linux-kernel - Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works

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Date:	Mon, 10 Sep 2012 08:55:07 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	fes@...gle.com, aarcange@...hat.com, riel@...hat.com,
	yvugenfi@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mikew@...gle.com, yinghan@...gle.com,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio-balloon spec: provide a version of the "silent
 deflate" feature that works

Il 10/09/2012 08:47, Michael S. Tsirkin ha scritto:
> On Mon, Sep 10, 2012 at 08:38:09AM +0200, Paolo Bonzini wrote:
>> Il 10/09/2012 08:03, Michael S. Tsirkin ha scritto:
>>> On Mon, Sep 10, 2012 at 07:50:13AM +0200, Paolo Bonzini wrote:
>>>> Il 09/09/2012 00:22, Michael S. Tsirkin ha scritto:
>>>>>> Almost.  One is "the guest, if really needed, can tell the host of
>>>>>> pages".  If not negotiated, and the host does not support it, the host
>>>>>> must break the guest (e.g. fail to offer any virtqueues).
>>>>>
>>>>> There is no way in spec to break the guest.
>>>>> You can not fail to offer virtqueues.
>>>>
>>>> You can always return 0 for the first queue.
>>>
>>> I don't think guest drivers recover gracefully from this.
>>> Do they?
>>
>> No, that's the point ("break the guest" is really "break the driver").
> 
> You can just stop VM then. No need for a side channel.

Keeping the VM running, just with no balloon driver is preferrable.

>>>>>> The other is "the guest, though, would prefer not to do so".  It is
>>>>>> different because the guest can proceed in a fallback mode even if the
>>>>>> host doesn't offer it.
>>>>>
>>>>> I think I get what your proposed SILENT means what I do not get
>>>>> is the motivation. It looks like a premature optimization to me.
>>>>
>>>> The motivation is to let the driver choose between two behaviors: the
>>>> current one where ballooning is only done on request, and a more
>>>> aggressive one.
>>>
>>> Yes but why is being silent any good? Optimization?
>>> Any data to show that it will help some workload?
>>
>> Idle guests can move cache pages to the balloon.  You can overcommit
>> more aggressively, because the host can madvise away a lot more memory.
> 
> IMHO this feature needs more thought. E.g. how will this work with assignment?

Revert to normal cooperative ballooning.

> If we build something let's build it in a way that plays nicely
> with other features.

Yes, that's the point of SILENT. :)

Paolo
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