Re: Future support of 5-level paging in Xen

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 01/03/2017 12:32 PM, anshul makkar wrote:
>
>
> On 08/12/16 23:40, Boris Ostrovsky wrote:
>>
>>
>> On 12/08/2016 05:21 PM, Andrew Cooper wrote:
>>> On 08/12/2016 19:18, Stefano Stabellini wrote:
>>
>>>
>>>> Of course even the largest virtual machine today (2TB on Amazon AFAIK)
>>>> is not close to reaching the current memory limit, but it's just a
>>>> matter of time.
>>>
>>> /me things Oracle will have something to say about this.  I'm sure
>>> there
>>> was talk about VMs larger than this at previous hackathons. XenServer
>>> functions (ish, so long as you don't migrate) with 6TB VMs, although
>>> starting and shutting them down feels like treacle.
>>
>>
>>
>> I've been working (on and off) with SGI to get one of their 32TB
>> boxes to boot and I don't think that works. We've fixed a couple of
>> bugs but I don't think Xen can boot with that much memory. We
>> successfully booted with just under 8TB but couldn't do it with the
>> full system. The machine has been taken from us for now so this work
>> is on hold.
>>
>> This is on OVM, which is 4.4-based, we haven't tried (IIRC) latest bits.
>>
>> (BTW, speaking of slow starting and shutting down very large guests ---
>> have you or anyone else had a chance to look at this? My
>> investigation initially pointed to scrubbing and then to an insane
>> number of hypercall preemptions in relinquish_memory()).
>>
> I had a quick look at it when I was working on support for large guest
> and found that scrubbing was indeed one of the issue. Just haven't got
> time to look at it in more details. Hopefully in near future, might
> work on it.

If you are interested I can share with you the prototype code that I
have for improving scrubbing performance. Ping me when/if you start
looking into this.

-boris

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