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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.11.16 at 10:45, <srn@xxxxxxxxx> wrote:
> 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?

No, I'm not saying this, but please note that you didn't post the
question on xen-users, but xen-devel.

> 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.

I view this as a bug too, fwiw. Me previously bringing up the topic
didn't result in any activity though, presumably because 32-bit
guests are being considered rare enough these days.

>> 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?

I don't know, that's a question for xl folks I guess.

Jan


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