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Re: [Xen-users] Increase Domain 0 memory

  • To: xen-users@xxxxxxxxxxxxxxxxxxx
  • From: Peter Milesson <miles@xxxxxxxx>
  • Date: Tue, 14 Feb 2012 12:31:04 +0100
  • Delivery-date: Tue, 14 Feb 2012 11:31:42 +0000
  • List-id: Xen user discussion <xen-users.lists.xensource.com>

Hi Mark,

For example see this:


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.


On 14.2.2012 11:34, Mark van Dijk wrote:

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.


KC, the syntax for dom0_mem can be specified as follows:



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.

  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

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