linux-kernel - Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization

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Message-ID: <20160309170438.GB9715@rkaganb.sw.ru>
Date:	Wed, 9 Mar 2016 20:04:39 +0300
From:	Roman Kagan <rkagan@...tuozzo.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"Li, Liang Z" <liang.z.li@...el.com>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	"ehabkost@...hat.com" <ehabkost@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"quintela@...hat.com" <quintela@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"amit.shah@...hat.com" <amit.shah@...hat.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"rth@...ddle.net" <rth@...ddle.net>, <riel@...hat.com>
Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration
 optimization

On Wed, Mar 09, 2016 at 05:41:39PM +0200, Michael S. Tsirkin wrote:
> On Wed, Mar 09, 2016 at 05:28:54PM +0300, Roman Kagan wrote:
> > For (1) I've been trying to make a point that skipping clean pages is
> > much more likely to result in noticable benefit than free pages only.
> 
> I guess when you say clean you mean zero?

No I meant clean, i.e. those that could be evicted from RAM without
causing I/O.

> Yea. In fact, one can zero out any number of pages
> quickly by putting them in balloon and immediately
> taking them out.
> 
> Access will fault a zero page in, then COW kicks in.

I must be missing something obvious, but how is that different from
inflating and then immediately deflating the balloon?

> We could have a new zero VQ (or some other option)
> to pass these pages guest to host, but this only
> works well if page size matches the host page size.

I'm afraid I don't yet understand what kind of pages that would be and
how they are different from ballooned pages.

I still tend to think that ballooning is a sensible solution to the
problem at hand; it's just the granularity that makes things slow and
stands in the way.

Roman.