linux-kernel - Re: [RFC 2/2] virtio_balloon: auto-ballooning support

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Message-ID: <20130513153513.GA4981@redhat.com>
Date:	Mon, 13 May 2013 18:35:13 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Luiz Capitulino <lcapitulino@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	aquini@...hat.com, amit.shah@...hat.com, anton@...msg.org
Subject: Re: [RFC 2/2] virtio_balloon: auto-ballooning support

On Mon, May 13, 2013 at 11:22:58AM -0400, Rik van Riel wrote:
> On 05/13/2013 11:16 AM, Michael S. Tsirkin wrote:
> 
> >However, there's a big question mark: host specifies
> >inflate, guest says deflate, who wins?
> 
> If we're dealing with a NUMA guest, they could both win :)
> 
> The host could see reduced memory use of the guest in one
> place, while the guest could see increased memory availability
> in another place...
> 
> I also suspect that having some "churn" could help sort out
> exactly what the working set is.
> 
> >At some point Google sent patches that gave guest
> >complete control over the balloon.
> >This has the advantage that management isn't involved.
> 
> I believe the Google patches still included some way for the
> host to initiate balloon inflation on the guest side, because
> the guest internal state alone is not enough to see when the
> host is under memory pressure.
> 
> I discussed the project with the Google developers in question
> a little over a year ago, but I do not remember whether their
> pressure notification went through qemu, or directly from the
> host kernel to the guest kernel...

So increasing the max number of pages in balloon
indicates host memory pressure to the guest?
Fair enough but I wonder whether there's a way to
make it more explicit in the interface, somehow.


> >And at some level it seems to make sense: why set
> >an upper limit on size of the balloon?
> >The bigger it is, the better.
> 
> Response time.
> 
> If too much of a guest's memory has been removed, it can take
> too long for the guest to react to user requests, be it over
> the web or ssh or something else...

Absolutely. But it's a Guest issue. Host does not care.
If Guest wants to shoot itself in the foot it has
many other ways to do this.

> -- 
> All rights reversed
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