Re: [Xen-users] 32Bit domus and RAM between 128GB and 168 GB

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Wed, Jan 4, 2017 at 6:00 PM, Austin S. Hemmelgarn
<ahferroin7@xxxxxxxxx> wrote:
> On 2017-01-04 09:50, Jan Marquardt wrote:
>>
>> Hi,
>>
>> unfortunately we still have a lot of paravirtual guests with 32 Bit OS
>> and are currently running in some problems.
>>
>> As far as I understand the documentation in xend-config.sxp, if a
>> physical machine has between 128 GB and 168 GB RAM, 32 Bit guests should
>> be able to consume up to 168 GB.
>>
>> # 32-bit paravirtual domains can only consume physical
>> # memory below 168GB. On systems with memory beyond that address,
>> # they'll be confined to memory below 128GB.
>> # Using total_available_memory (in GB) to specify the amount of memory
>> reserved
>> # in the memory pool exclusively for 32-bit paravirtual domains.
>> # Additionally you should use dom0_mem = <-Value> as a parameter in
>> # xen kernel to reserve the memory for 32-bit paravirtual domains, default
>> # is "0" (0GB).
>>
>> In our case we have four systems with 160 GB RAM, but the guests are not
>> able to start if there are already guests running which are consuming
>> 128 GB RAM in sum.
>>
>> Is this a bug or do we miss something?
>
> 168GB seems like a really odd number, and I'd be very inclined to believe
> that this is a typo in the documentation (that is, it should say 'physical
> memory below 128GB').

I've just been having a chat with Andy Cooper, and apparently the
comment is actually not a typo; the explanation is rather complicated
but the summary is that the actual practical limit for modern 32-bit
Linux guests is 128 GB anyway.

I don't think this has been documented well anywhere -- this is
probably something we should change.

Jan,  Can you report the exact error message you got?  At a bare
minimum we should report a useful error message when people encounter
this situation.

 -George

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