Re: [Xen-devel] [PATCH] libxc: try to find last used pfn when migrating

[Juergen Gross <jgross@xxxxxxxx>]

On 27/11/15 16:01, David Vrabel wrote:
> On 27/11/15 14:50, Juergen Gross wrote:
>> For migration the last used pfn of a guest is needed to size the
>> logdirty bitmap and as an upper bound of the page loop. Unfortunately
>> there are pv-kernels advertising a much higher maximum pfn as they
>> are really using in order to support memory hotplug. This will lead
>> to allocation of much more memory in Xen tools during migration as
>> really needed.
>>
>> Try to find the last used guest pfn of a pv-domu by scanning the p2m
>> tree from the last entry towards it's start and search for an entry
>> not being invalid.
>>
>> Normally the mid pages of the p2m tree containing all invalid entries
>> are being reused, so we can just scan the top page for identical
>> entries and skip them but the first one.
> 
> This has been fixed in Linux by "x86/xen/p2m: hint at the last populated
> P2M entry" 98dd166ea3a3c3b57919e20d9b0d1237fcd0349d which is tagged for
> stable.
> 
> Do we really need a toolstack fix as well?

xl migrate will use much less resources for a domain with a 3.x kernel
started with max_mem being much larger than mem. E.g. in case you start
a domain on a small stand-by system and migrate it later to the large
production system and want to balloon it up there.

Additionally there was a discussion this week on irc regarding this
topic and concern was raised this could block dom0 responsiveness.


Juergen


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