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

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Message-ID: <20160304122456-mutt-send-email-mst@redhat.com>
Date:	Fri, 4 Mar 2016 12:36:58 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Li, Liang Z" <liang.z.li@...el.com>
Cc:	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	Roman Kagan <rkagan@...tuozzo.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>
Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration
 optimization

On Fri, Mar 04, 2016 at 10:11:00AM +0000, Li, Liang Z wrote:
> > On Fri, Mar 04, 2016 at 09:12:12AM +0000, Li, Liang Z wrote:
> > > > Although I wonder which is cheaper; that would be fairly expensive
> > > > for the guest wouldn't it? And you'd somehow have to kick the guest
> > > > before migration to do the ballooning - and how long would you wait for
> > it to finish?
> > >
> > > About 5 seconds for an 8G guest, balloon to 1G. Get the free pages
> > > bitmap take about 20ms for an 8G idle guest.
> > >
> > > Liang
> > 
> > Where is the time spent though? allocating within guest?
> > Or passing the info to host?
> > If the former, we can use existing inflate/deflate vqs:
> > Have guest put each free page on inflate vq, then on deflate vq.
> > 
> 
> Maybe I am not clear enough.
> 
> I mean if we inflate balloon before live migration, for a 8GB guest, it takes about 5 Seconds for the inflating operation to finish.

And these 5 seconds are spent where?

> For the PV solution, there is no need to inflate balloon before live migration, the only cost is to traversing the free_list to
>  construct the free pages bitmap, and it takes about 20ms for a 8GB idle guest( less if there is less free pages),
>  passing the free pages info to host will take about extra 3ms.
> 
> 
> Liang

So now let's please stop talking about solutions at a high level and
discuss the interface changes you make in detail.
What makes it faster? Better host/guest interface? No need to go through
buddy allocator within guest? Less interrupts? Something else?


> > --
> > MST