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Re: [Xen-devel] [PATCH RFC v2] Add SUPPORT.md



On 11/01/2017 05:10 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Oct 24, 2017 at 04:22:38PM +0100, George Dunlap wrote:
>> On Fri, Sep 15, 2017 at 3:51 PM, Konrad Rzeszutek Wilk
>> <konrad.wilk@xxxxxxxxxx> wrote:
>>>> +### Soft-reset for PV guests
>>>
>>> s/PV/HVM/
>>
>> Is it?  I thought this was for RHEL 5 PV guests to be able to do crash 
>> kernels.
>>
>>>> +### Transcendent Memory
>>>> +
>>>> +    Status: Experimental
>>>> +
>>>> +[XXX Add description]
>>>
>>> Guests with tmem drivers autoballoon memory out allowing a fluid
>>> and dynamic memory allocation - in effect memory overcommit without
>>> the need to swap. Only works with Linux guests (as it requires
>>> OS drivers).
>>
>> But autoballooning doesn't require any support in Xen, right?  I
>> thought the TMEM support in Xen was more about the trancendent memory
>> backends.
> 
> frontends you mean? That is Linux guests when compiled with XEN_TMEM will
> balloon down (using the self-shrinker) to using the normal balloon code
> (XENMEM_decrease_reservation, XENMEM_populate_physmap) to make the
> guest smaller. Then the Linux code starts hitting the case where it starts
> swapping memory out - and that is where the tmem comes in and the
> pages are swapped out to the hypervisor.

Right -- so TMEM itself actually consists of this ephemeral and
non-ephemeral memory pools.  Autoballooning is just a trick to get Linux
to put the least-used pages into one of the pools.

How about this:

---
Transcendent Memory (tmem) allows the creation of hypervisor memory
pools which guests can use to store memory rather than caching in its
own memory or swapping to disk.  Having these in the hypervisor can
allow more efficient aggregate use of memory across VMs.
---

 -George

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