep domUs off those reserved cpus.
> Yes, one can balloon dom0, see below. What made you think otherwise? Are
> you using a Xen version that has been built before 2006?
>
> There are enough misunderstandings already wrt Xen. So please just
> don't answer unless you're certain or else say "I THINK that ..."
>
> Also, you say locking dom0 memory is good for performance? Why? Do give
> us some reasoning to support this apparent "fact". Does it apply under
> all circumstances?
>
> Then dom0 CPU pinning. dom0_max_vcpus=1 is enough to have dom0 use
> one CPU. You don't *have* to pin the CPU. Maybe you want to share one
> of your Quadcore CPU cores sometimes but not all the time.
>
> False info is so much more annoying than no info at all.
>
> Anyway.
>
> KC, the syntax for dom0_mem can be specified as follows:
>
> dom0_mem=[min:<min_amt>,][max:<max_amt>,][<amt>]
>
> example:
> dom0_mem=min:1G,max:4G,1G
> dom0_mem=min:1G,max:-1G,1G
>
> The second example explained: if your system has 16G RAM then you allow
> dom0 to use 15G at most.
>
> <min_amt>: The minimum amount of memory which should be allocated for dom0.
> <max_amt>: The maximum amount of memory which should be allocated for dom0.
> <amt>:     The precise amount of memory to allocate for dom0.
>
> Notes:
>   1.<amt>  is clamped from below by<min_amt>  and from above by available
>      memory and<max_amt>
>   2.<min_amt>  is clamped from above by available memory and<max_amt>
>   3.<min_amt>  is ignored if it is greater than<max_amt>
>   4. If<amt>  is not specified, it is calculated as follows:
>      "All of memory is allocated to domain 0, minus 1/16th which is reserved
>       for uses such as DMA buffers (the reservation is clamped to 128MB)."
>
> Each value can be specified as positive or negative:
>   If +ve: The specified amount is an absolute value.
>   If -ve: The specified amount is subtracted from total available memory.
>
> This has been commited in 2006, see xen-unstable commit
> 2191:444496ecb14e.
>
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