Re: [Xen-devel] memory hotplug for domUs

[Jim Fehlig <jfehlig@xxxxxxxx>]

On 01/20/2017 09:06 AM, Juergen Gross wrote:
> On 20/01/17 16:54, Ian Jackson wrote:
> > Juergen Gross writes ("memory hotplug for domUs"):
> > > We first thought to enhance "xl mem-set", but after some more thinking
> > > about it I'd rather add a new xl command, e.g. "mem-add" (we could later
> > > even add "mem-remove" to support memory unplug).
> > Why ?  Why would xl mem-set not automatically do the right thing ?
> How would you specify the numa node to add the memory to?
And the host numa node providing the memory?

> Especially for removal of memory ballooning and hotplug are _very_
> different.
I agree. libvirt already models ballooning and hot(un)plug differently.

For ballooning, typical VM configuration is

  <memory unit='KiB'>16777216</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <memballoon model='virtio|xen'>

'virsh setmaxmem' and 'virsh setmem' allow adjusting runtime and/or persistent 
configuration.
For hot(un)plug (currently only supported by the qemu driver) typical 
configuration is
  <maxMemory slots='16' unit='KiB'>16777216</maxMemory>
  <devices>
    <memory model='dimm'>
      <source>
        <nodemask>0-1</nodemask>
      </source>
      <target>
        <size unit='KiB'>1048576</size>
        <node>0</node>
      </target>
    </memory>
    ...
</devices>

which describes a VM with 16 dimm slots with total capacity of 16G. One slot is 
filled with a 1G memory module whose memory must come from host numa nodes 0 or 
1, and is provided to numa node 0 in the VM.
'virsh attach-device' and virsh detach-device' allow adding or removing memory 
modules from the dimms. AIUI, under the covers qemu uses ACPI memory hotplug.
> > This problem with lack of support (and indeed requirementse to do
> > hotplug things) is a detail that in the long term should not be a
> > problem, and in the meantime can be dealt with by a force option.
> I think it would be clearer with new commands for hotplugging. I
> don't feel very strong about this, so in case everyone else is fine
> with handling everything via mem-set I won't object.
If ACPI memory hotplug is indeed the goal, then I agree new commands should be 
used since it is quite different than ballooning.
Regards,
Jim


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel