Re: [Xen-devel] initial ballooning amount on HVM+PoD

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.01.14 at 17:54, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Fri, 2014-01-17 at 16:26 +0000, Jan Beulich wrote:
>> >>> On 17.01.14 at 17:08, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>> > On Fri, 2014-01-17 at 16:03 +0000, Jan Beulich wrote:
>> >> max_pfn/num_physpages isn't that far off for guest with less than
>> >> 4Gb, the number calculated from the PoD data is a little worse.
>> > 
>> > On ARM RAM may not start at 0 and so using max_pfn can be very
>> > misleading and in practice causes arm to balloon down to 0 as fast as it
>> > can.
>> 
>> Ugly. Is that only due to the temporary workaround for there not
>> being an IOMMU?
> 
> It's not to do with IOMMUs, no, and it isn't temporary.
> 
> Architecturally on ARM its not required for RAM to be at address 0 and
> it is not uncommon for it to start at 1, 2 or 3GB (as a property of the
> SoC design).
> 
> If you have 128M of RAM at 0x80000000-0x88000000 then max_pfn is 0x88000
> but target pages is just 0x8000, if current_pages is initialised to
> max_pfn then the kernel immediately thinks it has to get rid of 0x800000
> pages.

And there is some sort of benefit from also doing this for virtual
machines?

>> And short of the initial value needing to be architecture specific -
>> can you see a calculation that would yield a decent result on ARM
>> that would also be suitable on x86?
> 
> I previously had a patch to use memblock_phys_mem_size(), but when I saw
> Boris switch to get_num_physpages() I thought that would be OK, but I
> didn't look into it very hard. Without checking I suspect they return
> pretty much the same think and so memblock_phys_mem_size will have the
> same issue you observed (which I confess I haven't yet gone back and
> understood).

If there's no reserved memory in that range, I guess ARM might
be fine as is.

Jan


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