Re: [Xen-devel] Can't always start 32 bit domains after 64 bit domains

[Sarah Newman <srn@xxxxxxxxx>]

On 11/21/2016 12:20 AM, Jan Beulich wrote:
>>>> On 19.11.16 at 22:22, <srn@xxxxxxxxx> wrote:
>> My current understanding is that on a server with more than 168GiB
>> of memory, I should still be able to around 128GiB of 32-bit PV
>> domUs, regardless of what order the domUs are started in.
> 
> You don't clarify what you base this understanding of yours on; I don't
> think this is the case. What exact memory will be allocated for 64-bit
> guests isn't very predictable. In particular, the preference of allocating
> 1Gb pages as long as available may result in allocations eating into the
> low 128Gb of memory earlier than you expect. You may want to analyze
> system state by looking at debug key output at relevant points in time.

Because whenever I read about the 168GiB limit and 32 bit domains there was no 
mention that the initial 128GiB might not all be available if there
were also 64 bit domains. Given how little visibility there typically is into 
where memory is physically allocated, there is no way to know this
doesn't work as a normal user until it doesn't. Are you saying all Xen users 
have to be developers too?

In my opinion this is a bug until/unless it's very clearly documented that it's 
not supported behavior to mix 32 and 64 bit PV domains when there is
more than 168GiB RAM, or maybe just period if that's too complicated to explain 
to people. I don't know where to find that documentation. It seems
like https://wiki.xenproject.org/wiki/Xen_Project_Release_Features might be the 
right place, I'm not sure. If you know of a better place I'd love to know.

> Back in the xend days someone here had invented a (crude) mechanism
> to set aside memory for 32-bit PV domains, but I don't think dealing with
> this situation in xl has ever seen any interest.

If I wanted to add that, where would it go?

Thanks, Sarah

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